Heating Load Calculator Widget

Give readers a quick heat-loss estimate for a room or a simple single-storey building. They enter the dimensions, window share, insulation level or their own U-values, air leakage and the design indoor and outdoor temperatures, and see the heat output a boiler, furnace or heat pump must supply, split by walls, windows, roof, floor and air.

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<iframe src="https://a2z.tools/embed/w/heating-load-calculator" title="Heating Load Calculator by A2Z Tools" width="100%" height="1000" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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How it works

Heat leaves a building in two ways, and the widget adds both. Fabric loss is U x A x dT for each element: the walls (perimeter x ceiling height, less the window share), the windows, the roof and the floor (each the plan area). Air leakage is the heat needed to warm incoming outdoor air: 0.33 x ACH x volume x dT in watts with metres - the coefficient in the UK SAP 10.2 worksheet - or 0.018 x ACH x volume x dT in BTU/h with feet, from 0.075 lb/ft³ of air x 0.24 BTU/lb.°F. Three presets supply U-values: an uninsulated older building (BRE RdSAP 10 defaults of 1.7 for a 200-280 mm brick wall, 2.3 for an uninsulated roof and 4.8 for single glazing, with 0.7 for the floor), a partly insulated building with typical mid-range values, and new build at the UK Approved Document L 2021 limits (wall 0.26, roof 0.16, floor 0.18, windows 1.6 W/m²K). Or enter your own. A 10 x 8 m single storey built to the 2021 limits loses 2.44 kW at a 24 K difference.

Calculation method

  • Walls: A = 2 x (L + W) x H x (1 - window share); windows: A = 2 x (L + W) x H x window share; roof = floor = L x W
  • Fabric loss = (U_wall A_wall + U_window A_window + U_roof A_roof + U_floor A_floor) x (T_in - T_out)
  • Air leakage (metric) = 0.33 x ACH x V(m³) x dT, in W
  • Air leakage (imperial) = 0.018 x ACH x V(ft³) x dT(°F), in BTU/h
  • 1 W/m²K = 0.17611 BTU/h.ft².°F; 1 kW = 3,412.142 BTU/h
  • Presets (W/m²K, wall / roof / floor / window): older uninsulated 1.7 / 2.3 / 0.7 / 4.8; partly insulated 0.6 / 0.4 / 0.45 / 2.8; new build 0.26 / 0.16 / 0.18 / 1.6

Worked examples

New-build bungalow

Inputs: 10 x 8 x 2.5 m, 15% windows, new build (UK 2021 limits), 0.5 ACH, 21 °C in, -3 °C out

Result: 2.44 kW (8,328 BTU/h): fabric 1,649 W, air leakage 792 W

UA = 68.69 W/K; 68.69 x 24 + 0.33 x 0.5 x 200 x 24.

Same footprint, uninsulated

Inputs: 10 x 8 x 2.5 m, 15% windows, uninsulated older building, 1 ACH, 21 °C in, -3 °C out

Result: 12.02 kW (41,015 BTU/h)

Single glazing and an uninsulated roof dominate the loss.

US house with custom U-values

Inputs: 40 x 30 x 8 ft, 15% windows, U wall 0.08, roof 0.03, floor 0.05, window 0.35 BTU/h.ft².°F, 0.5 ACH, 70 °F in, 10 °F out

Result: 19,042 BTU/h (5.58 kW): air leakage 5,184 BTU/h

0.018 x 0.5 x 9,600 ft³ x 60 °F for the air.

A simplified planning estimate, not a heating system design. Size equipment with ACCA Manual J (US) or EN 12831 (Europe) by a qualified designer.

Limitations

  • One rectangular zone with the full indoor-outdoor difference across every element; floors over the ground or a crawl space really see a smaller difference.
  • Thermal bridges, doors, rooflights, mechanical ventilation, heat recovery and internal gains are not modelled.
  • The partly insulated preset is a typical mid-range set of U-values, not taken from a standard.
  • Gives the steady-state design loss, not annual energy use or running cost.

Where publishers use it

  • Heat pump installers showing homeowners why insulation comes before sizing
  • Retrofit and energy-efficiency blogs comparing single glazing with modern windows
  • Building-services and HVAC course pages teaching the U x A x dT method
  • Garden-office, cabin and workshop builders choosing a heater
  • Self-builders checking a rough boiler or furnace size before a full design

Questions

How much heat does a new-build single storey lose?

A 10 x 8 m plan with 2.5 m ceilings, 15% windows, UK 2021 limiting U-values and 0.5 ACH loses 1,649 W through the fabric and 792 W through air leakage at 21 °C inside and -3 °C outside - 2.44 kW in total, or 30.5 W per m² of floor.

What difference does insulation make?

The same 10 x 8 m building with uninsulated brick walls, an uninsulated roof, single glazing and 1 ACH needs 12.02 kW - almost five times as much. The breakdown bar shows which element to improve first.

Where do the 0.33 and 0.018 come from?

Both are the heat capacity of air per unit volume. In metric, 0.33 W per m³ per K per air change is the ventilation coefficient used in the UK SAP 10.2 calculation. In imperial units, 0.075 lb/ft³ x 0.24 BTU/lb.°F gives 0.018 BTU/h per ft³ per °F per air change.

What design temperatures should I use?

Indoors, the temperature you heat to - commonly 21 °C (70 °F) for living rooms. Outdoors, the local heating design temperature, which national standards publish (for example the climate data tables in EN 12831 annexes or ASHRAE climatic design conditions); it is a cold-day value, not the coldest record.

What is a typical air leakage rate?

Air changes per hour from infiltration vary widely with construction quality. A blower-door test gives the real figure; without one, compare a few values to see how sensitive the result is - in the example, each 0.5 ACH adds 792 W.

Why recommend Manual J or EN 12831?

Those methods split the building into rooms, use separate temperatures for ground floors and unheated spaces, add thermal bridges and ventilation systems, and set safety margins. This widget treats the whole envelope as one zone exposed to the full outdoor difference, which overestimates floor loss and ignores bridges.

Sources

  1. Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments - Table 4.1 - UK Government (gov.uk) . Limiting U-values for new dwellings: roof 0.16, wall 0.26, floor 0.18, window 1.6, doors 1.6 W/m²K - the new-build preset. Checked 2026-10-01.
  2. RdSAP 10 Specification (9 June 2025) - Tables 13, 16 and 24 - BRE Group, for the UK Government . Default U-values for existing dwellings: brick wall 200-280 mm 1.7, roof with no insulation 2.3, single glazing 4.8 W/m²K - the older-building preset. RdSAP has no single default for uninsulated floors; 0.7 is a typical value.
  3. SAP 10.2 - The Government's Standard Assessment Procedure for Energy Rating of Dwellings (11-04-2023) - BRE, for the UK Government . Ventilation heat loss = 0.33 x air change rate x dwelling volume (W/K), the metric infiltration coefficient.
  4. NEAT Engineering Manual (ORNL/CON-469/R1) - Oak Ridge National Laboratory, US Department of Energy Weatherization Assistance Program . Air density 0.075 lb/ft³ and the 1.08 = 60 x cp factor for CFM, i.e. 0.018 BTU/h per ft³ per °F per air change used for imperial infiltration.

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A2Z Tools Heating Load Calculator
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