BTU calculator
Enter the room's size, how sunny it is, how many people use it and whether it is a kitchen. The calculator reads the room air conditioner size off the ENERGY STAR sizing chart and applies its adjustments, in BTU per hour, tons and kilowatts.
BTU calculator
Each regular occupant past two adds 600 BTU/hr.
The chart assumes an 8 ft (2.4 m) ceiling, as ENERGY STAR does. A taller room, a top floor under a hot roof or a poorly insulated room may need more.
Air conditioner size
7,200 BTU/hr
- Tons
- 0.6
- Chart row
- 150–250 ft²
- Size up
- 8k
- Kilowatts
- 2.11 kW
The room from above
Where it sits among room AC sizes
Where the BTUs come from
The method
- Floor area: length × width, in square feet (the chart is in ft²; a metric room is converted, 1 m² = 10.764 ft²).
- Read the chart: ENERGY STAR gives a capacity for each band of area, from 5,000 BTU/hr for 100 up to 150 ft² to 34,000 BTU/hr for 2,000 up to 2,500 ft².
- Sun: take off 10% for a heavily shaded room, add 10% for a very sunny one.
- People: add 600 BTU/hr for each person past two who regularly uses the room.
- Kitchen: add 4,000 BTU/hr if the unit cools a kitchen.
- Pick a unit: the next size up on the chart. Tons are BTU/hr ÷ 12,000; kilowatts are BTU/hr ÷ 3,412.
Worked example
A 14 × 12 ft bedroom is 168 ft², in the chart's 150 up to 250 ft² band: 6,000 BTU/hr. It gets the afternoon sun, so add 10% (600). Three people sleep there, one past two, so add 600 more: 7,200 BTU/hr, 0.6 tons or 2.11 kW. The next size on the chart is an 8,000 BTU/hr unit. The same room as a kitchen would need 11,200 BTU/hr, a 12,000 BTU/hr unit.
In metric, a 4 × 3.5 m room is 14 m², or 150.7 ft²: just inside the 150 ft² band, so 6,000 BTU/hr (1.76 kW) before any adjustment.
| Room | Area | Normal sun | Very sunny | Unit for a sunny room |
|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 5,000 BTU/hr | 5,500 BTU/hr | 6,000 |
| 10 × 12 ft | 120 ft² | 5,000 BTU/hr | 5,500 BTU/hr | 6,000 |
| 12 × 12 ft | 144 ft² | 5,000 BTU/hr | 5,500 BTU/hr | 6,000 |
| 12 × 14 ft | 168 ft² | 6,000 BTU/hr | 6,600 BTU/hr | 7,000 |
| 14 × 16 ft | 224 ft² | 6,000 BTU/hr | 6,600 BTU/hr | 7,000 |
| 16 × 20 ft | 320 ft² | 8,000 BTU/hr | 8,800 BTU/hr | 9,000 |
| 20 × 20 ft | 400 ft² | 10,000 BTU/hr | 11,000 BTU/hr | 12,000 |
| 20 × 25 ft | 500 ft² | 12,000 BTU/hr | 13,200 BTU/hr | 14,000 |
| 24 × 25 ft | 600 ft² | 14,000 BTU/hr | 15,400 BTU/hr | 18,000 |
Why bigger is not better
- An oversized unit leaves the room clammy: in ENERGY STAR's words, it "will cool the room before it has a chance to remove the humidity", and it costs more to buy and run.
- Undersized is the other failure: on the hottest afternoons the unit runs flat out and never reaches the set temperature. If you sit between two sizes, the sun and the people decide it.
- Measure the room you will actually cool. If a doorway or an arch opens onto another room with no door, count both rooms' floor area.
- Placement matters too: keep lamps and TVs away from the unit's thermostat (their heat makes it run longer), and in a corner choose a unit that can throw its air the right way.
Every room in a plan
A floor plan gives you every room's exact area at once. Draw the house in FloorGen, read each room's area off the plan, and size a unit for each room that needs one. How to measure a room covers getting the dimensions right.
Questions
What size air conditioner do I need for a 12 × 12 room?
A 12 × 12 ft room is 144 ft², which the ENERGY STAR chart puts at 5,000 BTU/hr. Add 10% if it is very sunny (5,500, so a 6,000 BTU/hr unit) and 600 BTU/hr for each regular occupant past two.
How many square feet does a 10,000 BTU air conditioner cool?
On the ENERGY STAR chart, 10,000 BTU/hr is the size for 400 up to 450 ft², with an 8 ft ceiling and normal sun. In a very sunny room it covers less.
How many BTU is a ton of air conditioning?
12,000 BTU per hour. A 6,000 BTU/hr window unit is half a ton; a 24,000 BTU/hr unit is two tons.
How do I convert BTU/hr to kilowatts?
Divide by 3,412. A 12,000 BTU/hr unit is about 3.5 kW of cooling. This is cooling output, not the electricity the unit draws, which is much less.
Do high ceilings change the size?
They can. ENERGY STAR's chart assumes an 8 ft ceiling; a room with a much taller ceiling has more air to cool, so lean towards the next size up and check the manufacturer's guidance.