BTU Calculator
The air-conditioner size in BTU per hour and kW for a room, from the ENERGY STAR sizing chart with its sun, people and kitchen adjustments, or a heater size from a rule of thumb you can change.
Result…
How it's calculated
Chart size from floor area (ENERGY STAR, 8 ft ceiling) × ceiling height ÷ 8 ft when higher ± 10% for heavy shade or a very sunny room + 600 BTU/h per person above two + 4,000 BTU/h in a kitchen 1 kW = 3412.14 BTU/h; 1 ton = 12,000 BTU/h
Sizes a room air conditioner the way ENERGY STAR's sizing guide does: find the floor area on its chart (100 ft² to 2,500 ft², based on an 8 ft ceiling), then reduce 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU/h for each regular occupant above two and 4,000 BTU/h for a kitchen. A ceiling higher than 8 ft is scaled up in proportion; that step is this calculator's assumption, not part of the chart. Heater mode uses a watts-per-area rule of thumb that you set. Results are in BTU per hour, kW and tons.
Your numbers, step by step
About the BTU calculator
This BTU calculator sizes a room air conditioner in BTU per hour, kW and tons of cooling. It follows the ENERGY STAR room air conditioner sizing guide: find the room's floor area on its chart, which runs from 100 to 2,500 ft² and assumes an 8 ft ceiling, then take 10% off for a heavily shaded room, add 10% for a very sunny one, add 600 BTU/h for each regular occupant above two and 4,000 BTU/h if the unit is in a kitchen. A ceiling higher than 8 ft is scaled up in proportion, which is this calculator's own assumption rather than part of the chart. Heater mode uses a watts-per-area rule of thumb that you set. Metric rooms are converted to square feet for the chart, and 1 kW = 3412.14 BTU/h. Every adjustment is listed in the steps.
How to use it
- Choose “Room air conditioner” or “Room heater”, and metric or imperial units.
- Enter the “Room length”, “Room width” and “Ceiling height”.
- For cooling, set “Sun”, the “People who usually use the room” and “Is it a kitchen?”.
- For heating, check the “Heater watts per floor area” rule of thumb and change it for your insulation and climate.
- Read the size in BTU/h and kW, then pick the nearest unit at or just above it.
Worked example
A bedroom 5 m × 4 m is 20 m², about 215.3 ft², so the ENERGY STAR chart gives 6,000 BTU/h. The 2.4 m ceiling is under 8 ft, so there is no ceiling change. The room is very sunny, adding 10% (600 BTU/h), and three people use it, adding 600 BTU/h for the third. The total is 7,200 BTU/h, about 2.11 kW or 0.6 tons.
Updated
Questions people ask
How many BTU do I need for a room?
Look up the floor area on the ENERGY STAR chart, then adjust for sun, people and kitchen use. For example, a 400 to 450 ft² room starts at 10,000 BTU/h.
How do I convert BTU per hour to kW?
Divide by 3412.14. A 12,000 BTU/h air conditioner is about 3.52 kW of cooling, which is also 1 ton of refrigeration.
Is a bigger air conditioner better?
No. ENERGY STAR notes that an oversized unit cools the room before it removes the humidity, leaving it damp and clammy, and it uses more energy. Choose the size nearest to the calculated figure.
How is a heater sized in this calculator?
By a rule of thumb: watts per unit of floor area, starting at 10 W per ft² (about 108 W per m²), scaled up for a ceiling over 8 ft. It is a starting point you should change for poor insulation or a cold climate.
When is a BTU calculator not enough?
For a whole home, a central system, or a room above 2,500 ft², the chart does not apply. An HVAC professional's load calculation accounts for insulation, windows and climate, and gives the size to buy.