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Heat Pump Sizing Calculator

Size a heat pump to a known heat loss, and find the balance point where supplementary heat starts to be needed.

Computed in your browser — nothing you enter is uploaded. Figures are presented in United States units and terminology, and each formula’s basis is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Capacity required at design temperature

8 kW at design

Check your inputs

The derate is a linear planning approximation. Use the manufacturer's published capacity table for the actual machine before committing.

Nameplate capacity at 7 °C
8 kW
Capacity at the design temperature
6.4 kW
Shortfall needing supplementary heat
1.6 kW
Balance point
32 °F
Capacity retained at design
80 %

With the figures above, the capacity required at design temperature comes to 8 kW at design. The method behind this is well established, though site conditions and material batches will move it somewhat. This is presented for United States. Building in another market? Change the selector above and the units and terminology follow.

Add the equipment this sizes

This result is a specification — 8 kW at design — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Regulatory standards & verification citations

  • Heat pump capacity falls with outdoor temperature; manufacturers publish capacity tables at 47/17/5 °F (8/−8/−15 °C)
  • ACCA Manual S — equipment selection against the calculated load, including balance-point analysis for heat pumps
  • MCS 3005 (UK) requires sizing to the calculated design heat loss rather than to the existing boiler output

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Still deciding? Heat Pump vs Air Conditioner — the factors that actually differ, with no invented prices.

Frequently asked questions

Why can I not size a heat pump like a boiler?
Because a boiler's output does not change with the weather and a heat pump's does. A 24 kW boiler delivers 24 kW at −10 °C; an 8 kW heat pump delivers perhaps 5.5 kW there. Sizing from the old boiler also inherits its oversizing — boilers were routinely fitted at two or three times the actual heat loss, and copying that produces a heat pump that costs far more than it needs to and cycles badly.
What is the balance point?
The outdoor temperature at which the heat pump's falling capacity exactly meets the building's rising demand. Above it the heat pump covers everything; below it something else has to make up the difference. A balance point near the design temperature means very little supplementary running; one well above it means the immersion or backup heater carries a large share of the season.
Is supplementary heat a failure?
No, it is a design choice. Sizing for the coldest hour of the year leaves a machine badly oversized for the other 99%, cycling and running inefficiently. Accepting a few dozen hours of backup is usually cheaper both to install and to run. What matters is knowing how many hours, which is what the balance point tells you.
Will my existing radiators work?
Often not without change, and this calculator does not answer it. A heat pump runs at 35-50 °C flow against a boiler's 70-80 °C, and a radiator's output falls sharply at lower flow temperature — frequently to half. Emitter sizing is usually the binding constraint in a retrofit, and it is a room-by-room question.