Heat pump size calculator: what size heat pump do I need?

1. 1Choose the whole home or one room, then enter the heated floor area (or the room's length and width) and the ceiling height.
2. 2Pick how well the home is insulated — by the year it was built, or by what has been insulated since.
3. 3Choose the indoor temperature and the design temperature where you live, or enter your own.
4. 4Switch on hot water if the heat pump will heat that too, and read the size in kW, BTU/h and tons.

Installers in Germany and the Netherlands start a heat pump estimate with a figure per m² of floor: about 40 W/m² for a new home, 60–100 W/m² for a partly insulated one and 100–160 W/m² for an old home without insulation (Bundesverband Wärmepumpe; Bosch). Those figures are for their climate — about −10 °C outside and 20 °C inside, a 30-degree difference — and for rooms about 2.5 m high. The calculator divides them by 30 to get the loss per degree, then multiplies by your own temperature difference and ceiling height, so the same home needs less in London (−3 °C) and more in Chicago (−20 °C).

The design temperature is not the coldest night on record but the cold the heating must still beat on all but a few hours a year: the UK figures (CIBSE Guide A, used by MCS) and the US ones (ASHRAE) are the temperatures that 99.6 % of the hours in a year are warmer than. A heat pump that covers the heat loss at that temperature keeps the home warm nearly all the time without a big backup heater.

• Insulate and draught-proof first: every watt per m² you save makes the heat pump smaller and cheaper to run.
• Check your radiators: a heat pump heats the water less hot than a boiler, so some rooms may need bigger radiators or underfloor heating.
• If you heat with gas or oil now, a year of bills is a good cross-check — an installer can work back from it to the heat loss.
• Ask for the room-by-room heat-loss calculation in writing and compare its total with this estimate.

• MCS Heat Load Calculator – external design temperatures from CIBSE Guide A (2015), 99.6 %: London −3.0 °C, Birmingham −5.1, Manchester −4.5, Edinburgh −5.4, Glasgow −5.6, Belfast −3.2, Cardiff −3.1, Leeds −3.3; living rooms 21 °C
• MCS – MIS 3005-D, the heat pump design standard
• MCS – MIS 3005-D clause 5.5: the heat pump should provide at least 100 % of the calculated heat loss
• 42watt – Bundesverband Wärmepumpe, Leitfaden Wärmepumpendimensionierung (11/2025): 30–50 W/m² new or renovated, 60–100 partly renovated, 100–160 unrenovated before 1977
• Bosch Home Comfort – rules of thumb 40 W/m² new build, 75 around 2000, 160 around 1970, and why they usually come out too high
• Passipedia (Passive House Institute) – a passive house has a heating load of at most 10 W/m²
• Bundesverband Wärmepumpe – climate map, design temperature by postcode (DIN/TS 12831-1)
• heiznorm.de – DIN/TS 12831-1 design temperatures by city: Berlin −11.1 to −13.3 °C, Hamburg −8.2 to −10.5, Munich −11.1 to −13.9, Cologne −7.0 to −8.7, Frankfurt −8.3 to −10.1, Dresden −12.1 to −14.0
• ISSO 51 (2017) sample heat-loss calculation – base design temperature −10 °C for the Netherlands
• FGIA – ASHRAE 99.6 % heating design temperatures, US cities (2005 Handbook): New York 12.8 °F, Philadelphia 11.6, Washington 15.9, Boston 7.7, Chicago −5.0, Denver −4.0, Detroit 0.2, Minneapolis −14.9, Atlanta 18.8, Seattle 23.8
• ACCA – Manual J, the ANSI-recognised standard for residential heating and cooling loads
• Rechner-Portal – hot-water allowance 0.25 kW per person (DIN 4708 basis: 40 litres a day per person heated by 35 K)
• NEEP – cold-climate air-source heat pump list (output at 5 °F, 17 °F and 47 °F)

Formulas last reviewed: September 30, 2026 ·How we check our formulas