Heat loss estimate

A quick whole-house heat loss in kW for a simple, rectangular home, from published UK U-values and design temperatures. A starting point for boiler sizing; you make the decision.

Size of the home

Lengthmetres, outside or insideWidthmetresRoom heightmetres, one storeyFloorsheated storeys, 1 to 4

External walls on the width0, 1 or 2External walls on the length0, 1 or 2

The building

Age of the homewhen it was builtWalls

WindowsLoft insulation

Ground flooris the lowest floor on the ground?

YesNo, a heated home below

Nearest locationfor the design temperature

Optional

Floor typenot known: solid for this ageWindow aream², blank: 20 % of the external wallFloor insulationmm, blank: 0 mm for this ageAir changes per hourblank: 0.5Hot water allowanceW, for cylinder reheat, blank: 0Extension or loft roomW, worked out separately, blank: 0

Whole-house heat loss

U-values used, W/m²K
Windows-
Walls-
Roof-
Floor-
Heat loss per degree, W/K
Windows-
Walls-
Roof-
Floor-
Watts
Temperature difference-
Fabric-
Ventilation-
Hot water-
Extension-

Type the length and width to start.

How it is worked out

External wall area: the width times the external walls on the width, plus the length times the external walls on the length, times the room height and the number of floors.
Window area: what you measured, or 20 % of the external wall area. The 20 % is our assumption, not a standard.
Heat loss per degree (W/K): each area times its U-value. Windows, the rest of the wall, the roof over the footprint and the floor under it. No floor loss when a heated home is below, no roof loss when one is above.
Temperature difference: 21 °C inside minus the design temperature of the nearest location.
Fabric loss: the W/K total times the temperature difference.
Ventilation loss: 0.33 × air changes × volume × temperature difference.
Total: fabric plus ventilation, plus any hot water allowance and extension you add, shown in kW to one decimal place.

Wall U-values follow the age of the home; measured solid brick in homes built before 1976 uses the value for its thickness. Unknown loft insulation and floor insulation use the values assumed for the age of the home. The ground floor U-value is worked out from the floor area, the exposed perimeter, the wall thickness and the insulation.

Good to know

This is a quick whole-house estimate for a simple rectangular home, not a room-by-room heat loss calculation (BS EN 12831). A heat pump installed under MCS needs a full room-by-room calculation, for example with the free MCS Heat Load Calculator.

It does not choose a boiler. For a combi boiler, the hot water flow rate usually decides the size, not this figure.

The U-values come from the England tables; Scotland and Northern Ireland use slightly different ones. A home well above its nearest weather station is colder: allow about 0.6 °C for every 100 m.

Windows default to 20 % of the wall. Measure them for a better figure.

For a home that is not a rectangle, split it into rectangles and add the results.

You check the result and make the decision.

Everything is calculated in your browser. Nothing you type is sent or stored.

Sources

U-values from the RdSAP 10 Specification (BRE, June 2025). External design temperatures from CIBSE Guide A, as published by MCS: design conditions. For this simplified whole-house estimate we assume 21 °C inside and a default of 0.5 air changes per hour, based on the minimum rate in the CIBSE Domestic Heating Design Guide (ventilation rates); a room-by-room design uses different temperatures per room, and real ventilation can be higher.

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