This BTU calculator estimates the cooling capacity a room air conditioner needs and the heating capacity for a space heater or heat source, based on room size, ceiling height, sun exposure, occupants and your winter climate. It uses the ENERGY STAR room air conditioner sizing chart as its starting point.
How to Use the BTU Calculator
- Measure the room’s length and width in feet.
- Enter the ceiling height. Leave it at 8 feet for a standard ceiling.
- Choose the sun exposure: average, very sunny, or heavily shaded.
- Enter how many people usually use the room at the same time.
- Choose your winter climate for the heating estimate.
- Tick the kitchen box if the unit will cool a kitchen, then read the cooling and heating results.
How the Calculation Works
Cooling. The calculator multiplies length by width to get square feet, then looks up the base BTU rating from the ENERGY STAR chart shown below. If the room is larger than 2,500 square feet, it uses 14 BTU per square foot instead and shows a note recommending a professional load calculation.
The base figure is then adjusted in this order:
- If the ceiling is taller than 8 feet, the base is multiplied by the ceiling height divided by 8. A 10 foot ceiling multiplies it by 1.25.
- A very sunny room adds 10 percent. A heavily shaded room subtracts 10 percent.
- For more than two regular occupants, 600 BTU is added for each extra person.
- A kitchen adds 4,000 BTU for the heat from cooking appliances.
The final cooling figure is rounded to the nearest 100 BTU per hour.
Heating. The heating estimate multiplies the floor area by a BTU per square foot figure for your climate: 30 for mild, 40 for moderate, 50 for cold and 60 for very cold winters. Ceilings over 8 feet scale it up the same way as cooling. The result is rounded to the nearest 100 BTU. The electric heater equivalent divides the heating BTU by 3.412, since one watt of electric heat produces about 3.412 BTU per hour, and rounds to the nearest 100 watts.
Worked example with the default values: a 15 x 20 foot room with an 8 foot ceiling, average sun, 2 people and a moderate winter.
- Area: 15 x 20 = 300 square feet
- Chart lookup: 300 square feet falls in the over 250 up to 300 row, so the base is 7,000 BTU
- Adjustments: 8 foot ceiling, average sun and 2 people change nothing, so cooling is 7,000 BTU per hour
- Heating: 300 x 40 = 12,000 BTU per hour
- Electric equivalent: 12,000 / 3.412 = 3,517 watts, rounded to 3,500 watts
Change the same room to very sunny and the cooling result becomes 7,000 x 1.10 = 7,700 BTU.
Room AC Sizing Chart and Adjustments
This is the ENERGY STAR room air conditioner sizing chart the calculator uses for its base cooling figure.
| Room area | Cooling capacity needed |
|---|---|
| Up to 150 sq ft | 5,000 BTU/hr |
| Over 150 up to 250 sq ft | 6,000 BTU/hr |
| Over 250 up to 300 sq ft | 7,000 BTU/hr |
| Over 300 up to 350 sq ft | 8,000 BTU/hr |
| Over 350 up to 400 sq ft | 9,000 BTU/hr |
| Over 400 up to 450 sq ft | 10,000 BTU/hr |
| Over 450 up to 550 sq ft | 12,000 BTU/hr |
| Over 550 up to 700 sq ft | 14,000 BTU/hr |
| Over 700 up to 1,000 sq ft | 18,000 BTU/hr |
| Over 1,000 up to 1,200 sq ft | 21,000 BTU/hr |
| Over 1,200 up to 1,400 sq ft | 23,000 BTU/hr |
| Over 1,400 up to 1,500 sq ft | 24,000 BTU/hr |
| Over 1,500 up to 2,000 sq ft | 30,000 BTU/hr |
| Over 2,000 up to 2,500 sq ft | 34,000 BTU/hr |
| Condition | Adjustment |
|---|---|
| Ceiling taller than 8 ft | Multiply by ceiling height / 8 |
| Very sunny room | Add 10% |
| Heavily shaded room | Subtract 10% |
| More than 2 regular occupants | Add 600 BTU per extra person |
| Kitchen | Add 4,000 BTU |
| Heating, mild / moderate / cold / very cold winter | 30 / 40 / 50 / 60 BTU per sq ft |
Tips and Common Mistakes
- Bigger is not better. An oversized air conditioner cools the room quickly and shuts off before it removes much humidity, which can leave the room cold and clammy. Pick the size closest to the result rather than jumping several sizes up.
- Measure the space the air actually reaches. If a room opens fully into another without a door, the unit has to cool both. Include the connected area in your measurements.
- Insulation and windows change the picture. A poorly insulated room, a top floor under a hot roof, or large single pane windows may need the higher end of the range. A well insulated room may do fine at the lower end.
- Check your outlet and circuit. Larger window units and many electric heaters draw a lot of current. Check the unit’s plug type and amperage, and avoid running a heater on an extension cord.
- Whole house systems need a load calculation. This calculator is for single rooms. Sizing central air or a heat pump for a whole home should be based on a Manual J load calculation done by an HVAC professional.
Frequently Asked Questions
How many BTU do I need for a 300 square foot room?
The ENERGY STAR chart lists 7,000 BTU for a room over 250 and up to 300 square feet with an 8 foot ceiling and average conditions. Add 10 percent for a very sunny room, and more for a kitchen or extra occupants.
What size air conditioner do I need for a 12 x 12 bedroom?
A 12 x 12 room is 144 square feet, which falls in the 5,000 BTU row of the chart. A very sunny bedroom would come out at 5,500 BTU, so a 6,000 BTU unit is a reasonable choice in that case.
How do I convert BTU to watts for a space heater?
Divide BTU per hour by 3.412. A room needing 5,000 BTU of heat would need about 1,465 watts, which is close to the common 1,500 watt space heater.
Why does a kitchen need more cooling?
Stoves, ovens, dishwashers and refrigerators all release heat into the room. The ENERGY STAR guidance adds 4,000 BTU when a unit is used in a kitchen to account for that extra load.