Heating Load Calculator (Design Heat Loss)

Estimate a building's design heat loss in watts and kilowatts from floor area, ceiling height, U-values for walls, roof, windows and floor, window share, air changes per hour and indoor and outdoor design temperatures.

Last updatedHow we build & check our tools
%
Results will appear here once you enter your values.

How the heat loss is calculated

The calculator estimates design heat loss: the heat a building loses on a cold design day, which a heating system must replace to hold the indoor temperature. It works in metric units. Enter floor area in m², ceiling height in m, U-values in W/m²K, and indoor and outdoor design temperatures in °C.

Heat escapes two ways. Fabric loss through each surface is U-value × area × temperature difference (ΔT), summed for walls, roof, windows and floor. Ventilation loss from air leaking in and out is 0.33 × air changes per hour × building volume × ΔT, where 0.33 W/m³K is the heat capacity of air divided by 3,600 seconds.

Wall area is estimated by assuming a square footprint: perimeter = 4 × √floor area, multiplied by ceiling height. Your window percentage is taken out of that wall area and costed at the window U-value, and the rest is treated as opaque wall. Roof and floor losses both use the full floor area, so the model describes a single-storey box.

  • Total heat loss in W and kW, split into walls, roof, windows, floor and ventilation
  • W per m², useful for comparing buildings of different sizes
  • Suggested boiler size: total heat loss plus a 20% margin, in kW

Worked example with the default values

The defaults describe a well-insulated 150 m² single-storey home with 2.7 m ceilings, walls at U 0.30, roof at U 0.18, double glazing at U 1.4, floor at U 0.25, windows on 20% of the walls, 0.5 air changes per hour, 21 °C inside and −5 °C outside, giving ΔT = 26 K.

  • Perimeter 4 × √150 ≈ 49.0 m; wall area ≈ 132.3 m², of which 26.5 m² is glazing and 105.8 m² opaque wall
  • Walls: 0.30 × 105.8 × 26 ≈ 825 W
  • Roof: 0.18 × 150 × 26 = 702 W
  • Windows: 1.4 × 26.5 × 26 ≈ 963 W
  • Floor: 0.25 × 150 × 26 = 975 W
  • Ventilation: 0.33 × 0.5 × 405 m³ × 26 ≈ 1,737 W

Total heat loss is about 5,203 W (5.2 kW), or 34.7 W per m², and the suggested boiler size with a 20% margin is 6.2 kW. For readers who size equipment in imperial units, 1 W is 3.412 BTU/h, so the loss is roughly 17,750 BTU/h and the margin-included size about 21,300 BTU/h.

Ventilation is a third of the total here. Doubling air changes to 1.0 per hour raises the total to about 6.9 kW, while swapping the windows for older U 2.8 glazing raises it to about 6.2 kW, which shows where improvements pay off.

Common mistakes to avoid

Mixing units. U-values must be in W/m²K and areas in m². An imperial U-factor in Btu/(h·ft²·°F) is 5.678 times smaller than the metric figure, so a window rated U-0.30 imperial is about U 1.70 metric. Entering it unconverted understates window loss more than fivefold.

Using an average winter temperature. The outdoor figure should be your local design temperature, the cold condition a system is sized for, not the seasonal average. Using a mild value shrinks ΔT and undersizes the system.

Guessing air changes. Infiltration is often the largest single loss. A blower-door test gives a measured air change rate; as a rough guide, very airtight new homes are well under 0.5 ACH in normal conditions while older draughty houses can exceed 1.0.

  • Multi-storey buildings: the model applies roof and floor losses to the full floor area and assumes a square, single-storey footprint. For two storeys, calculate per floor or adjust the areas.
  • Adding extra margins twice: the boiler figure already includes 20%. Stacking further safety factors leads to oversized, short-cycling equipment.
  • Ignoring gains: the result excludes solar and internal gains from people and appliances, so it is a conservative design figure, not an annual energy use estimate.

Where to find the inputs

U-values. Building plans, energy performance certificates and product data sheets list U-values. If you only know an R-value in metric units (m²K/W), U = 1 ÷ R; add the layers' R-values first for built-up walls or roofs. For windows, use the whole-window U-value from the label rather than the centre-of-glass figure.

Design temperatures. Local building codes, heating design guides such as CIBSE Guide A in the UK, or the ASHRAE climatic design data used in North America give the outdoor design temperature for your location. Typical indoor set points are 20–21 °C for living areas.

Air changes per hour. Use a blower-door result converted to natural conditions if you have one. Otherwise pick a conservative value for the building's age and airtightness, then test a higher and lower value to see how sensitive the result is.

  • Treat the output as a whole-building estimate for comparing options and sanity-checking quotes.
  • Room-by-room sizing of radiators or ducts needs a detailed calculation such as EN 12831 or ACCA Manual J, usually done by a heating designer.

Frequently Asked Questions

Common questions about the Heating Load Calculator (Design Heat Loss)

Here they mean the same thing: the rate, in watts, at which a building loses heat on a cold design day, which is the output a heating system must supply to keep the indoor temperature steady. The calculator adds a 20% margin to that figure for its suggested boiler size. Annual heating energy use is different; it depends on how many cold hours occur over a season, not just the design day.
Learn More

Sources & References

International System of Units (SI): energy, work and heat

Energy, work and heat is measured in the joule (J); 1 cal = 4.184 J. Conversions between SI and other units use exact, internationally agreed factors maintained by NIST.
→Source: NIST — International System of Units (SI)

International System of Units (SI)

Authoritative definitions for energy, work and heat, from the BIPM SI Brochure (9th edition), the defining reference for the SI.
→Source: BIPM — SI Brochure (9th ed.)

Explore More Tools

Continue your financial journey with these related calculators

energy

Solar Panel Output Calculator

Estimate how many kWh your solar panels will produce per day, month and year from panel wattage, peak sun hours, system losses and roof orientation.

Try it now
energy

Home Energy Savings Calculator

Estimate what solar panels, a home battery and a heat pump would save on your electric and gas bills using your own energy rates, sun region and incentives, then see the break-even year and 25-year net savings.

Try it now
energy

AC BTU Calculator

Find the right air conditioner size in BTU for a room using the ENERGY STAR sizing chart, adjusted for ceiling height, sun exposure, occupants and kitchens.

Try it now
energy

Furnace Size Calculator

Estimate the furnace BTU size for your home from square footage, climate zone, insulation, ceiling height and AFUE efficiency.

Try it now
energy

Generator Size Calculator

Add up running and starting watts for the appliances you need during an outage and get a recommended generator size with a safety buffer.

Try it now
energy

Solar Battery Size Calculator

Size a solar battery bank in kWh and amp-hours from daily energy use, days of autonomy, depth of discharge, inverter efficiency and battery temperature.

Try it now