BTU Calculator
Calculate the BTU capacity you need to heat or cool any room. Enter square footage, ceiling height, and climate zone for accurate HVAC sizing.
Formula & How It Works
BTU = Area (sq ft) × BTU per sq ft (climate) + Adjustments
The base rule is 20-40 BTU per square foot depending on climate zone. Multiply by a ceiling height factor (height/8). Add 4,000 BTU for kitchens (appliance heat), 4,000 BTU for heavy occupancy or full sun exposure, and subtract 4,000 BTU for shaded rooms. One ton of AC capacity equals 12,000 BTU/hr.
BTU Sizing Reference Table
Standard 8-ft ceiling, mixed climate (30 BTU/sq ft):
| Room Size (sq ft) | Cooling BTU | AC Tons | Recommended Unit |
|---|---|---|---|
| 150 sq ft | 5,000 BTU | 0.4 ton | Window unit |
| 250 sq ft | 7,000 BTU | 0.6 ton | Window unit |
| 400 sq ft | 12,000 BTU | 1.0 ton | Window / mini-split |
| 600 sq ft | 18,000 BTU | 1.5 ton | Mini-split |
| 800 sq ft | 24,000 BTU | 2.0 ton | Mini-split / central |
| 1,000 sq ft | 30,000 BTU | 2.5 ton | Central AC |
| 1,500 sq ft | 45,000 BTU | 3.5 ton | Central AC |
| 2,000 sq ft | 60,000 BTU | 5.0 ton | Central AC |
BTU Sizing Factors
- Climate zone: Hot climates need 35-40 BTU/sq ft; cold climates need 40+ BTU/sq ft for heating
- Insulation: Poorly insulated homes may need 50% more BTU capacity
- Windows: Large south-facing windows add significant heat load in summer
- Kitchen: Add 4,000 BTU for cooking appliances and heat output
- Occupancy: Each person adds approximately 400-600 BTU of heat
- 1 ton = 12,000 BTU: AC capacity is traditionally measured in tons of ice equivalent
Frequently Asked Questions
How many BTU do I need for a 400 sq ft room?
A 400 sq ft room with standard 8-ft ceilings in a mixed climate needs approximately 12,000 BTU (1 ton) of cooling capacity. In hot climates, increase to 14,000-16,000 BTU. If it is a kitchen, add 4,000 BTU. A 12,000 BTU mini-split or window unit is ideal.
How do I calculate BTU for my whole house?
Multiply your total square footage by 30 BTU/sq ft (mixed climate) as a starting point. A 2,000 sq ft home needs about 60,000 BTU (5 tons). Adjust for ceiling height, insulation quality, sun exposure, and number of occupants. For accurate sizing, have an HVAC professional perform a Manual J load calculation.
What does 1 ton of AC mean in BTU?
One ton of air conditioning equals 12,000 BTU per hour. The term comes from the amount of heat needed to melt one ton of ice in 24 hours. A 3-ton AC unit provides 36,000 BTU/hr of cooling capacity, suitable for a 1,200-1,500 sq ft home.
What happens if my AC is oversized?
An oversized AC cools the room too quickly and shuts off, a problem called short cycling. This reduces dehumidification, causes temperature swings, increases energy bills, and shortens equipment life. It is better to slightly undersize than oversize. Properly sized units run longer cycles for better comfort and humidity control.
Do I need more BTU for heating or cooling?
In most US climates, heating requires more BTU than cooling. A heat pump or furnace for a 2,000 sq ft home may need 60,000-80,000 BTU for heating vs 36,000-48,000 BTU for cooling. In hot climates like Florida, cooling loads dominate. Always size for the larger load when selecting equipment.