Renewables
Heat Pump Sizing Calculator
Indicative heat pump kW from floor area and fabric
Updated 12 July 2026 · Live
What this tool does
Gives an indicative heat pump size in kW from floor area, ceiling height, fabric standard and the design temperature difference.
Enter floor area, ceiling height, a fabric standard and the design temperature difference; the calculator returns an indicative heat pump size in kW.
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Formula Used
How the heat pump sizing works
The heated volume is floor area times ceiling height. Multiplying volume by a heat-loss factor for the fabric standard and by the design temperature difference gives the heat loss in watts, divided by 1000 for kilowatts. The factor falls as insulation improves, so a well-sealed home needs a much smaller heat pump than a draughty one of the same size.
What you enter
Enter floor area, ceiling height, a fabric standard and the design temperature difference; the calculator returns an indicative heat pump size in kW. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
The kW figure is a first-pass size to sense-check quotes, not a design. It sizes the heat requirement only — it says nothing about flow temperatures, radiators or the electrical supply.
Safe by design
This calculator does the arithmetic of sizing — quantities, geometry and cost. It deliberately stops at the line a certified installer must sign off. It will never tell you a cable or fuse size, an earthing arrangement, a battery charge voltage, or a roof structural load: those are regulated calculations where a wrong number causes fire, electrocution or collapse. All electrical work must be carried out by a qualified, certified installer, and this page is not an installation guide.
Assumptions and limits
This is a whole-building rule of thumb. The design standard is a room-by-room MCS heat-loss survey that accounts for each wall, window and air-change rate. Flow-temperature design, emitter sizing and the electrical connection are all separate and should be set by a qualified heating engineer or MCS-registered installer.
Using it with the rest of BuildMetricLab
This calculator is one step in planning a system. Pair it with the other solar sizing, geometry and cost tools to go from an energy figure to a panel count, a layout and a costed bill of materials. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Gives an indicative heat pump size in kW from floor area, ceiling height, fabric standard and the design temperature difference. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or tariffs are embedded.
Frequently asked questions
Is this an MCS heat-loss calculation?
No. It is a whole-building rule of thumb for sanity-checking. A full room-by-room MCS survey is the design standard and can differ significantly.
What design temperature difference should I use?
It is the indoor target minus your coldest outdoor design temperature — often around 20–25°C in temperate climates. Use a difference figure, not an absolute temperature.
Why does the fabric standard matter so much?
Heat loss scales with the fabric factor. A well-insulated home can need less than half the heat pump size of a poorly insulated one of the same volume.
Does this set the flow temperature?
No. Flow temperatures, emitter sizing and electrical supply are design decisions for a qualified heating engineer, not part of this sizing estimate.
Spotted something off?
Calculations or display — let us know.