Find the cooling a room needs from its area, ceiling height, sun, people and kitchen, using the ENERGY STAR sizing chart.
A room needs roughly 20 to 25 BTU of cooling per hour for each square foot of floor. A 300 square foot room (28 m²) needs 7,000 BTU/h, a 450 square foot room needs 10,000 BTU/h and a 1,000 square foot space needs 18,000 BTU/h. These figures are the ENERGY STAR room air conditioner sizing chart, which this calculator uses: the chart gives the capacity for the floor area in bands from 100 to 2,500 square feet, and the table under the result lists every band in square feet and square metres. Type the area in either unit and the page converts it with 1 ft² = 0.09290304 m².
ENERGY STAR lists four adjustments, and they are applied in this order. A heavily shaded room takes 10% off the chart value, and a very sunny room adds 10%. When more than two people use the room, each extra person adds 600 BTU/h. A kitchen adds 4,000 BTU/h for the heat from cooking. For a sunny 400 ft² living room used by 4 people that is 9,000 × 1.10 = 9,900, plus 2 × 600 = 1,200, so 11,100 BTU/h, and the next common size up is 12,000. The formula box lists every step for your numbers.
The chart assumes an 8 foot (2.44 m) ceiling. This page scales the chart value by ceiling height divided by 8 ft, so a 10 foot ceiling adds 25%. That scaling is this tool's own extension of the chart, which says nothing about taller rooms, so treat the taller-room figure as a rougher guess. The result is also shown in kilowatts of cooling (1 BTU/h = 0.29307107 W) and in tons of cooling, where 1 ton is 12,000 BTU/h. The line labelled unit size gives the next common size sold, such as 8,000 or 12,000 BTU/h. Choosing one size up is normal; choosing much larger switches the unit on and off too often.
This is a quick estimate for a room air conditioner, not a Manual J load calculation. Manual J, published by ACCA, adds climate, insulation, window area and direction, air leaks and ducts, and it is the method contractors use for central systems and heat pumps. This page does not ask about climate or insulation, and it gives cooling only, not a heating load. In a very hot climate, with poor insulation or large west-facing windows, the true need can be higher. Spaces over 2,500 ft² (232 m²) are outside the chart and are not calculated.
7,000 BTU/h for a standard 8 foot ceiling, 2 people and normal sun, from the ENERGY STAR chart. A 350 ft² room needs 8,000 and a 400 ft² room needs 9,000.
Multiply BTU/h by 0.00029307107. 12,000 BTU/h is 3.52 kW of cooling. That is heat moved, not electricity used: a typical air conditioner draws less electrical power than its cooling capacity.
12,000 BTU/h, which is 3.52 kW. A 1.5 ton unit is 18,000 BTU/h and a 2 ton unit is 24,000 BTU/h.
Yes. ENERGY STAR adds 4,000 BTU/h for a kitchen. A 150 ft² kitchen that would be 5,000 BTU/h otherwise needs 9,000 BTU/h.
No. An oversized unit cools the room quickly and shuts off before it has removed the moisture, which leaves the air cool and clammy. Pick the next common size above the result, for example 8,000 BTU/h when the result is 7,177.
No. The area, height and other values stay in this tab and are not stored or sent. Reloading puts the 25 m² example back. The page only counts that a result was shown.