BTU Calculator
Size an air conditioner or heater for any room. Adjust for ceiling height, sun, people, kitchens and climate to get a realistic BTU figure.
Formulas shown on this page · Last reviewed October 2026
BTU formula
base = area (sq ft) × 20 × ceiling height ÷ 8
BTU/h = base × sun factor × climate factor + 600 × (people − 2) + 4,000 (kitchen)
Sun factor is 0.9 for heavily shaded rooms and 1.1 for very sunny ones. Climate factor is 0.9 for cool regions, 1.0 for moderate and 1.1 for hot regions. The heating figure uses roughly 30–50 BTU per square foot depending on climate and is only a rough guide, as insulation and window quality matter a lot.
Room size to AC size
Average conditions, 8 ft ceiling, two occupants, moderate climate.
| Room area | Cooling need | Common unit size |
|---|---|---|
| 150 sq ft | 3,000 BTU | 5,000 BTU (smallest size) |
| 300 sq ft | 6,000 BTU | 6,000 BTU |
| 400 sq ft | 8,000 BTU | 8,000 BTU |
| 500 sq ft | 10,000 BTU | 10,000 BTU |
| 700 sq ft | 14,000 BTU | 14,000 BTU |
| 1,000 sq ft | 20,000 BTU | 24,000 BTU |
Practical tips
- A 10 ft ceiling adds 25% to the load: a 300 sq ft room goes from 6,000 to 7,500 BTU, so an 8,000 BTU unit.
- If the room is open to a hallway or another room, include that area as well.
- Check the EER or CEER rating. Higher numbers mean lower running costs, so compare with our electricity cost calculator.
- For whole-home central systems, a professional Manual J load calculation is more accurate than any rule of thumb.
AC size by square feet
150 sq ft · 200 sq ft · 250 sq ft · 300 sq ft · 350 sq ft · 400 sq ft · 450 sq ft · 500 sq ft · 600 sq ft · 700 sq ft · 800 sq ft · 1,000 sq ft · 1,200 sq ft · 1,500 sq ft · 1,800 sq ft · 2,000 sq ft · 2,500 sq ft
Frequently asked questions
How many BTU do I need per square foot?
A common guideline, consistent with ENERGY STAR guidance, is about 20 BTU per square foot for an 8 ft ceiling. A 300 sq ft room needs about 6,000 BTU/h of cooling.
How many BTU for a kitchen or sunny room?
Add 4,000 BTU for a kitchen because of the heat from appliances, and about 10% for a very sunny room. A 300 sq ft sunny kitchen needs 6,000 × 1.1 + 4,000 = 10,600 BTU, so a 12,000 BTU unit.
What does a ton of air conditioning mean?
One ton equals 12,000 BTU per hour. A 24,000 BTU unit is a 2-ton system, and a 1,500 sq ft home at 20 BTU per sq ft needs about 30,000 BTU, or 2.5 tons.
Is a bigger air conditioner better?
No. An oversized unit cools the air too quickly and shuts off before it removes humidity, leaving the room cold but clammy and wasting energy. Choose the closest size at or just above the calculated figure.