BTU Calculator
Calculate the BTU per hour and tonnage needed to heat or cool a room from its dimensions and ceiling height. Sizes air conditioners, mini-splits, and heaters.
How this calculator works
Cooling and heating load scales with the volume of the space and the climate. The calculator multiplies floor area by a per-square-foot BTU factor, then adjusts for ceiling height above the standard 8 feet.
Formula: BTU/hr = length × width × BTU-per-sq-ft × (ceiling ÷ 8). Tons = BTU/hr ÷ 12,000.
Worked example
A 12 ft × 15 ft room, 8-foot ceiling, 20 BTU per square foot:
- Area: 12 × 15 = 180 sq ft
- Load: 180 × 20 × (8 ÷ 8) = 3,600 BTU/hr
- In tons: 3,600 ÷ 12,000 = 0.30 tons
Reference (8 ft ceiling, 20 BTU/sq ft)
| Room area | BTU/hr | Typical unit |
|---|---|---|
| 150 sq ft | 3,000 | Small window AC |
| 300 sq ft | 6,000 | Bedroom mini-split |
| 500 sq ft | 10,000 | Living-room unit |
How to use
Measure the room’s length, width, and ceiling height. Raise the BTU factor for sunny, hot, or high-heat rooms and lower it for well-insulated or shaded ones, then match the result to a unit’s rating.
Frequently asked questions
How many BTUs do I need per square foot?
About 20 BTU per square foot is the common rule of thumb for cooling a standard room. Adjust up for hot, sunny climates, big windows, or a kitchen; adjust down for shaded, well-insulated, or northern spaces. The calculator lets you set the per-square-foot factor.
What is a ton of cooling?
One ton equals 12,000 BTU per hour — a holdover from the ice trade. Central air and heat pumps are rated in tons; the calculator converts your BTU load to tons so you can match a unit.
Does ceiling height matter?
Yes — you're conditioning a volume, not just a floor. The calculator scales the load by ceiling height relative to a standard 8 feet, so a 10-foot ceiling adds about 25% to the load.
Why does an oversized unit cause problems?
An air conditioner that's too big cools the air fast but shuts off before removing humidity, leaving the room cold and clammy and short-cycling the compressor. Right-sizing — not oversizing — gives comfort and efficiency.
Is this a full Manual J load calculation?
No — this is a fast square-footage estimate for a single room. A whole-house system should be sized with a Manual J calculation that accounts for insulation, windows, orientation, and infiltration. Use this to sanity-check a room unit or a quote.
Estimates only. Verify quantities with your supplier before purchasing.