Heating Load Calculator

Work out heating load instantly with clear inputs, formula shown and shareable results.

About Heating Load Calculator

Design heat loss splits into two independent paths: conduction through the building fabric, given by U x A x ΔT, and the energy needed to warm outdoor air entering by ventilation or infiltration, given by 1.08 x CFM x ΔT. Adding them yields the boiler or furnace output the building needs on the coldest design day.

Formulas

Heat loss

Q = U x A x ΔT + 1.08 x CFM x ΔT

U: Area-weighted U-value
A: Envelope area
ΔT: Indoor minus outdoor design temperature

Kilowatt conversion

kW = BTU/h / 3412.142

Worked example

With the default values below, Heating Load Calculator returns Total heating load of 13,752. Change any input above and every figure here is recalculated in your browser.

Inputs used

Envelope area (walls, roof, glazing)minimum 1 ft²: 2,200 ft²
Average U-valueminimum 0.005 BTU/h·ft²·°F: 0.08 BTU/h·ft²·°F
Design temperature differenceminimum 1 °F: 45 °F
Ventilation / infiltration airflowminimum 1 CFM: 120 CFM

Results

Total heating load: 13,752 BTU/h
Fabric loss (U x A x ΔT): 7,920 BTU/h
Ventilation loss: 5,832 BTU/h
Load in kilowatts: 4.03 kW

Step by step

1Values used
2Heat loss
3Kilowatt conversion
4Total heating load= 13,752 BTU/h
5Fabric loss (U x A x ΔT)= 7,920 BTU/h
6Ventilation loss= 5,832 BTU/h
7Load in kilowatts= 4.03 kW

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Verify against a full room-by-room heat loss calculation and local energy code before sizing equipment.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Verify against a full room-by-room heat loss calculation and local energy code before sizing equipment.

Frequently Asked Questions

How is the Heating Load figure worked out?

Q = U x A x ΔT + 1.08 x CFM x ΔT. Design heat loss splits into two independent paths: conduction through the building fabric, given by U x A x ΔT, and the energy needed to warm outdoor air entering by ventilation or infiltration, given by 1.08 x CFM x ΔT.

Where does the 1.08 constant come from?

It is the product of air density, specific heat and 60 minutes per hour at sea level standard conditions, so 1.08 x CFM x ΔT gives sensible BTU/h directly.

What values do I need to enter?

This calculator takes 4 inputs: Envelope area (walls, roof, glazing), Average U-value, Design temperature difference, Ventilation / infiltration airflow. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.