Free Online Calculators
Estimated heat load by floor area & insulation standard for heat pump
This calculator estimates a building's heat load from floor area and insulation standard: area × a flat 120, 80, 50 or 10 W/m². It shows the heat load in kW and a heat pump size rounded up in 0.8 kW steps. A 150 m² house from 1980–2000 comes to 12.0 kW; proper sizing needs a room-by-room calculation to EN 12831-1.
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Heat load is the sum of all heat losses from a room: transmission losses through walls, roof, floor, windows and doors, plus ventilation losses from air exchange. Each surface is multiplied by its U-value and by the temperature difference between the indoor design temperature and the outdoor design temperature, then added up.
You calculate it room by room and add the results to get the total for the system. For each room you need the areas and U-values of the enclosing surfaces, the indoor and outdoor design temperatures, the air change rate and the room volume; the total then determines boiler or heat pump size, while the individual room loads size the radiators or the air flow.
Air flow follows from the room's heating or cooling load divided by the product of air density, specific heat of air and the supply-to-room temperature difference. In imperial units this is the familiar sensible heat equation, so a larger temperature difference between supply air and room air means fewer CFM for the same load.
Work out the load in watts first (areas times U-values times temperature difference, plus ventilation losses) and then convert watts to BTU per hour: 1 W is about 3.412 BTU/h. Short-cut rules based only on floor area, like the calculator on this page with its 120, 80, 50 or 10 W/m², are rough estimates that ignore window area and ceiling height.
20,000 BTU/h is about 5.9 kW. With the calculator's flat values that heats roughly 49 m² (about 530 sq ft) in an uninsulated pre-1980 building at 120 W/m², 73 m² (790 sq ft) at 80 W/m², 117 m² (1,260 sq ft) when well insulated at 50 W/m² and about 586 m² (6,300 sq ft) in a passive house at 10 W/m². Window area, ceiling height and climate shift these figures.
For heating, usually yes. 12,000 BTU/h is about 3.5 kW, while 200 sq ft (18.6 m²) needs about 2.2 kW even at the calculator's highest value of 120 W/m² for uninsulated buildings, and about 0.9 kW when well insulated at 50 W/m². Oversizing causes short cycling and uneven temperatures. For cooling, the load depends on sun, occupants and appliances, which this calculator does not cover.
For heating, 12,000 BTU/h (about 3.5 kW) covers roughly 29 m² (315 sq ft) at 120 W/m², 44 m² (470 sq ft) at 80 W/m², 70 m² (760 sq ft) at 50 W/m² and 352 m² (3,790 sq ft) in a passive house at 10 W/m², using the calculator's insulation values. Cooling capacity depends on solar gain, occupants and appliances and is not calculated here.
A building's heat load is the heating power it needs to stay warm on a very cold winter day. It is given in kilowatts (kW) and is the starting point for choosing a heat pump or boiler. This free heat load calculator gives a quick estimate from two inputs: the heated floor area and the insulation standard of the building.
Enter the living area in m² and choose one of four insulation standards. Each comes with a fixed specific heat load:
The total heat load is floor area × specific heat load ÷ 1,000, in kW. The recommended heat pump size is that load rounded up to the next multiple of 0.8 kW. The calculator does not ask for ceiling height, indoor or outdoor temperatures, U-values, window areas or ventilation; these are covered only by the flat W/m² value. To convert to BTU/h, multiply kW by about 3,412.
Heat load shown by the calculator for typical floor areas (in kW):
| Insulation standard | 100 m² | 150 m² | 200 m² |
|---|---|---|---|
| pre-1980, uninsulated (120 W/m²) | 12.0 | 18.0 | 24.0 |
| 1980–2000 / partly insulated (80 W/m²) | 8.0 | 12.0 | 16.0 |
| post-2000 / well insulated (50 W/m²) | 5.0 | 7.5 | 10.0 |
| passive house (10 W/m²) | 1.0 | 1.5 | 2.0 |
A house from the 1990s with 150 m² of living area: with the 1980–2000 standard (80 W/m²) the calculator shows a heat load of 12.0 kW and a recommended heat pump of 12.0 kW. After an upgrade to well insulated (50 W/m²), the load falls to 7.5 kW and the recommendation to 8.0 kW. The same house uninsulated and built before 1980 comes out at 18.0 kW, with 18.4 kW recommended, more than twice the renovated value.
For designing a heating system, the design heat load is calculated room by room, in Europe to EN 12831-1. It adds transmission losses through walls, windows, roof and floor (simplified Q = U × A × ΔT) and ventilation losses. That requires the U-values and areas of all building elements, the local design outdoor temperature, indoor temperatures and the air change rate. This calculator does none of that and does not replace a heat loss calculation by an energy assessor or heating engineer.
A heat pump should match the real heat load. An oversized unit cycles on and off frequently, which lowers efficiency and wears the compressor; an undersized one relies more on its electric backup heater. The calculator's figure helps you judge whether a quoted unit size is plausible before you ask for a full calculation.
It gives a rough estimate from floor area and a flat W/m² value. Two houses of the same size and age can differ a lot depending on window area, shape, exposure and renovations. For system design you need a room-by-room calculation by a professional.
Heat load is power (kW) on a very cold day. Annual heating demand is energy (kWh per year) over the whole season. The calculator shows only the heat load.
You can enter the floor area of one room and get area × W/m². Room-specific factors such as exterior walls, windows or the desired temperature are not included, so radiator or underfloor heating sizing needs a room-by-room calculation.
Yes, it is free and needs no registration.