How the two losses are worked out
Fabric loss is the U-value of each element multiplied by its area and by the design temperature difference, added together for walls, roof and floor. The floor area is used for the roof as well, which suits a simple plan.
The US figure uses the standard 1.08 Btu/h per cubic foot per minute per F, with the air change rate converted to cubic feet per minute.
The total is the sum of the two. The split matters: in a well-insulated house ventilation is often the larger share, and in a poorly insulated one the fabric dominates.
Worked example
A 1,600 sq ft house with 1,200 sq ft of wall at 0.032, a roof at 0.027 and a floor at 0.03, at a 70 F design difference, loses 38.4 + 43.2 + 48 = 129.6 Btu/h per degree, which is 9,072 Btu/h through the fabric. Ventilation at half an air change an hour adds 8,064 Btu/h, so the total is 17,136 Btu/h.
The result sizes a heat source. Compare it with the boiler size calculator, which also has to allow for hot water.
What the design difference means
The design difference is the indoor temperature you want minus the coldest outdoor temperature the system is sized for. A larger difference gives a bigger heat loss and a bigger heat source.
In the UK the conventional winter design temperature is about minus 1 °C, which gives 21 K from a 20 °C room. In the US the 99 per cent design temperature is used, which varies from about 0 °F in the north to 40 °F in the south.
Sizing to the coldest day on record wastes money on a boiler that runs part-loaded all year. Sizing to a milder day leaves the house cold on the few days that matter.
Limits of this calculator
It treats the walls as one average U-value with the windows included. A house with a lot of glazing needs the windows counted separately, because their U-value is several times the wall's.
It does not model thermal bridging, solar gain, internal gains, party walls to heated neighbours or the ground temperature under the floor.
It gives a steady-state figure for the coldest design day. It does not predict annual energy use, which depends on the weather through the year.
To turn the heat loss into a heat source size, use the US boiler size calculator.
Inputs for the US heat loss calculator
Floor area
Ground floor area of the home, which is also used as the roof area.
Ceiling height
Floor to ceiling height, used to find the air volume for the ventilation loss.
Exposed wall area
Total external wall area including windows, not the perimeter of the floor.
Wall U-value
Average U-value of the walls with openings included. A modern cavity wall is about 0.18 W/m²K.
Roof U-value
U-value of the roof, or of the ceiling below an unheated loft.
Floor U-value
U-value of the ground floor, which depends on the insulation and the floor construction.
Air changes per hour
How often the whole air volume is replaced in an hour. Older homes leak more.
Design temperature difference
Indoor design temperature minus the outdoor design temperature, 70 F for a cold climate.
CALCULATOR QUESTIONS
US heat loss calculator questions
Is this the same as a SAP or Manual J calculation?
No. Those are full assessments with bridging, gains and room-by-room detail. This is a steady-state estimate.
Why is ventilation such a large share?
Because every air change carries heat out. In a well-insulated home it can be half the total loss.
What design temperature should I use?
The coldest temperature your system is sized for, not the coldest on record. Local convention is minus 1 °C in the UK and the 99 per cent value in the US.
Should I count windows separately?
If the glazing is a large share of the wall, yes. This page folds them into the wall U-value.