BTU Calculator
Your estimate
- Tons
- 0.58
- Nearest AC size (BTU/h)
- 8,000
- Area
- 300 ft²33.33 yd² · 27.87 m²
Air-conditioner size from the ENERGY STAR chart, or the heating power for a room.
Updated 2026-10-20 · Based on: NEC Chapter 9 Table 8 and 310.16; DIN VDE 0100-520 for metric.
How to use the BTU Calculator
- Enter the room length, width and ceiling height.
- Cooling: set sun exposure, people and whether it is a kitchen.
- Heating: choose how well the room is insulated.
- Pick the nearest AC size at or above the result.
Formula
Cooling: ENERGY STAR chart (e.g. 250–300 ft² → 7,000 BTU/h), scaled for ceiling height; +10 % for a sunny room, −10 % if shaded, +600 BTU/h per person beyond two, +4,000 for a kitchen.
Heating: area × watts per m² (40 for a new build, 80 average, 140 for an old uninsulated house).
Worked example
With the values preset in the calculator above: (Cooling (AC))
- Length
- 15 ft
- Width
- 20 ft
- Height
- 8 ft
- Sun
- Normal
- People in the room
- 2
- Kitchen
- No
- Sizing rule
- ENERGY STAR chart (US)
- Cooling / heating power
- 7,000 BTU/h (2.05 kW)
- Tons
- 0.58
- Nearest AC size (BTU/h)
- 8,000
- Area
- 300 ft² (33.33 yd²)
BTU per square foot
Roughly 20 BTU/h per ft² for cooling in a standard room. A 15 × 20 ft room (300 ft²) needs 7,000 BTU/h — a 7,000–8,000 BTU unit.
AC size for common rooms
| Length (ft) | Width (ft) | Cooling / heating power |
|---|---|---|
| 10 | 12 | 5,000 BTU/h |
| 12 | 15 | 6,000 BTU/h |
| 15 | 20 | 7,000 BTU/h |
| 20 | 20 | 9,000 BTU/h |
| 20 | 25 | 12,000 BTU/h |
| 25 | 30 | 18,000 BTU/h |
| 30 | 40 | 21,000 BTU/h |
Frequently asked questions
What size AC for 400 square feet?
9,000 BTU/h from the ENERGY STAR chart, before sun and occupancy adjustments.
How many BTU is one ton?
12,000 BTU/h.
Is a bigger AC better?
No — an oversized unit cycles on and off and leaves the room clammy. Size it to the room.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.