🌡 BTU Calculator
Estimate the heating or cooling output a room needs. The base figure comes from the volume, then it is adjusted for insulation, climate, sun exposure, windows and how many people use the room.
Sizing heating and cooling for a room
A BTU, British thermal unit, is the energy needed to raise one pound of water by one degree Fahrenheit. Heating and cooling equipment is rated in BTUs per hour, which is a rate rather than a quantity. The question this answers is how much of that rate a given room needs.
1 kW = 3,412 BTU/h 1 ton of cooling = 12,000 BTU/h
The "ton" in air conditioning is a historical unit: the cooling a ton of ice provides melting over 24 hours. A 3-ton unit is 36,000 BTU/h, and nothing about it weighs three tons.
The base figure
Rough rules of thumb based on volume: around 140 BTU per cubic metre for cooling and 185 for heating in a temperate climate. A 5 by 4 metre room with a 2.5 m ceiling is 50 m³, so about 7,000 BTU/h of cooling or 9,250 of heating before adjustments.
| Room area | Approximate BTU/h | Equivalent kW |
|---|---|---|
| 10 m² (110 ft²) | 3,500 | 1.0 |
| 15 m² (160 ft²) | 5,250 | 1.5 |
| 20 m² (215 ft²) | 7,000 | 2.1 |
| 30 m² (325 ft²) | 10,500 | 3.1 |
| 45 m² (485 ft²) | 15,750 | 4.6 |
| 60 m² (645 ft²) | 21,000 | 6.2 |
The adjustments that matter
- Insulation. A well-insulated modern room needs about 15% less; an old draughty one with single glazing needs 25% more. This is the largest single factor.
- Sun exposure. A south-facing room with large windows needs about 10% more cooling; a shaded north-facing one about 10% less.
- Occupancy. Each person beyond the first two adds roughly 600 BTU of cooling load. A meeting room for twelve is a very different calculation from a bedroom.
- Kitchens. Add about 4,000 BTU for the cooking appliances.
- Ceiling height. Already handled if you calculate by volume, which is why volume beats floor area for anything other than a standard ceiling.
Why oversizing is a real mistake
With heating, an oversized boiler mostly wastes money. With cooling it actively performs worse. An air conditioner removes humidity as a side effect of running; an oversized unit reaches the set temperature quickly, shuts off, and never runs long enough to dehumidify. The result is a room that is cold and clammy, with the unit short-cycling, which wears the compressor and uses more electricity than a correctly sized one.
Match the calculated figure rather than rounding generously upward. If you are between two unit sizes, the smaller one running longer usually gives better comfort than the larger one cycling.
Heating is a different calculation
This gives a per-room estimate suitable for choosing a radiator or an electric heater. Sizing a whole-house boiler or heat pump properly requires a heat loss survey, which calculates the loss through each wall, window, floor and roof from their construction and area, plus air changes per hour. Volume-based rules of thumb routinely oversize boilers by 50% or more, which is why so many houses have a boiler that short-cycles all winter.
Useful conversions
| From | To | Multiply by |
|---|---|---|
| BTU/h | Watts | 0.293 |
| BTU/h | Kilowatts | 0.000293 |
| Kilowatts | BTU/h | 3,412 |
| Tons of cooling | BTU/h | 12,000 |
| Horsepower (HVAC, Asia) | BTU/h | about 9,000 |
A worked example
A 5 by 4 m living room with a 2.5 m ceiling, average insulation, normal sun, two occupants, needing cooling. Volume is 50 m³, base load 50 × 140 = 7,000 BTU/h. No adjustments apply, so the requirement stays 7,000 BTU/h, which is 2.05 kW. A 7,000 BTU unit is the right choice; a 12,000 BTU unit would cool the room faster and leave it damp.
Two neighbours worth knowing: the square footage calculator, for the question this one leads into, and the volume calculator, which answers it from the other direction.
Frequently Asked Questions (FAQs)
How many BTUs do I need for my room?
Roughly 140 BTU per cubic metre for cooling and 185 for heating, then adjust for insulation, sun exposure and occupancy. A 20 m² room with a standard ceiling needs about 7,000 BTU/h of cooling, which is a little over 2 kW.
What happens if the air conditioner is too big?
It cools quickly, shuts off, and never runs long enough to remove humidity, so the room feels cold and clammy. The short cycling also wears the compressor and uses more electricity than a correctly sized unit running steadily.
How do I convert BTU to kilowatts?
Divide by 3,412. A 9,000 BTU/h unit is 2.64 kW of output. Going the other way, multiply kilowatts by 3,412. Note that this is output, not electrical consumption, which is lower for a heat pump or air conditioner.
What is a ton of air conditioning?
Twelve thousand BTU per hour, from the historical rate at which a ton of ice absorbs heat as it melts over a day. A 3-ton unit delivers 36,000 BTU/h. It describes cooling capacity, not weight.
Does ceiling height matter?
Considerably, which is why calculating by volume beats calculating by floor area. A room with a 3.5 m ceiling needs 40% more than the same floor area at 2.5 m, and rules of thumb quoted per square metre quietly assume a standard height.
Is this enough to size a boiler?
No. A whole-house heating system needs a proper heat loss survey, accounting for wall and window construction, floor and roof losses and air changes per hour. Volume rules of thumb routinely oversize boilers by half, which causes short cycling and wastes fuel.