✦ Free & No Login Required

Heat Loss Calculator

Calculate room and whole-house heat loss in Watts, BTU/hr and kW. Transparent U-values, location-aware design temperatures, and equipment sizing — all in one place.

Units
Building Type
Heating System
°C
°C
m
m
m

🧱 Building Envelope & Thermal Construction

Floor Area
20.0m²
Volume
48.0m³
Design ΔT
25.0K
⚡ Live Calculation Result
713 W
0.71 kW • 2,433 BTU/hr

Heat Loss Breakdown

713
Watts

Loss Category Summary

🧱
Fabric / Transmission
Walls, windows, doors, roof, and floor conduction
440 W
1,502 BTU/hr
💨
Infiltration & Ventilation
Air leakage and fresh air ventilation losses
238 W
811 BTU/hr
🔗
Thermal Bridging Allowance
Linear junction heat bypass (approx. 8–10%)
35 W
120 BTU/hr
Total Heat Loss Rate
713 W
2,433 BTU/hr
🔥
Radiator Output
713 W
at ΔT50
⚙️
Boiler Sizing
~0.9 kW
room contribution
♻️
Heat Pump Radiator Sizing
1,191 W
radiator output at ΔT30
✦ Overview

What Is a Heat Loss Calculator?

A heat loss calculator estimates the rate at which thermal energy escapes from a building or room through its walls, windows, doors, roof, floor, and via air leakage and ventilation.

The result — expressed in Watts, kilowatts, or BTU/hr — represents the heating power required to maintain a comfortable indoor temperature during the coldest design conditions for your location.

Who Uses a Heat Loss Calculator?

  • ✓ Homeowners — sizing replacement radiators and evaluating insulation upgrades
  • ✓ Heating Installers — specifying boilers, heat pumps, and emitter sizing accurately
  • ✓ HVAC Engineers — verifying room-by-room steady-state thermal loads
  • ✓ Energy Assessors — benchmarking building energy efficiency and EPC ratings
  • ✓ DIY Renovators — planning room additions, extensions, and draught-proofing
Climatic Delta T Engine BS EN 12831

📍 London, United Kingdom

Heating Design Reference Conditions

Indoor Design Temp 21°C (70°F)
Outdoor Design Temp −4°C (25°F)
Design ΔT 25 K (ΔT)
Heating Degree Days 2,200 HDD
Data Source CIBSE Guide A / DIN EN 12831
Interactive Building Envelope Explorer

Interactive Building Heat Loss Cutaway

Click any zone in the building cutaway below to inspect heat loss pathways, typical U-values, and recommended retrofit remedies.

Click any hotspot to inspect Outdoor: −4°C
Roof & Loft (25%) Internal 21°C Walls (35%) Window Window Draughts Ground Floor (10%)
🧱 35% of Total Loss

External Walls

Typically the largest surface area of any building envelope. Solid uninsulated masonry transfers over 7× more energy per m² than modern insulated cavity walls.

Typical U-Value: 0.28 W/m²K (Insulated) vs 2.10 W/m²K (Solid)
Recommended Fix: Cavity Wall Bead / External Wall Insulation
⚡ Engineering Methodology

How Heat Loss Is Calculated

Every calculation is based on transparent, peer-reviewed building physics principles conforming to BS EN 12831 and ISO 52016.

🧱 BS EN ISO 6946

Fabric Transmission Heat Loss

Thermal energy conducted directly through solid building surfaces based on surface area, composite U-value, and internal-to-external temperature delta.

Q = Σ(U × A × ΔT)
Q = heat loss rate (W) · U = thermal transmittance (W/m²K) · A = area (m²) · ΔT = temperature difference (K)
💨 EN 12831 Vent

Ventilation & Infiltration Heat Loss

Heat energy carried away by natural infiltration through cracks and gaps plus controlled fresh air exchange required for indoor air quality.

Q = 0.33 × n × V × ΔT
n = air changes per hour (ACH) · V = room volume (m³) · 0.33 = volumetric heat capacity of air (Wh/m³K)
🔗 ISO 10211 (Ψ)

Thermal Bridging Heat Loss

Additional two-dimensional and three-dimensional conductive heat bypass occurring at structural junctions, lintels, corners, and window reveals.

Q_tb = Σ(L × Ψ × ΔT)
L = junction length (m) · Ψ (psi) = linear thermal transmittance (W/mK)
Σ Peak Total

Total Design Heat Loss Rate

The instantaneous sum of all fabric conduction, air exchange, and junction bypass losses representing peak heating system demand.

Q_total = Q_fab + Q_vent + Q_tb
Output in Watts (W), kilowatts (kW), and BTU/hr for universal compatibility.
🧪 Live Sandbox

Interactive Heat Loss Formula Sandbox

Adjust the U-value, surface area, and temperature difference below to see the fundamental equation Q = U × A × ΔT calculated in real time.

0.28 W/m²K
0.10 (Passivhaus) 3.00 (Solid Brick)
20 m²
2 m² (Window) 150 m² (Whole Wall)
25 K (ΔT)
5 K (Mild) 45 K (Sub-Zero Freeze)
Calculated Fabric Heat Transmission
140 W
≈ 478 BTU/hr (0.14 kW)
📐 Step-by-Step Guide

How to Measure a Room for Heat Loss

Follow these 5 straightforward steps to gather accurate room dimensions, boundary conditions, and envelope parameters for precision heating calculations.

01 Geometry

Step 1: Measure Room Dimensions

Measure internal length, width, and clear ceiling height from finished floor to ceiling with a laser measure or tape.

Floor Area = Length × Width
Volume = Floor Area × Height
02 Wall Orientation

Step 2: Count External Walls

Identify how many walls face the outdoor air versus heated adjacent rooms. Only exterior-facing walls lose heat.

  • • 1 Wall (Terrace/Mid)
  • • 2 Walls (Corner/End)
  • • 3 Walls (Extension)
  • • 4 Walls (Detached/Bungalow)
03 Openings

Step 3: Measure Windows & Doors

Measure opening dimensions for all exterior glazing and doors to calculate gross vs net external wall areas accurately.

Gross Wall Area: = External Walls (External Walls:)
Net Wall Area: = Gross Wall Area: − (Windows & Glazing + External Doors)
04 Boundaries

Step 4: Identify Ceiling & Floor Boundaries

Determine if there is an unheated loft or flat roof above, and whether the floor is a ground slab, crawlspace, or heated room below.

  • • Well Insulated 270mm (U = 0.16)
  • • Conditioned Floor Above (U = 0.00)
  • • Standard Insulated Floor (U = 0.25)
  • • Conditioned Floor Below (U = 0.00)
2.5D Blueprint Visualizer

Interactive Room Geometry & Area Calculator

Adjust the sliders below to explore how room proportions, ceiling height, and external wall count affect gross versus net surface areas.

20.0 m² Windows & Glazing L: 5.0m W: 4.0m H: 2.4m
Floor Area
20.0 m²
Volume
48.0 m³
Ext Walls
21.6 m²
Understanding Results

Understanding Your Heat Loss Results

Learn how heating metrics correlate, how to convert between Watts, BTU/hr, and kW, and benchmark your property against building standards.

⚡

Watts (W)

The standard metric unit for instantaneous heating power. Used globally for radiator ratings, underfloor loops, and heat emitter sizing.

1 W = 3.412142 BTU/hr
🔥

BTU per Hour (BTU/hr)

British Thermal Units per hour. Widely used in the United States and the UK plumbing trade for radiator catalog sizing.

1,000 BTU/hr = 293.07 W = 0.293 kW
⚙️

Kilowatts (kW)

Equal to 1,000 Watts. Standard rating unit for central heating boilers, heat pumps, chillers, and whole-house thermal loads.

1 kW = 1,000 W = 3,412 BTU/hr
Live Unit Converter

Universal Heating Power Converter

Type into any box below to convert instantly between Watts (W), Kilowatts (kW), and BTU per hour (BTU/hr).

Property Age & Energy Benchmarks

Older, uninsulated solid brick properties typically require 120–160 W/m² of heating power, while modern Part L compliant homes require only 35–50 W/m².

Ultra-low-energy Passivhaus dwellings can operate comfortably at 10–15 W/m², requiring only minimal heating even during freezing conditions.

Heat Loss per Square Metre (W/m² Ratio)

Divide your total heat loss by the floor area. A result below 50 W/m² indicates exceptional thermal efficiency, while over 100 W/m² suggests urgent retrofit needs.

Building Age / Era Specific Heat Loss (W/m²) Specific Heat Loss (BTU/hr·ft²)
Pre-1919 Solid Brick (Uninsulated) 120 – 160 W/m² 38 – 51 BTU/hr·ft²
1930s – 1970s Uninsulated Cavity 90 – 120 W/m² 29 – 38 BTU/hr·ft²
1980s – 1990s Partially Insulated 60 – 80 W/m² 19 – 25 BTU/hr·ft²
Modern Build (Building Regs 2022+) 35 – 50 W/m² 11 – 16 BTU/hr·ft²
Passivhaus / Super-Insulated 10 – 15 W/m² 3.2 – 4.8 BTU/hr·ft²

Values at standard design condition (ΔT = 25 K). Actual performance varies by geometric exposure.

🔍 Thermal Elements

What Affects Building Heat Loss?

Every building element contributes to total heat loss. Understanding each element helps prioritize cost-effective insulation and heating upgrades.

🧱
35%

External Walls

Typically the largest single heat-loss surface area in any building. U-values range from 0.18 W/m²K for modern cavities to 2.10 W/m²K for uninsulated solid masonry.

2.10 W/m²K → 0.28 W/m²K
🏠
25%

Roofs & Loft Ceilings

Warm air rises naturally due to buoyancy (stack effect). Uninsulated roofs account for up to 25% of total home heat loss.

2.30 W/m²K → 0.16 W/m²K
🪟
15%

Windows & Glazing

Glass conducts heat 4 to 8 times faster per square metre than insulated walls. Upgrading to Low-E double or triple glazing dramatically cuts radiative loss.

4.80 W/m²K → 0.80 W/m²K
💨
15%

Air Leakage & Draughts

Uncontrolled infiltration through gaps, service penetrations, floorboards, and loose joinery brings cold outdoor air directly indoors.

1.50 ACH → 0.40 ACH
🪵
10%

Ground & Exposed Floors

Conduction through ground slabs or suspended timber floors over cold crawlspaces can account for 10–15% of space heating demand.

0.80 W/m²K → 0.22 W/m²K
🚪
5%

External Doors

Exterior doors introduce both direct conductive heat transmission and infiltration leakage around perimeters, frames, and letterboxes.

3.00 W/m²K → 1.10 W/m²K
🌬️
MVHR

Ventilation Strategy

Controlled ventilation is essential for indoor air quality and moisture control. MVHR systems reclaim up to 90% of heat from outgoing exhaust air.

100% loss → 90% recovery
🧭
A/V

Building Exposure & Geometry

Detached homes have four exposed walls and lose substantially more heat than terraced houses or mid-floor flats protected by heated neighbours.

4 Ext. Walls → 2 Ext. Walls
🧱
Inspecting Element: 35%

External Walls

Uninsulated / Poor:
2.10 W/m²K
➔
Modern / Insulated:
0.28 W/m²K
86% Less Loss
🌐 Climatic Parameters

Design Temperature for Heat Loss

The winter design temperature difference (ΔT) is the fundamental multiplier for every heat loss calculation. It ensures heating systems never fail during cold snaps.

Indoor Design Temperature (T_indoor)

Typically set to 21°C (70°F) for primary living rooms, lounges, and bathrooms, and 18°C (64°F) for bedrooms and utility spaces per CIBSE Guide A recommendations.

Outdoor Design Temperature (T_outdoor)

Based on 99.6% statistical winter meteorological percentiles. It represents the temperature that local outdoor conditions will exceed 99.6% of the winter hours.

Global 99.6% Benchmarks ASHRAE / CIBSE
Location Outdoor Design (°C) Design ΔT (at 21°C)
London, UK -4°C 25 K
Birmingham, UK -3.4°C 24.4 K
Manchester, UK -3.1°C 24.1 K
Edinburgh, UK -7°C 28 K
Dublin, IE -3°C 24 K
New York, US -13°C 34 K
Climatic Selector

Climatic Location Lookup & Thermal Delta Visualizer

Select your regional city below to inspect official design temperatures and see the corresponding temperature differential on the gradient scale.

Selected Location:

London, UK

Degree Days 2,120 HDD
Design ΔT: 25.0 K (ΔT)
❄️ Outdoor: -4°C ← Thermal Lift → 🌡️ Indoor: 21°C
🧱 U-Values & R-Values

Thermal Transmittance & Resistance Reference

U-values (W/m²K) quantify the rate of thermal transmittance through composite assemblies, while R-values (m²K/W) measure thermal resistance.

Transparent U-Values vs Vague Presets

Unlike consumer calculators that hide critical assumptions behind generic 'Poor', 'Average', or 'Good' labels, our engine displays explicit numerical U-values for complete engineering transparency.

U-Value (Metric) vs R-Value (Imperial)

Europe and the UK use U-values (W/m²K, lower is better), whereas North America uses R-values (hr·ft²·°F/BTU, higher is better). The mathematical relationship is inverse: U = 1 / R_si.

BS EN ISO 6946 Reference U ↔ R Values
Building Element Typical U-Value Rating
Insulated cavity wall 0.28 W/m²K Part L 2021
Solid brick (uninsulated) 2.10 W/m²K Uninsulated
Double glazing 1.40 W/m²K Standard Double
Triple glazing Low-E 0.80 W/m²K Triple Glazed
Insulated roof (270mm) 0.16 W/m²K 270mm Loft
Ground floor (insulated) 0.22 W/m²K 100mm Floor PIR
Composite door 1.20 W/m²K Composite Door
Wall Cross-Section

Multi-Layer Wall Assembly & Thermal Gradient Simulator

Select a construction archetype below to visualize the physical layer cross-section and observe how heat drops across the thermal boundary.

Assembly Type: Modern Insulated Cavity Wall (Building Regs Part L)
Calculated Assembly U-Value: 0.28 W/m²K
Outer Brick 102mm Insulation 100mm Blockwork 100mm Plaster −4°C +21°C
⚡ Heating Systems

Heating System & Emitter Sizing

Your heat loss calculation directly determines heating equipment sizing. Different heat emitters and generators require specific temperature regimes.

🔥 BS EN 442

Radiator Sizing

Ensure each room's radiator output matches heat loss at the design flow temperature. Radiators running on heat pumps need correction factors.

Standard: ΔT50 / Heat Pump: ΔT30
⚙️ Condensing 94%

Boiler Sizing

Aggregate total whole-house heat loss plus domestic hot water (DHW) cylinder allowances. Avoid severe oversizing to preserve seasonal efficiency.

Return Temp ≤ 54°C Dew Point
🌀 Ducted Air

Warm Air Furnace Sizing

Size forced-air furnaces to match total building thermal envelope demand while balancing ductwork CFM airflow and duct static pressure.

AFUE Rating: 96%+ High Efficiency
♻️ SCOP 3.8 – 4.6

Heat Pump Sizing

Heat pumps operate most efficiently at lower flow temperatures (35°C–45°C). Precise sizing avoids short-cycling and backup heater usage.

MCS Sizing: 100% Peak Design Load
⚡ 100% Direct

Direct Electric Heaters

100% efficient at point of use with a direct 1:1 conversion between thermal loss (Watts) and electric panel heater wattage.

Infrared Panels & Storage Heaters
🌊 35°C Ultra-Low

Underfloor Heating (UFH)

Operates at gentle low temperatures (30°C–35°C) across large floor surface areas, limited to ~75–100 W/m² for physiological foot comfort.

Output: 70–100 W/m² Floor Area
🛠️ Live Emitter & Plant Sizing Engine

Interactive Heat Emitter & Flow Temperature Sizer

Adjust total room heat loss to calculate required flow temperatures, catalog radiator corrections, and electrical consumption across different heating systems.

8.0 kW
2 kW (Small Flat) 25 kW (Large Home)
45°C Flow (ΔT25)
35°C (UFH) 45°C (Heat Pump) 75°C (Boiler)
Recommended Unit Size: 8.0 kW Unit
Radiator Sizing Factor: 2.46 × ΔT50 Catalog
Est. Seasonal Efficiency: SCOP 3.80 (380% Eff)
Target Min. Pipe Size: 28mm Primary Copper
Delta T Physics

Delta T (ΔT) & Heat Pump Radiator Sizing

Radiator catalog ratings are traditionally published at ΔT50 (75°C flow). When retrofitting heat pumps at ΔT30 (45°C flow), radiators produce ~51% of rated heat.

What Is Radiator Delta T (ΔT)?

Delta T is the difference between the mean water temperature in the radiator and the surrounding room air temperature. Lower water temperature reduces heat output exponentially.

ΔT = ((Tflow + Treturn) / 2) − Troom
Example: ((75 + 65) / 2) − 20 = ΔT50

The Radiator Correction Exponent Formula

Radiator heat emission is governed by the standard BS EN 442 formula with an exponential correction factor of 1.3.

Qactual = QΔT50 × (ΔTactual / 50)1.3
At ΔT30: (30 / 50)1.3 = 0.514 (51.4%)
Operating ΔT Flow / Return Temperatures Output Factor (vs ΔT50) Standard Heating Application
ΔT60 85°C / 75°C 1.26× Older non-condensing gravity/oil boilers
ΔT50 75°C / 65°C 1.00× Standard UK/European manufacturer catalog rating
ΔT40 65°C / 55°C 0.75× High-efficiency condensing gas/LPG boilers
ΔT30 50°C / 40°C 0.51× Standard Air Source Heat Pump (ASHP) target
ΔT25 45°C / 35°C 0.40× Ultra-efficient low-temp Heat Pump / Ground Source

Values at standard design condition (ΔT = 25 K). Actual performance varies by geometric exposure.

Correction Curve

Interactive Radiator Flow Temperature & Output Curve

Drag the slider to test different central heating flow temperatures and view real-time radiator size multipliers and upgrade recommendations.

45°C
35°C (Air Source Heat Pump (ΔT30)) 75°C (Gas / Oil Boiler (ΔT50))
Operating Delta T
ΔT20.0 K
Output vs Catalog
30.3%
Radiator Oversizing Multiplier Required
3.30×

Requires ~1.94× catalog radiator output. Upgrade single panel (Type 11) to Type 22.

♻️

Rule 1: Lower Flow Equals Higher Efficiency

Every 1°C reduction in heat pump flow temperature increases the seasonal coefficient of performance (SCOP) by approximately 2.5%, saving substantial electricity.

📈

Rule 2: Upgrade to Type 22 or Type 33 Radiators

Instead of increasing radiator wall length, install double-panel double-convector (Type 22) or triple-panel (Type 33) units to double thermal output in the same footprint.

📐

Rule 3: Balance Systems Room by Room

Hydronic balancing via lockshield valves ensures equal heat distribution, eliminating cold rooms and allowing the entire house to run on lower flow temperatures.

Engineering Standards

Heat Loss Calculator vs Engineering Standards

Compare how our transparent web-native heat loss calculator matches up against formal industry standards like ACCA Manual J, BS EN 12831, and CIBSE Guide A.

🇺🇸

ACCA Manual J (United States)

The official standard for residential equipment sizing in North America. Features comprehensive solar gain matrices and duct loss coefficients.

🇪🇺

BS EN 12831 (United Kingdom & Europe)

The European standard for calculating design heat load. Features room-by-room steady-state transmission and ventilation thermal loss algorithms.

🇬🇧

CIBSE Guide A (Environmental Design)

The UK authoritative benchmark for thermal design, climatic weather percentiles, air exchange rates, and internal comfort temperatures.

Standard Selector

Standards Recommendation Wizard

Select your project objective and location below to identify which calculation methodology is best suited for your project.

Recommended: Web-Native Tool

Our Free Online Heat Loss Calculator

Ideal for homeowners, heating installers, and energy assessors. Delivers instant, transparent room-by-room Watts and BTU/hr sizing informed by physical steady-state conduction and CIBSE Guide A principles.

Key Feature Our Heat Loss Calculator ACCA Manual J (USA) BS EN 12831 (UK / EU)
Primary Purpose Radiator sizing, heat pump checks, insulation ROI, room estimates US permit submissions, code compliance, warranty sign-off Commercial & residential design heating load compliance
Calculation Method Steady-state conduction (Q = U·A·ΔT) + air exchange Multi-zone heat balance with solar radiation matrices Room-by-room steady-state transmission & ventilation load
U-Value Transparency Explicit numerical U-values with instant manual override Component tables with assembly R-values Standard multi-layer thermal resistance summation
Accessibility & Cost 100% Free, instant client-side execution, no account needed $300–$800 fee for certified HVAC engineer survey Proprietary engineering suite licenses
Retrofit Strategies

How to Reduce Building Heat Loss

Reducing your property's heat loss allows you to install smaller, cheaper heating systems and permanently cut annual heating bills.

🏠
−25%

Loft & Attic Insulation

Upgrading from bare or shallow ceiling insulation to 270mm mineral wool (U = 0.16) cuts up to 25% of total home heat loss with rapid payback.

Before: 2.30 W/m²K
After: 0.16 W/m²K
Payback: 2 – 4 Years
🧱
−30%

Cavity & External Wall Insulation

Insulating external walls with cavity beads or external EPS renders saves up to 30% of space heating energy by addressing the home's largest surface area.

Before: 1.50 – 2.10 W/m²K
After: 0.18 – 0.28 W/m²K
Payback: 3 – 6 Years
💨
−15%

Draught-Proofing & Air Sealing

Sealing cracks, perimeter joints, letterboxes, and suspended floor edges reduces unwanted cold air infiltration by up to 15% with minimal upfront cost.

Before: 1.5 – 2.5 ACH
After: 0.5 – 0.7 ACH
Payback: < 1 Year
🪟
−12%

Double & Triple Low-E Glazing

Replacing single pane or older blown double glazing with modern argon-filled Low-E units (U = 1.2 to 0.8) cuts conductive window heat loss in half.

Before: 4.80 W/m²K
After: 1.20 – 0.80 W/m²K
Payback: 8 – 15 Years
⬇️
−8%

Ground & Suspended Floor Insulation

Installing rigid PIR insulation beneath ground floorboards or floor screeds stops cold subterranean air draughts, saving 8–10% of heat demand.

Before: 0.80 W/m²K
After: 0.20 W/m²K
Payback: 4 – 8 Years
🌬️
−85%

Mechanical Ventilation with Heat Recovery (MVHR)

Balanced whole-house mechanical ventilation continuously supplies fresh filtered air while extracting stale air, reclaiming up to 90% of heat.

Before: 100%
After: 85–92%
Payback: 5 – 10 Years
Savings Simulator

Interactive Retrofit Energy Savings Simulator

Select energy efficiency improvements below to simulate cumulative heat loss reduction, peak load decrease, and estimated annual financial savings.

Simulated Peak Heat Loss
10,000 W
Baseline (Uninsulated)
Super Insulated (~2 kW) Baseline (10 kW)
Estimated Annual Energy Bill Savings: £0 / $0 / yr

Ready to Model Your Own Home's Energy Upgrades?

Use our live interactive calculator to compare before-and-after heat loss scenarios with custom wall, window, and roof insulation values.

Open Heat Loss Calculator ↑
🧰 Dedicated Calculation Engines

Explore the Heat Loss Calculator Suite

Access our complete family of specialized thermal engineering tools for rooms, whole houses, radiators, heat pumps, and insulation assemblies.

🏠
Single Room

Room Heat Loss Calculator

Calculate precise heat loss for individual rooms to size replacement radiators.

Open Dedicated Tool →
🏘️
Whole Building

Whole House Calculator

Aggregate whole-building heat demand to size central heating boilers and heat pumps.

Open Dedicated Tool →
📋
Multi-Zone + CSV

Room-by-Room Calculator

Comprehensive multi-room inventory with cumulative heat loss schedules.

Open Dedicated Tool →
📐
ISO 6946

U-Value Calculator

Calculate composite U-values for multi-layer wall, roof, and floor assemblies.

Open Dedicated Tool →
🔄
4-Way Converter

R-Value to U-Value Converter

Instantly convert between imperial R-values (hr·ft²·°F/BTU) and metric U-values.

Open Dedicated Tool →
🔥
BS EN 442

Radiator Sizing Calculator

Size panel radiators for ΔT50, ΔT40, and low-temperature ΔT30 heat pumps.

Open Dedicated Tool →
⚙️
Combi & System

Boiler Sizing Calculator

Size condensing gas, oil, or LPG boilers with domestic hot water allowances.

Open Dedicated Tool →
♻️
MCS & SCOP

Heat Pump Sizing Calculator

Determine design kW capacity and seasonal coefficient of performance (SCOP).

Open Dedicated Tool →
🌡️
80+ Cities Database

Design Temperature Lookup

Search regional 99.6% winter design outdoor temperatures across the globe.

Open Dedicated Tool →
💨
Blower Door & MVHR

Air Infiltration / ACH Calculator

Estimate ventilation and air leakage heat loss based on air changes per hour.

Open Dedicated Tool →
Case Studies

Heat Loss Calculation Worked Examples

Review complete room-by-room calculation breakdowns across four common building archetypes to verify the underlying mathematical models.

Example 1

Small Bedroom (780 W)

Dimensions: 3.0m × 3.0m × 2.4m
External Walls: 2
Glazing Area: 1 × double (~1.5 m²)
Design ΔT: 25 K
780 W
2,661 BTU/hr
Example 2

Living Room (2,150 W)

Dimensions: 5.0m × 4.0m × 2.4m
External Walls: 2
Glazing Area: 2 × double (~3.0 m²)
Design ΔT: 25 K
2,150 W
7,336 BTU/hr
Example 3

3-Bed Semi-Detached House (8.06 kW)

Dimensions: 8.5m × 5.0m × 4.8m
External Walls: 3
Glazing Area: Glazing (~18 m²)
Design ΔT: 25 K
8.06 kW
27,500 BTU/hr
Example 4

Well-Insulated Passivhaus New Build (4.20 kW)

Dimensions: 10.0m × 6.0m × 4.8m
External Walls: 4
Glazing Area: Triple (~22 m²)
Design ΔT: 25 K
4.20 kW
14,330 BTU/hr
Active Scenario Inspector

Small Bedroom (780 W) (780 W)

Fabric Loss: 560 W + Air Infiltration: 220 W = Total: 780 W

Frequently Asked Questions

Find clear, expert answers to common questions about heat loss calculations, heating system sizing, U-values, and building thermal efficiency.

Enter the room dimensions, number of external walls, window and door sizes, insulation levels, and the indoor/outdoor design temperatures. The calculator applies the formula Q = U × A × ΔT for each building element, adds ventilation and infiltration losses, and gives you the total heat loss in Watts, kW, and BTU/hr.
The core fabric heat loss formula is Q = Σ(U × A × ΔT), where U is the thermal transmittance of a building element (W/m²K), A is its area (m²), and ΔT is the temperature difference between inside and outside (K). Ventilation heat loss uses Q = 0.33 × n × V × ΔT, where n is the air change rate and V is the room volume.
Use the heating design temperature for your location — typically the 99% or 99.6% winter design condition from sources like CIBSE Guide A (UK), ASHRAE Fundamentals (US), or your national building code. Our calculator includes a location lookup to suggest the appropriate value, but you can always override manually.
A U-value (W/m²K) measures how much heat passes through one square metre of a building element for every degree of temperature difference. Lower U-values mean better insulation. Unlike calculators that use vague labels like 'poor' or 'good', we display the actual U-value used in the calculation so you can verify and adjust it.
This calculator uses principles informed by recognised standards including ACCA Manual J, BS EN 12831, and CIBSE guidelines. It provides a Manual J-style estimate suitable for preliminary sizing and comparison. For official Manual J compliance — typically required for code submissions or equipment warranties in the US — a professionally reviewed calculation using certified software is recommended.
Yes. Heat pumps typically operate at lower flow temperatures (35–45°C) compared to boilers (60–80°C). This means a radiator's heat output at ΔT30 is significantly lower than at ΔT50. Our calculator shows radiator requirements at multiple ΔT values so you can size correctly for heat pump systems.
Online calculators provide estimates based on the inputs and assumptions you provide. Accuracy depends on the quality of your input data — particularly U-values, room dimensions, and air tightness. For most domestic radiator sizing, a well-configured online calculation is adequate. For complex buildings, regulatory submissions, or large installations, a professional survey and calculation is recommended.
Cold outside air that leaks into the building through gaps, cracks, and openings must be heated to room temperature. This infiltration heat loss can represent 15–25% of the total, especially in older buildings with poor air sealing. Excluding it would significantly underestimate your heating requirement.

About This Heat Loss Calculator

Built by thermal engineering specialists to provide transparent, accurate, and accessible building heat loss calculations for homeowners and heating professionals worldwide.

Why We Built This Calculator

Many online calculators hide calculations behind generic 'rule-of-thumb' estimates that lead to oversized boilers or undersized heat pumps. We believe heating sizing should be fully transparent and mathematically verifiable.

Engineering-Grade Methodology

All calculations adhere strictly to ISO 52016, BS EN 12831, and CIBSE Guide A steady-state principles, integrating transparent numerical U-values, room volume ventilation, and linear thermal bridging allowances.

Designed for Real-World Sizing

Whether sizing radiators for low-temperature heat pumps at ΔT30, specifying condensing gas boilers, or planning whole-house insulation retrofits, our calculations reflect manufacturer design standards.

100% Free & Client-Side Privacy

All calculation algorithms run directly in your browser. No sign-up, personal data submission, or tracking is required. Your room dimensions and property information remain completely private.