❄️BTU Calculator

About this tool

Enter the room's length and width in feet or metres, choose average sun, heavy shade or very sunny, set the number of people who usually use the room and tick if it is a kitchen. You get the recommended cooling capacity in BTU per hour, the area, and the equivalent in tons and kilowatts.

How AC sizing works

Cooling capacity is measured in British thermal units per hour; one ton of cooling is 12,000 BTU/h and one kilowatt about 3,412 BTU/h. The base figure comes from the ENERGY STAR sizing table for room air conditioners — 5,000 BTU for up to 150 sq ft, 6,000 for 150–250, 8,000 for 300–350, 12,000 for 450–550, rising to 34,000 for 2,500 sq ft — and the same guidance's adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU for each regular occupant beyond two, and 4,000 more for a kitchen. Bigger is not better: an oversized unit cools the air in short bursts without running long enough to remove humidity, leaving the room cold and clammy, while an undersized one runs constantly. High ceilings, poor insulation, large west-facing windows and hot climates push the need up, and whole-house systems or heat pumps should be sized with a proper load calculation by an installer. The electricity cost calculator estimates running cost, the area calculator handles irregular rooms, the temperature converter switches °C and °F, and the unit converter covers other units.

Frequently asked questions