BTU Calculator
BTU (British Thermal Unit) per hour is the standard measure of air conditioner and heater capacity. Too few BTU and the unit runs constantly without reaching temperature; too many and it short-cycles, leaving the room clammy.
This calculator starts from the 20 BTU per ft² rule and adjusts for the factors that actually move the number: insulation, sun, kitchens and people.
Enter valid values to see the results.
Formula
Base rate 20 BTU/h per ft² (≈ 215 BTU/h per m²). Adjustments: good insulation −10%, poor +20%; sunny +10%, shaded −10%; kitchen +4,000 BTU; each occupant beyond two +600 BTU.
How to measure
- Measure the room floor area (length × width) — use the Room Area Calculator if the shape is complex.
- Judge insulation honestly: single glazing or uninsulated walls count as poor.
- Count the people who regularly use the room at the same time.
Worked example
Practical tips
- When between two sizes, choose the smaller for humid climates (better dehumidification) and the larger for dry heat.
- High ceilings above 2.7 m (9 ft) add about 10% per extra metre of height.
- Rooms above uninsulated garages or under flat roofs need 10–20% more.
- Portable AC units deliver about 30% less effective cooling than their rating — size up accordingly.
Waste allowance
No waste applies, but round the result up to the next standard unit size (7,000 / 9,000 / 12,000 / 18,000 / 24,000 BTU).
Quick BTU reference by room size (average conditions)
| Room area | BTU/h | kW |
|---|---|---|
| 10 m² / 108 ft² | 5,000 | 1.5 |
| 20 m² / 215 ft² | 7,000 | 2.1 |
| 30 m² / 323 ft² | 9,000 | 2.6 |
| 40 m² / 430 ft² | 12,000 | 3.5 |
| 55 m² / 592 ft² | 14,000 | 4.1 |
Common mistakes
- 1 Sizing by area alone and ignoring sun exposure — a west-facing room can need 25% more than a shaded one.
- 2 Buying the biggest unit “to be safe”: oversizing causes short cycles, poor dehumidification and higher bills.
- 3 Forgetting kitchens add roughly 4,000 BTU/h from cooking appliances.
- 4 Using cooling BTU rules for heating in cold climates — heating loads there are usually higher.
Frequently asked questions
How many BTU per square metre?
About 215 BTU/h per m² (20 per ft²) for average conditions. Adjust up for sun and poor insulation, down for shade and good insulation.
What size AC for a 20 m² room?
Around 7,000 BTU/h (2 kW) in average conditions. A sunny kitchen of the same size can need 12,000 BTU/h.
Is 12,000 BTU enough for a living room?
12,000 BTU/h (1 ton) covers about 40–50 m² (430–550 ft²) in average conditions — enough for most living rooms.
What happens if my AC is too powerful?
It cools the air before removing humidity, then shuts off. The room feels cold and clammy, and the compressor wears out faster from constant restarts.
Does this work for heating?
As a lower bound, yes. In cold climates heating loads are typically 30–50% higher than cooling loads — check a heating-specific calculation for boiler or furnace sizing.