BTU Calculator
How many BTUs you need to cool and heat each room, adjusted for sun, people, kitchens and high ceilings, with the right window AC size or central AC tonnage.
Cooling: about 20 BTU per square foot (ENERGY STAR chart): a 300 sq ft room needs 7,000 BTU, 500 sq ft needs 12,000. Add 10% for a very sunny room, subtract 10% if heavily shaded, add 600 BTU per person beyond two and 4,000 BTU for a kitchen. Heating: about 30–60 BTU per square foot depending on climate.
ENERGY STAR room AC sizing chart
| Room area | Cooling capacity |
|---|---|
| 100–150 sq ft | 5,000 BTU/h |
| 150–250 sq ft | 6,000 BTU/h |
| 250–300 sq ft | 7,000 BTU/h |
| 300–350 sq ft | 8,000 BTU/h |
| 350–400 sq ft | 9,000 BTU/h |
| 400–450 sq ft | 10,000 BTU/h |
| 450–550 sq ft | 12,000 BTU/h |
| 550–700 sq ft | 14,000 BTU/h |
| 700–1,000 sq ft | 18,000 BTU/h |
| 1000–1,200 sq ft | 21,000 BTU/h |
| 1200–1,400 sq ft | 23,000 BTU/h |
| 1400–1,500 sq ft | 24,000 BTU/h |
| 1500–2,000 sq ft | 30,000 BTU/h |
| 2000–2,500 sq ft | 34,000 BTU/h |
Heating BTU per square foot by climate
| Climate | BTU per sq ft | 1,500 sq ft home |
|---|---|---|
| Hot (climate zones 1–2) | 30 | 45,000 BTU/h |
| Warm (zone 3) | 35 | 52,500 BTU/h |
| Mixed (zone 4) | 45 | 67,500 BTU/h |
| Cold (zone 5) | 50 | 75,000 BTU/h |
| Very cold (zones 6–7) | 60 | 90,000 BTU/h |
For a whole-house system, ask your contractor for an ACCA Manual J load calculation; it accounts for insulation levels, windows, air leakage and duct losses room by room.
Frequently asked questions
How many BTU do I need per square foot?
About 20 BTU per square foot for cooling in a typical room, per the ENERGY STAR chart, and about 30–60 BTU per square foot for heating depending on how cold your winters are and how well insulated the house is.
What size AC do I need for a 12 × 12 room?
144 sq ft falls in the 100–150 sq ft band, so 5,000 BTU. Go up 10% if the room gets strong afternoon sun.
How many BTU for a 500 square foot room?
12,000 BTU (one ton) by the ENERGY STAR chart, before adjusting for sun, extra people or a kitchen.
How many BTU is a ton of air conditioning?
12,000 BTU per hour. A 3-ton central AC delivers about 36,000 BTU/h.
Is a bigger air conditioner better?
No. An oversized AC cools the room quickly and shuts off before it removes humidity, leaving it clammy, and it wears out sooner. Pick the size that matches the load.
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Cooling sizes and adjustments (−10% shade, +10% sun, +600 BTU per person over two, +4,000 BTU for kitchens) follow the ENERGY STAR room air conditioner sizing guide; ceilings over 8 ft scale by room volume (our addition). Heating uses common BTU-per-square-foot rules by IECC climate zone with an insulation factor. These are estimates; a Manual J calculation is the standard for equipment sizing.