BTU Calculator

Please enter length, width, ceiling height and people.
–square feet
–BTU per hour needed
–tons of cooling
–nearest standard size (BTU)

Estimate only. Climate, insulation and windows also affect the size you need. Ask an HVAC professional for a full load calculation.

How to use this calculator

Enter the room length, width and ceiling height. Choose how sunny the room is, how many people use it and whether it is a kitchen. Press Calculate to get the cooling capacity in BTU per hour and the nearest standard air conditioner size.

The formula

BTU = area (sq ft) × 20 × (ceiling height ÷ 8) × sunlight factor
Add 600 BTU for each person beyond two, and 4,000 BTU for a kitchen.
Tons = BTU ÷ 12,000

Example: A 15 ft × 20 ft room is 300 square feet. At 20 BTU per square foot with an 8 ft ceiling, normal sunlight and two people, it needs about 6,000 BTU.

Choosing the right size air conditioner from your BTU result

This calculator estimates the cooling capacity a room needs, in BTU per hour. It starts with about 20 BTU per square foot, then adjusts for ceiling height, sunlight, extra people and kitchens. The result is the size to look for when shopping for a window, portable or mini-split unit.

What the result means in practice

Air conditioners come in set sizes, so you will rarely find one that matches your number exactly. Choose the size closest to your result, usually rounding up slightly rather than down. The tons figure is mainly useful for central systems and mini-splits, which are often sized in tons.

Real-world examples

A 12 ft by 15 ft bedroom is 180 sq ft. With an 8 ft ceiling and normal sunlight, it needs about 3,600 BTU. With a 9 ft ceiling, it needs about 4,050 BTU. Many of the smallest window units are around 5,000 BTU, which would cover either case. A 15 ft by 20 ft room with 4 people instead of 2 needs about 6,000 plus 1,200 BTU, or about 7,200 BTU.

Main factors that change the size

  • Sun exposure: West- and south-facing rooms with large windows gain more heat.
  • Ceiling height: Taller ceilings mean more air to cool.
  • Occupants: Each person adds body heat.
  • Heat sources: Kitchens, computers and TVs add heat that the unit must remove.
  • Insulation and climate: A hot attic above the room or a very hot climate can raise the need.

Tips and common mistakes

  • Do not oversize. A unit that is too big cools the air quickly, then shuts off before removing humidity, leaving the room cool but clammy.
  • Do not undersize either. A small unit will run nonstop on hot days and still fall short.
  • For open floor plans, measure the full connected space, not just one room.
  • Portable units often cool less effectively than window units of the same rated BTU, so compare their newer adjusted ratings if listed.

Buying and planning advice

Check the unit’s efficiency rating, as a more efficient model costs less to run. Make sure your outlet and circuit can handle the unit. Larger window units may need a 240-volt or dedicated circuit, which should be installed by a licensed electrician to meet local codes. For whole-home central air, ask an HVAC pro for a Manual J load calculation rather than adding up rooms.

Frequently asked questions

How many BTU do I need per square foot?
A common rule of thumb is about 20 BTU per square foot for a room with an 8 foot ceiling and normal sunlight.
What size air conditioner do I need for 300 square feet?
About 6,000 to 8,000 BTU, depending on sunlight, ceiling height and how many people use the room.
How many BTU are in a ton of cooling?
One ton of cooling equals 12,000 BTU per hour.
Is a bigger air conditioner better?
No. An oversized unit cools too quickly, shuts off early and removes less humidity.
Why add BTU for a kitchen?
Cooking adds a lot of heat, so a kitchen needs about 4,000 extra BTU.
Does an air conditioner also heat a room?
A standard air conditioner only cools. Some window units and many mini-splits are heat pumps, which can run in reverse to provide heat. Others include electric resistance heating strips. Check the product description for “heat pump” or “heat” mode, and note the separate heating BTU rating, which is often listed apart from cooling capacity.
How many BTU do I need to cool a whole house?
Adding up room-by-room estimates gives a rough idea, but whole-house systems should be sized with a professional Manual J calculation. That method accounts for insulation, windows, air leakage, duct location and local climate. Central systems are sized in tons, where 1 ton equals 12,000 BTU per hour.
Can one window unit cool two rooms?
Only if the rooms are connected by a wide opening and air can move freely between them. A window unit cools best in the room where it sits. A fan can help push cool air into a nearby room, but closed doors and hallways block most of it. Size the unit for the combined area of both spaces.