BTU calculator

What a BTU is

A British Thermal Unit is the energy needed to raise one pound of water by one degree Fahrenheit. Heating and cooling equipment is rated in BTU per hour, which is a rate of energy transfer — a power, not a quantity, despite the name.

One kilowatt is about 3,412 BTU/h, and one "ton" of air conditioning is 12,000 BTU/h. That last unit comes from the cooling power of a ton of ice melting over 24 hours, which is a genuinely nineteenth-century way to describe an air conditioner.

Bigger is not better

This is the single most important thing about sizing an air conditioner. An oversized unit cools the air to the thermostat setting very quickly and then shuts off — a pattern called short cycling.

The problem is that dehumidification takes time. Moisture condenses on the evaporator coil only while the unit runs, so a system that runs in short bursts removes far less water from the air. The result is a room that is cold and clammy rather than cool and comfortable. Short cycling also wears the compressor faster and uses more energy than steady running.

An undersized unit runs continuously and never quite reaches the target on the hottest days, which is uncomfortable but less damaging. If you must err, err small for cooling.

The factors that matter

• Floor area — the base of the estimate, roughly 20 BTU/h per square foot for cooling and 30 for heating in a temperate climate.
• Ceiling height — you heat and cool volume, not floor. A 3 m ceiling adds 25% over a standard 2.4 m one.
• Insulation — the largest single variable. A well-sealed modern room can need a third less than a draughty solid-walled one of the same size.
• Glazing and orientation — a large south-facing window is a substantial heat gain in summer and a heat loss in winter.
• Occupancy — each person contributes roughly 100 W of body heat, so a room used by six needs meaningfully more cooling than one used by two.
• Kitchens — cooking appliances add several thousand BTU/h of cooling load.

Where this estimate stops

This is a rule-of-thumb sizing guide, of the kind used to choose a portable air conditioner or a single radiator. It is not a heat loss calculation.

A proper survey — Manual J in the US, or an EN 12831 calculation in Europe — works through every wall, window, floor and ceiling separately, using their measured U-values, the local design temperature, and air change rates. For a whole-house heating system, a heat pump, or anything you are paying to install, that survey is worth its cost. Undersizing a heat pump makes it run on expensive backup heat; oversizing wastes capital and cycles the compressor.

Common questions