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BTU Calculator
Estimate the air conditioner capacity a room needs. Enter the room’s length and width and the calculator starts from 20 BTU per square foot, then adjusts for ceiling height, sun exposure, the number of people and whether the room is a kitchen.
- Tons of cooling
- 0.5 tons
- Cooling capacity in kilowatts
- 1.76 kW
- Floor area
- 300 ft²
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Enter the room’s length and width.
- Enter the ceiling height.
- Choose the sun exposure and the usual number of occupants.
- Tick the kitchen box if it applies, then pick the nearest unit size.
Worked examples
A 20 × 15 ft room with an 8 ft ceiling, average sun and two people
- Floor area
- 300 ft²
- Cooling capacity needed
- 6,000 BTU/h
- Tons of cooling
- 0.5 tons
- Cooling capacity in kilowatts
- 1.76 kW
A very sunny 25 × 20 ft kitchen with a 10 ft ceiling and four people
- Floor area
- 500 ft²
- Cooling capacity needed
- 18,950 BTU/h
- Tons of cooling
- 1.58 tons
Where the adjustments come from
The 20 BTU per square foot starting point and the adjustments — 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU for each occupant beyond two and 4,000 BTU for a kitchen — are the standard room air conditioner sizing guidance published by ENERGY STAR. The ceiling-height scaling is a simple proportion added for rooms taller than 8 ft.
Typical window and portable units come in steps such as 5,000, 6,000, 8,000, 10,000, 12,000, 14,000 and 18,000 BTU. Choose the closest size at or just above your result.
Do not oversize
A unit that is too large cools the air quickly and shuts off before it has removed much humidity, leaving the room cold and clammy and cycling on and off more than it should. Slightly under is usually better than far over.
This is a room-by-room estimate. Whole-house central systems should be sized with a full load calculation (ACCA Manual J) that accounts for insulation, windows, climate and ductwork; a qualified HVAC professional and local codes govern that choice.
Questions people ask
How many BTU do I need for a 300 sq ft room?
About 6,000 BTU per hour for an average room with an 8 ft ceiling and two people. Add 10% if it is very sunny, and 4,000 BTU if it is a kitchen.
How many BTU is a 1 ton air conditioner?
12,000 BTU per hour. A 2 ton unit is 24,000 and a 3 ton unit is 36,000.
What size room will a 12,000 BTU air conditioner cool?
Up to about 600 sq ft under average conditions at 20 BTU per square foot; less if the room is sunny, crowded or has a high ceiling.