Case 1
- Room floor area (square feet)
- 150sq ft
3,000BTU/h
Open with these valuesAir Conditioner BTU Calculator
6,000BTU/h
Result: 6,000 BTU/hTwenty BTU per hour for every square foot of floor: a 300 sq ft room comes out at 6000 BTU/h. Enter square feet, not square metres — one square metre is 10.76 sq ft. It is a rule of thumb for an eight-foot ceiling and average insulation, not a heat-load calculation.
The numbers at a glance
| Room floor area (square feet) (sq ft) | Result (BTU/h) |
|---|---|
| 100.0 | 2,000 |
| 200.0 | 4,000 |
| 300.0Your value | 6,000 |
| 400.0 | 8,000 |
| 500.0 | 10,000 |
| 600.0 | 12,000 |
Worked examples
Case 2
- Room floor area (square feet)
- 300sq ft
6,000BTU/h
Open with these valuesCase 3
- Room floor area (square feet)
- 700sq ft
14,000BTU/h
Open with these valuesHow it's calculated
BTU/h = floor area in sq ft × 20
- StepMeasure the room in feet and multiply length by width for the floor area.
- StepWorking in metres? Multiply the square metres by 10.76 to get square feet.
- StepMultiply the square feet by 20 for the capacity in BTU per hour.
- ResultAdjust after: +10 % for a very sunny room, +600 per extra person, +4000 for a kitchen.
What this number means
Square feet, not square metres
The rule of 20 BTU per hour and the ENERGY STAR sizing chart behind it are both published per square foot. One square metre is 10.76 square feet, so a 28 m² room is about 300 sq ft.
Floor area is only the starting point
The figure assumes an eight-foot ceiling and average insulation, and it is a rule of thumb rather than a heat-load calculation. A taller ceiling, poor insulation or a sun-facing wall all move it.
Treat the result as a floor, not a ceiling
Against the published chart the rule is exact at the top and low at the bottom: 20 × 700 sq ft gives 14000 BTU/h as charted, but a 300 sq ft room is charted at 7000 where this returns 6000. Add 10 % for a very sunny room, 600 BTU/h per person beyond two and 4000 BTU/h for a kitchen.
Commonly misread
A bigger unit will cool the room better.
An oversized unit cools the air before it has removed the humidity, leaving the room cold and clammy and wasting energy. Matching the capacity to the room is the whole point of sizing it.
My room is 300 square metres, so I enter 300.
The field is square feet, not square metres. Multiply your square metres by 10.76 before entering them.
The number the calculator gives is the capacity to buy.
It is the bare area figure, and the published adjustments still go on top. Sun, extra people and a kitchen all add capacity, while heavy shade takes 10 % off.
Reference table
| Floor area (sq ft) | Roughly | Capacity (BTU/h) |
|---|---|---|
| 150 | 13.9 m² | 3000 |
| 250 | 23.2 m² | 5000 |
| 300 | 27.9 m² | 6000 |
| 500 | 46.5 m² | 10000 |
| 700 | 65.0 m² | 14000 |
Questions
How many BTU do I need to cool my room?
Multiply the floor area in square feet by about 20 BTU per hour. A 300 sq ft room comes out at 6000 BTU/h, and you add capacity on top for sun, extra people or a kitchen.
Why does this calculator work in square feet?
Because the rule of thumb and the ENERGY STAR sizing chart behind it are both published in square feet, and room air conditioners are sold against that chart. One square metre is 10.76 square feet, so a 28 m² room is about 300 sq ft.
How close is the rule to the official chart?
Close at the top and low at the bottom. The chart puts 550 to 700 sq ft at 14000 BTU/h, which 20 × 700 hits exactly, but it gives a 300 sq ft room 7000 where this rule returns 6000. Treat the result as a floor, not a ceiling.
What should I add for sun, people or a kitchen?
The published adjustments are plus 10 % for a very sunny room and minus 10 % for a heavily shaded one, plus 600 BTU per hour for each person beyond two, and plus 4000 BTU per hour if the unit cools a kitchen.
Is a bigger unit always better?
No. An oversized unit cools the air before it has removed the humidity, which leaves the room cold and clammy and wastes energy. Matching the capacity to the room is the whole point of sizing it.
Sources and last check
Information, not professional advice.