How to use the BTU calculator
- Enter the Room length (ft), Room width (ft) and Ceiling height (ft) of the space you want to cool.
- Set People who regularly use the room, then choose Sun exposure and Room type. Pick Kitchen if the unit will cool a kitchen.
- Optional: open Fine-tune for insulation and climate for an attic room, a top floor or a very hot summer.
- Read the Recommended BTU per hour and the nearest common unit sizes, then compare them with the BTU rating on the box.
How the BTU number is worked out
The base figure comes from the room air conditioner sizing chart on the ENERGY STAR room air conditioners page. It lists the capacity needed for each floor area, for example 6,000 BTU per hour for 150 up to 250 sq ft. ENERGY STAR then says to reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny one, add 600 BTU for each person beyond two, and add 4,000 BTU for a kitchen. Sizing charts like this typically assume 8 ft ceilings, so for taller rooms this tool multiplies the base by ceiling height divided by 8. That scaling, like the insulation and climate choices, is our own rule of thumb and not part of the ENERGY STAR chart. One ton of cooling is 12,000 BTU per hour.
Worked example with the default inputs
A 15 x 12 ft room is 180 sq ft, which falls in the 150 up to 250 sq ft row, so the base is 6,000 BTU. With 8 ft ceilings, normal sun, two people and no kitchen, nothing is added or removed. The tool shows Recommended: 6,000 BTU per hour, about 0.50 tons, which matches a 6,000 BTU unit. Turn the same room into a sunny kitchen with four people and you get 6,000 x 1.10 = 6,600, plus 1,200 for the two extra people, plus 4,000 for the kitchen, or 11,800 BTU per hour. The nearest sizes are 10,000 and 12,000.
Limits and edge cases
- This sizes cooling only. A heater's output depends on winter temperatures and insulation, so use the heater maker's guidance or a professional load calculation.
- The chart covers 100 to 2,500 sq ft. Smaller rooms use the smallest common unit, and larger spaces need an HVAC contractor.
- For an open floor plan, enter the combined area the unit has to cool.
- Portable units list a federal SACC rating, which is lower than the older ASHRAE number on many boxes. Compare this result with the SACC figure.
- Turn square meters into square feet with the area converter, change BTU per hour to watts with the power converter, and estimate running cost with the electricity cost calculator.
Frequently asked questions
What size air conditioner do I need for a 300 square foot room?
ENERGY STAR lists 8,000 BTU per hour for 300 up to 350 square feet. A 15 by 20 foot room is 300 square feet, so it needs about 8,000 BTU before any sun, people or kitchen adjustments.
How many BTUs per square foot do I need?
The ENERGY STAR chart works out to about 22 to 28 BTU per square foot for rooms between 250 and 550 square feet. Small rooms need more per square foot and large spaces need less, so a flat rule can miss.
Is a bigger air conditioner better?
No. ENERGY STAR notes that bigger is not always better. An oversized unit cools the air fast and shuts off before it has pulled much humidity out of the room.
How many BTUs is one ton of air conditioning?
One ton of cooling is 12,000 BTU per hour. A 12,000 BTU unit is a one-ton air conditioner, and the tool shows tons next to the BTU figure.
Sources: ENERGY STAR, Room Air Conditioners; U.S. Department of Energy, Test Procedures for Portable Air Conditioners (2016).