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What Size Heat Pump Do I Need? BTU by Square Footage, and Why That’s Not Enough
Square footage gets you a rough starting point. Everything that actually determines the answer — insulation, windows, air leakage, climate — is invisible in that number.
In this article
- The rough starting point
- Why this table can be wrong by a full ton
- Manual J is the real answer
- The oversizing problem
- Sizing for heating versus cooling
The internet is full of tables that map square feet to tons. They are useful as a sanity check and dangerous as a specification. Here is how to use one without getting burned.
The rough starting point
| Home size | Mild climate | Cold climate |
|---|---|---|
| 800 sq ft | 1.0–1.5 ton | 1.5–2.0 ton |
| 1,200 sq ft | 1.5–2.0 ton | 2.0–2.5 ton |
| 1,800 sq ft | 2.5–3.0 ton | 3.0–3.5 ton |
| 2,400 sq ft | 3.0–3.5 ton | 4.0–4.5 ton |
| 3,000 sq ft | 4.0–4.5 ton | 5.0 ton or two systems |
One ton equals 12,000 BTU per hour. A three-ton unit is a 36,000 BTU unit.
Why this table can be wrong by a full ton
Two 1,800 sq ft houses on the same street can have heat loads that differ by 40%. The variables that matter are insulation levels, window area and glazing type, air leakage rate, ceiling height, orientation, shading and how many people live there. A 1970s house with single-pane windows and a 2020 house with spray foam are not the same job.
Manual J is the real answer
A Manual J load calculation models your specific house room by room. Done properly it takes a contractor one to three hours and produces a heating load and a cooling load in BTU/h. Some charge $150–$400 for it standalone; most include it with a serious quote.
If a contractor will not produce one, that is your answer about the contractor.
The oversizing problem
Bigger is not safer. An oversized system reaches setpoint quickly and shuts off, which means it never runs long enough to pull humidity out of the air in summer. You end up with a house that is cold and clammy, a compressor that short cycles itself to an early death, and higher bills. With variable-speed equipment, modest oversizing is more forgiving — but 25% oversizing is still a design error, not insurance.
Sizing for heating versus cooling
In cold climates the heating load usually exceeds the cooling load, and sizing to the heating load can badly oversize the unit for summer. This is where variable-speed equipment earns its premium, because it can modulate down to a fraction of full capacity. The alternative is to size to the cooling load and accept that backup heat covers the coldest hours.
Get 3 local quotes before you commit
Homeowners who collect three written quotes typically save 12–18% on the same system. Tell us your ZIP and we will connect you with licensed installers in your area.
Connect this form to your lead partner (Modernize, Networx, HomeAdvisor). This channel often out-earns display ads in this niche.
Last checked August 2026. Prices are US ranges and vary by region — always confirm against written quotes.
