How to use the BTU calculator
For cooling, enter the room's size, how sunny it is and how many people usually use it. The calculator looks up ENERGY STAR's room air conditioner chart, applies its adjustments, and gives the capacity in BTU per hour and in tons. For heating, it estimates how fast the room loses heat on a cold night through its outside walls, windows and ceiling and through air leaks. That figure is the heater output you'd need to keep up.
The formulas
cooling = ENERGY STAR chart for the area, ±10% for shade or sun, +600 per person beyond 2, +4,000 for a kitchen
heating = Σ (area × ΔT ÷ R) + 0.018 × volume × ACH × ΔT
tons = BTU/h ÷ 12,000; kW = BTU/h ÷ 3,412
Worked example: cooling a sunny 20 × 12 ft bedroom for three people
- Area: 240 ft², which ENERGY STAR sizes at 6,000 BTU/h (150 up to 250 ft²).
- Very sunny: +10% gives 6,600.
- One person beyond two: +600 gives 7,200 BTU/h, or 0.6 ton. A 7,000 or 8,000 BTU unit fits.
Worked example: heating the same room
Two outside walls, 32 ft long and 8 ft high, with 30 ft² of windows (U 0.30), R-13 walls and R-38 attic insulation, at 70 °F inside and 10 °F outside (ΔT 60 °F), 0.5 air changes per hour:
- Walls: (256 − 30) × 60 ÷ 13 = 1,043 BTU/h.
- Windows: 30 × 0.30 × 60 = 540 BTU/h.
- Ceiling: 240 × 60 ÷ 38 = 379 BTU/h.
- Air leakage: 0.018 × 1,920 ft³ × 0.5 × 60 = 1,037 BTU/h.
- Total: about 3,000 BTU/h, or 0.88 kW. A 1,500 W heater (5,100 BTU/h) covers it with room to spare.
ENERGY STAR room AC chart
| Area (ft²) | BTU/h |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
ENERGY STAR notes the chart assumes 8 ft ceilings.
Limits of a room calculator
- Whole houses need a room-by-room ACCA Manual J load calculation; HVAC contractors do this, and many jurisdictions require it for new equipment.
- Effective R-values are lower than the insulation's label because studs and joists conduct heat. If you know the assembly's effective R-value, use it.
- Upstairs and west-facing rooms often need more cooling than the chart suggests.
Questions people ask
How many BTU do I need to cool a room?
ENERGY STAR's chart: 5,000 BTU for 100-150 sq ft, 6,000 for 150-250, 7,000 for 250-300, 8,000 for 300-350, 9,000 for 350-400, 10,000 for 400-450, 12,000 for 450-550, 14,000 for 550-700 and 18,000 for 700-1,000 sq ft, with larger sizes beyond. Add 10% for a very sunny room, take off 10% for a heavily shaded one.
How many BTU for a 12x12 room?
144 square feet falls in ENERGY STAR's 100-150 sq ft band: 5,000 BTU per hour. Add 600 BTU for each regular occupant beyond two, and 4,000 if it's a kitchen.
How many tons is my air conditioner?
One ton of cooling is 12,000 BTU per hour. A 24,000 BTU unit is 2 tons; a 9,000 BTU window unit is 0.75 ton.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and shuts off before it removes humidity, leaving the room cold and clammy, and it cycles more. Size it to the room.
How do I size a heater for a room?
Estimate the room's heat loss: each wall, window and ceiling loses heat in proportion to its area and the indoor-outdoor temperature difference, divided by its R-value, plus heat carried out by air leakage. This calculator does that. For a whole house, a contractor's ACCA Manual J calculation is the standard.
Sources and assumptions
- ENERGY STAR: room air conditioner sizing chart and adjustments (shade −10%, sun +10%, +600 BTU per person beyond two, +4,000 BTU for kitchens)
- Heat loss: conduction = area × ΔT ÷ R; air leakage = 0.018 BTU/ft³·°F × volume × air changes per hour × ΔT (air at 0.075 lb/ft³ × 0.24 BTU/lb·°F). Wall and ceiling R-values are your inputs; framing lowers a wall's effective R-value, so real losses are higher than the cavity R-value suggests.
- Whole-house heating and cooling equipment should be sized with ACCA Manual J.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.