BTU Calculator
Enter the size of the room and a few conditions. The calculator gives a rule-of-thumb estimate of the BTU per hour and the tons of cooling the room needs.
How to use this BTU Calculator
- Measure the floor area of the room in square feet.
- Keep the base BTU per square foot, or change it for a very well insulated or poorly insulated room.
- Enter the ceiling height. The standard 8 feet is the reference.
- Choose the sun exposure and enter how many people use the room.
- Choose yes for a kitchen, then press Calculate.
How to calculate BTU for a room
A BTU is a unit of heat. This calculator starts with a base figure per square foot and applies adjustments.
- Base BTU = area x BTU per sq ft
- Adjusted = base x (ceiling height / 8) x sun factor
- People = 600 BTU for each person above two
- Total = adjusted + people + kitchen allowance
- Tons = total BTU / 12,000
Worked example
A 300 sq ft room with an 8 foot ceiling, normal sun, two people and no kitchen use needs 300 x 20 = 6,000 BTU per hour. That is 0.50 tons of cooling. If the room has a 10 foot ceiling and gets a lot of sun, the base figure grows by 25% for height and 10% for sun, which gives 8,250 BTU per hour. Add a third person and the estimate rises by another 600 BTU.
Window units and portable units are sold in steps, so round up to the nearest size on the label. A unit that is too small runs all the time, while one that is far too large can cool the room too quickly without removing humidity. Rules of thumb do not account for windows, insulation, climate or duct losses, so treat the result as a starting point, and ask a professional for a whole-house system.
Assumptions & limits
- The 20 BTU per square foot figure is a common rule of thumb, not a standard. It is editable, and your climate and insulation matter.
- The adjustments are simple factors, not a full heat load calculation. Windows, insulation, air leaks and local climate are not modelled.
- One ton of cooling is 12,000 BTU per hour.
- The estimate is for one room. Do not add rooms together to size a whole-house system.
- For heating, climate and the heating system change the need a lot, so use the result as a rough guide only.
Frequently asked questions
How do I calculate BTU for a room?
Multiply the room area by about 20 BTU per square foot, then adjust for ceiling height, sun, people and kitchen use.
How many BTU do I need for 300 square feet?
About 6,000 BTU per hour by the rule of thumb, before adjustments.
How many BTU are in a ton?
One ton of cooling is 12,000 BTU per hour.
What happens if the air conditioner is too big?
It may cool too fast and cycle on and off, which can leave the room damp.
Can I use this for heating?
Only as a rough guide. Heating needs depend on climate and insulation, so ask a professional for a heating load.
Is this the same as a Manual J calculation?
No. Manual J is a full load calculation done by a professional. This is a quick estimate.
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Last updated: November 2026