BTU Calculator
Size heating and cooling capacity from room area and ceiling height.
How this calculator works
BTU = area × BTU per sq ft × (ceiling height ÷ 8)
This is a rule-of-thumb estimate. A proper Manual J load calculation accounts for insulation, window area and orientation, air leakage and occupancy, and often lands 20 to 30% below the rule of thumb.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ANSI/ACCA 2 Manual J (Residential Load Calculation) 2016, 8th edition — the load calculation this rule of thumb approximates, and which supersedes it for equipment selection
Standards are revised on a cycle. The edition above is the one this figure was checked against — confirm it against the edition your jurisdiction has adopted.
Worked example
With room area 1000 sq ft, ceiling height 8 ft, btu per square foot 25 , this calculator returns 25000 .
Heating and cooling load for rooms of typical size
Click through any row to load those measurements. The address bar keeps them, which makes the result something you can paste into a message.
| Job | Size | Capacity needed |
|---|---|---|
| Bedroom | 150 sq ft × 8 ft × 25 | 3,750 |
| Living room | 350 sq ft × 8 ft × 25 | 8,750 |
| Living room with a vaulted ceiling | 350 sq ft × 14 ft × 25 | 15,313 |
| Whole small house | 1200 sq ft × 8 ft × 25 | 30,000 |
| Sunroom, heavy glazing | 200 sq ft × 8 ft × 40 | 8,000 |
A Manual J calculation typically lands 20–30% below these figures. Size equipment from that, not from this.
Frequently asked questions
How many BTUs do I need per square foot?
For cooling, 20 to 30 depending on climate. For heating, 30 in the deep south up to 60 in the far north. These are rules of thumb — a Manual J calculation is the proper method.
How many BTUs are in a ton of cooling?
Twelve thousand. A 3 ton air conditioner delivers 36,000 BTU per hour. The term dates from the cooling equivalent of melting a ton of ice over 24 hours.
Is it bad to oversize an air conditioner?
Yes. An oversized unit cools the air fast then shuts off before removing humidity, leaving the house cold and clammy. Short cycling also wears the compressor and wastes energy.
Does ceiling height affect BTU requirements?
Directly. The load scales with volume, not floor area. A room with 10 ft ceilings needs 25% more capacity than the same footprint at 8 ft.