Heat Pump Size Calculator
Screen heat pump size by area and climate zone, in BTU, kW, and tons.
What it does
Heat pumps live or die by sizing. Too small and the house leans on costly backup strips all winter; too large and the unit short cycles, leaving rooms clammy and bills high. A quick area based estimate frames the conversation before anyone signs a quote.
The tool above gives that frame. Enter conditioned area with a mild, moderate, or cold zone in imperial or metric. It returns heating load in your primary unit, the electric equivalent, and tons. Defaults describe 1800 ft2 in a moderate zone, so the page opens solved at 63,000 BTU/h, 18.5 kW, and 5.25 tons.
The factors are deliberately coarse: 25, 35, and 45 BTU/h per ft2. They stand in for insulation, windows, leakage, and design temperatures that only a room by room Manual J can weigh. Metric math converts square meters at 10.7639 ft2 each, then applies the same factor, so 167.2 m2 lands at the same 18.5 kW and 5.25 tons (the BTU twin reads 62,990 from rounding).
Read all three rows, not just tons. Heating load sets winter survival, kW hints at panel and wire needs, and tons speak the language of equipment badges. Cold climate practice shows the spread: 2400 ft2 at 45 needs 108,000 BTU/h (9.00 tons), while 1200 ft2 mild needs only 30,000 BTU/h (2.50 tons).
Use the table for common sizes, then get the Manual J. Duct condition, refrigerant lines, defrost strategy, and backup heat all move the final pick. This page keeps shopping honest; it never replaces the load sheet the installer should show you.
How to use
- Pick imperial (ft2) or metric (m2).
- Enter the conditioned area.
- Pick mild, moderate, or cold climate.
- Read heating load in your primary unit.
- Read the equivalent plus tons.
- Book a Manual J check before buying.
Formula
- Area
- Conditioned floor area.
- Climate factor
- Screening load per ft2: 25 mild, 35 moderate, 45 cold.
- BTU/h
- Design heating need.
- Ton
- 12,000 BTU/h of capacity.
Climate factors are screening values (mild 25, moderate 35, cold 45 BTU/h per ft2), not a Manual J result. Metric area converts at 10.7639 ft2 per m2 before the same factor applies. Real sizing adds windows, leakage, duct loss, and local design temperatures plus backup heat planning.
Worked example
Example: 1800 ft2 in a moderate zone
An 1800 ft2 home in a moderate zone needs a heat pump estimate. Find heating load and tons.
- List inputs: 1800 ft2, moderate climate at 35 BTU/h per ft2.
- Heating = 1800 x 35 = 63,000 BTU/h.
- Electric equivalent = 63000 x 0.000293071 = 18.5 kW.
- Tons = 63000 / 12000 = 5.25 tons.
- Metric mirror: 167.2 m2 gives 18.5 kW and the same tons.
- A Manual J room check still sets the final badge size.
Reference table
| Area | Heating | kW | Tons |
|---|---|---|---|
| 1200 ft2 | 42,000 BTU/h | 12.3 kW | 3.50 tons |
| 1500 ft2 | 52,500 BTU/h | 15.4 kW | 4.38 tons |
| 1800 ft2 | 63,000 BTU/h | 18.5 kW | 5.25 tons |
| 2400 ft2 | 84,000 BTU/h | 24.6 kW | 7.00 tons |
Limits
- Screening only; Manual J decides.
- Ignores windows, leakage, and height.
- No duct loss or backup heat plan.
- Design temperatures vary by town.
Common mistakes
- Buying on tons alone.
- Using cooling rules for heating zones.
- Counting garages as conditioned area.
- Skipping backup heat in cold zones.
- Trusting square footage without a load sheet.
Try it yourself
Quick checks. Answers sit tucked under each question.
1200 ft2 in a mild zone?
30,000 BTU/h. Equivalent 8.8 kW. Tons 2.50 tons.
2400 ft2 in a cold zone?
108,000 BTU/h. Equivalent 31.7 kW. Tons 9.00 tons.
2000 ft2 moderate? Work it by hand.
Heating = 2000 x 35 = 70,000 BTU/h. Tons = 70000/12000 = 5.83 tons.
Questions
What does this estimate?
A fast area based estimate of design heating load plus the tons equivalent. It screens sizes before a Manual J room by room check, which remains required.
What climate factors are used?
Mild 25, moderate 35, cold 45 BTU/h per ft2. Cold zones need bigger heating numbers for the same floor area.
How are tons found?
Divide BTU/h by 12000. The example gives 63000 / 12000 = 5.25 tons.
How is kW found?
Multiply BTU/h by 0.000293071. The example gives 18.5 kW.
Does it work in square meters?
Yes. Pick metric and enter square meters: 167.2 m2 in the same zone gives 18.5 kW and 5.25 tons.
What does the shortcut ignore?
Insulation, windows, air leakage, ceiling height, and design temperatures. Tight homes need less; leaky glass boxes need more.