Calculators

BTU Calculator for Air Conditioners

What size air conditioner a room needs, in BTU per hour and tons, from its area, ceiling height, sun and use.

How it works

ENERGY STAR's room air conditioner chart gives a capacity for each band of floor area, assuming an 8 ft ceiling. Scale it for taller rooms, take off 10% for a heavily shaded room or add 10% for a very sunny one, add 600 BTU/h for each regular occupant beyond two, and add 4,000 BTU/h for a kitchen. The result is rounded up to the next common unit size.

BTU/h = chart(area) × height ÷ 8 × sun + 600 × (people − 2) + kitchen
BTU/h
British thermal units per hour of heat removed
Ton
12,000 BTU/h of cooling
kW
1 kW of cooling is 3,412 BTU/h

Worked example

A 16 × 20 ft living room with an 8 ft ceiling, very sunny, used by three people.

  1. 1Area: 16 × 20 = 320 sq ft; the chart gives 8,000 BTU/h
  2. 2Very sunny: 8,000 × 1.1 = 8,800
  3. 3One person over two: + 600 = 9,400 BTU/h

About 9,400 BTU/h, so a 10,000 BTU/h unit, or 0.78 tons.

Frequently asked questions

Is a bigger air conditioner better?

No. An oversized unit cools the air quickly and switches off before it has removed much humidity, so the room feels cold and damp. It also cycles on and off more, which wears it out and wastes energy. Pick the size that matches the room, rounded up to the next unit.

How many BTU per square foot?

Roughly 20 BTU/h per sq ft for a typical room, which is where ENERGY STAR's chart sits: 6,000 BTU/h for 250 sq ft. Large open areas need a little less per square foot; hot climates, poor insulation and big windows need more.

What does 1.5 ton AC mean?

Tons measure cooling capacity: one ton is 12,000 BTU/h, the heat needed to melt a ton of ice in a day. A 1.5 ton unit gives 18,000 BTU/h, about 5.3 kW, enough for roughly 700–1,000 sq ft by the ENERGY STAR chart.

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