Heat Pump Size Calculator
Get a recommended heat pump size for a room or whole home.
1. How large is your room?
2. How insulated is your room?
3. Where are you located closest to?
Indicative sizing guide only. Final sizing depends on window area, orientation, layout and airtightness that this calculator can't see. A professional in-home heat loss assessment confirms the exact model for your space.
How heat pump sizing actually works
A heat pump's job is to replace the heat a space loses to the outdoors as fast as it loses it. That loss rate depends mostly on volume (bigger spaces lose more heat) and how well insulated the space is (poor insulation loses heat much faster than a well-sealed new build). Get the size right and the unit cycles comfortably even on cold mornings; undersize it and it runs flat out without ever quite keeping up.
Region matters too. Every heat pump's rated capacity is measured at a mild 7°C outdoor temperature - the number on the spec sheet. Inverter units lose some of that capacity as it gets colder outside, so the same room needs a larger-rated unit in Central Otago than it does in Auckland to still perform on the actual coldest mornings, not just in the showroom.
Whole-home (ducted) sizing works differently to single-room sizing, not just at a bigger scale. A ducted system serving many rooms benefits from internal heat gains and load diversity - the whole house is rarely losing heat at its absolute worst in every room at once - so it's sized against a lower per-cubic-metre rate than a single standalone room unit would be.
Worked examples
Living room, partially insulated, Christchurch
A 5m x 4m living room with a 2.4m stud, partially insulated, no major glazing. Christchurch's colder winters push the recommendation up a size compared to the same room in Auckland - worth checking your own region rather than assuming a "standard" size travels between them.
Same room, large glazing
Add a large window area or ranch-slider to that same living room and the recommended size steps up again - glazing loses heat noticeably faster than an equivalent wall area, and it's a common reason a "should be fine" room ends up needing the next size up in practice.
Whole-home ducted, new build
A 140m² new-build home in Auckland, switched to whole home mode for a ducted system. New-build insulation standards and Auckland's mild winters both work in the same direction here, keeping the recommended ducted size well within a single standard unit for a home this size.
Frequently asked questions
How accurate is an online heat pump size calculator?
It's a good starting point for comparing options and budgeting, based on your room or home size, insulation level and region. It isn't a substitute for a proper in-home heat loss assessment, which accounts for window area, orientation, layout and airtightness in ways a calculator can't see from a few inputs.
Should I round down to save money on a smaller unit?
We'd recommend against it. An undersized heat pump runs flat out on the coldest mornings and struggles to keep up, which is both less comfortable and less efficient than a correctly sized unit cycling normally. Recommendations here are always rounded up to the next standard unit size, never down.
Why does the same room size need a bigger unit in some regions?
Heat pump capacity is rated at a mild outdoor temperature (7°C). In colder regions, an inverter unit has less spare capacity left on the coldest mornings, so the same room needs a larger-rated unit to still deliver comfortably when it matters, not just on the spec sheet.
What's the difference between single room and whole home mode?
Single room mode sizes one high wall or floor console unit for one space. Whole home mode sizes a ducted system covering your whole conditioned floor area, and accounts for the fact that a ducted system serving many rooms runs more efficiently overall than the sum of several standalone units would, due to internal heat gains and load diversity.
Do I need a bigger unit if I have a lot of windows?
Large glazed areas lose heat faster than an equivalent wall area, so yes - tick the glazing option and the recommendation sizes up accordingly. If your space is unusually glazed (e.g. floor-to-ceiling on multiple walls) an in-home assessment will give a more precise answer than the general allowance used here.
Related calculators
Sizing the circuit for a new heat pump install? These help with the electrical side.
Ready to move ahead? A qualified installer can confirm the exact model for your space.
Book a free in-home assessment