BTU Calculator

Start by choosing your mode: cooling if you are sizing an air conditioner, or heating if you are sizing a heater. The mode changes the base rate used, since heating a room generally needs more capacity per square foot than cooling it.

Enter the room length and width in feet. The calculator multiplies these to get the floor area, then applies the base rate. Set the ceiling height as well: most rooms are 8 feet, and anything taller increases the volume of air you have to condition.

Adjust the room conditions to match reality. In cooling mode, pick the sun exposure: a south-facing room in full sun runs hotter than a shaded north-facing one. In heating mode, pick the insulation quality, because a draughty, poorly insulated room loses heat faster. Choose the room type (a kitchen adds appliance heat) and the number of regular occupants, since each person gives off body heat.

The running cost fields are optional. Enter your electricity rate in pence per kilowatt-hour and the hours per day you expect the unit to run, and the calculator estimates the daily and monthly cost in your chosen currency. Everything updates live as you type, so there is no submit button to press.

The starting point is floor area times a base rate. This calculator uses the widely quoted rules of thumb of 20 BTU per square foot for cooling and 25 BTU per square foot for heating. For a 12 by 15 foot room that is 180 square feet, giving a base of 3,600 BTU/hr for cooling.

That base is then adjusted. Ceiling height scales the figure against an 8 foot standard, so a 10 foot ceiling multiplies the load by 1.25. In cooling mode a very sunny room adds about 10 percent and a shaded room subtracts about 10 percent. In heating mode, poor insulation adds about 20 percent while good insulation subtracts about 10 percent.

Two additive loads come next. A kitchen adds roughly 4,000 BTU/hr for cooking and appliance heat, and every occupant beyond the first two adds about 600 BTU/hr. The final figure is rounded to a sensible step so it lines up with real product capacities. The result is also converted to kilowatts (multiply BTU/hr by 0.000293) and, for cooling, to tons (divide BTU/hr by 12,000).

The headline figure is the recommended capacity in BTU/hr. When you shop for a unit, match this number as closely as you can rather than rounding far above it. Oversizing leads to short-cycling, where the unit blasts on and off, leaving humidity and uneven temperatures behind while burning extra energy.

The kilowatt figure is the same capacity in metric units, useful when comparing heat pumps and electric heaters sold in kW. For air conditioners, the cooling tons value helps when products are listed by tonnage rather than BTU.

The running cost is an indicative estimate based on the capacity, your run time, and your electricity rate. Real consumption depends on the efficiency rating of the unit, outdoor weather, and how you set the thermostat, so treat it as a guide for comparing options rather than an exact bill. A more efficient unit with the same capacity will cost less to run.