BTU Calculator

What a BTU actually is

One British thermal unit is the energy needed to raise a pound of water by one degree Fahrenheit — about 1055 joules. Air conditioners are rated in BTU per hour, which is a rate of heat removal, and the "per hour" is almost always dropped in advertising. A ton of cooling is 12,000 BTU/h, a unit that dates from the rate at which a ton of ice melts over a day. One kilowatt of cooling is about 3412 BTU/h.

Why bigger is not better

An air conditioner does two jobs: it lowers temperature and it removes moisture. Moisture removal only happens while the coil is cold and the fan is running, which means it needs run time. An oversized unit satisfies the thermostat in a few minutes, shuts off, and never runs long enough to dehumidify — so the room reads 22 °C and still feels damp and unpleasant. Short cycling also wears the compressor and uses more energy than steady operation. Matching the unit to the load, rather than buying the biggest one that fits the window, is the whole point of a sizing calculation.

Where the numbers come from

The baseline is the widely used guidance of roughly 20 BTU/h per square foot of floor area for a room with an eight-foot ceiling. Everything after that is an adjustment: higher ceilings add volume, poor insulation lets heat in faster, a sun-facing room gains solar heat through glass, each person beyond two adds about 600 BTU/h of body heat, a kitchen adds roughly 4,000 BTU/h for the cooker, and equipment adds its own electrical load almost entirely as heat. These are rules of thumb that get you to the right unit size on a shop shelf.

When a rule of thumb is not enough

For a whole house, a new build, or anything where you are committing to ducted equipment, a proper load calculation — Manual J in North America, or the equivalent local method — is the right tool. It accounts for the orientation and U-value of every window, the construction of each wall, infiltration rates, the local design temperatures and the internal gains room by room. A rule of thumb cannot know that your west-facing glazing is single-pane, and the error from that one fact can exceed the entire adjustment range here.

Heating is a different calculation

The heating figure above is a rough companion number, not a furnace specification. Heating load depends on the difference between indoor and outdoor design temperature, which varies enormously by climate — the same room might need half the cooling figure in a mild maritime climate and three times it somewhere with hard winters. Use it to get an order of magnitude, then size real heating equipment from local design temperatures.

Frequently Asked Questions