Heat Loss Calculator (walls, windows, roof, floor & ventilation)

Understanding the heat loss calculator — concept and principle

This calculator estimates how much heat a building loses through its walls, windows, roof and floor and through ventilation, at the coldest design temperature, and the heating energy needed over a year. Use it to size a heater or heat pump, compare insulation upgrades, or see where most heat escapes.

Fabric plus air: with the default inputs the result is 3,870 W (at 20 °C in, −10 °C out).

How it is calculated

Fabric loss for each part = U × A × ΔT, with U the U-value (W/m²K), A the area (m²) and ΔT = inside − outside temperature.
Ventilation loss = n × V × 1,200 ÷ 3,600 × ΔT, about 0.33 × n × V × ΔT, with n air changes per hour and V the heated volume.
Heat loss coefficient H (W/K) = Σ U·A + ventilation part.
Design heat loss = H × ΔT.
Yearly heating energy (kWh) = H × heating degree days × 24 ÷ 1,000.

The principle

Heat flows from warm to cold at a rate proportional to the temperature difference. The U-value says how many watts pass through one square metre for each degree of difference, so lower U-values mean better insulation. Air that leaks out or is ventilated takes its heat with it: warming one cubic metre of air by one degree takes about 1,200 joules. Adding all paths gives H, the building's total loss per degree. On the coldest design day, H times the temperature difference is the heater output needed.

Worked example

Default inputs: walls 80 m² at 0.35, windows and doors 15 m² at 1.6, roof 60 m² at 0.2, floor 60 m² at 0.25, volume 300 m³, 0.5 air changes, 20 °C inside, −10 °C outside, 2,800 degree days.

ΔT = 20 − (−10) = 30 K.
Walls 80 × 0.35 × 30 = 840 W; windows 15 × 1.6 × 30 = 720 W; roof 360 W; floor 450 W.
Ventilation = 0.5 × 300 × 1,200 ÷ 3,600 = 50 W/K × 30 = 1,500 W.
H = 28 + 24 + 12 + 15 + 50 = 129 W/K; design loss = 129 × 30 = 3,870 W.
Yearly energy = 129 × 2,800 × 24 ÷ 1,000 = 8,669 kWh.

Each point is the calculator re-run with only “Inside temperature (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Inside temperature (°C)	Design heat loss (W)
5	1,935
10	2,580
15	3,225
20	3,870
25	4,515
30	5,160
40	6,450

How to read the result

The breakdown shows where to act first. Here ventilation is the largest single loss, so draught-proofing or heat-recovery ventilation may pay off before more insulation. The design loss is the heater output needed on the coldest day; the yearly energy, divided by the system efficiency or heat pump COP, estimates fuel or electricity use.

Common mistakes

Counting window area twice, in the walls and the windows.
Using centre-of-glass U-values instead of whole-window values.
Guessing air changes; older houses leak much more than new ones.
Using a design temperature from a different climate.

Keep in mind: This simple method ignores thermal bridges, solar and internal gains and ground effects. Real heating system design needs local standards such as EN 12831 and a qualified professional.

Frequently asked questions

How do I calculate heat loss of a room?

Add U × A for every surface, add 0.33 × air changes × volume, and multiply by the temperature difference.

What is a U-value?

The heat flow through one square metre for each degree of temperature difference, in W/m²K.

How big a heat pump do I need?

Roughly the design heat loss at your coldest design temperature; a professional calculation is recommended.

Sources: ISO 13789:2017, Thermal performance of buildings — Transmission and ventilation heat transfer coefficients · EN 12831-1:2017, Energy performance of buildings — Method for calculation of the design heat load

Checked 9 Nov 2026. ISO 13789:2017 “Thermal performance of buildings — Transmission and ventilation heat transfer coefficients” (H_T = Σ U·A; H_V = ρ_a c_a V̇ with ρ_a c_a = 1,200 J/(m³·K)); design heat load method of EN 12831-1:2017; degree-day energy estimate as in CIBSE TM41 “Degree-days: theory and application” (2006).

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