AC Cooling Load Calculator (BTU by Room)
Estimate the cooling BTU/hr a room needs — by the BTU-per-square-foot rule or the ENERGY STAR room-size chart — from its area or length × width, ceiling height, occupants, sun, insulation, climate and kitchen use, with tons, kW, the unit size to buy, power draw and running cost.
How it works
Rule: BTU/hr = area × 25 (base rate) × (ceiling ÷ 8 ft) × sun × insulation × climate + 600 per person beyond two + 4,000 if kitchen · ENERGY STAR: chart BTU/hr for the area (e.g. 300–350 sq ft → 8,000) × the same factors + the same additions · sun 0.9 / 1 / 1.1 · insulation 0.9 / 1 / 1.15 · climate 0.9 / 1 / 1.2 · tons = BTU/hr ÷ 12,000 · kW = BTU/hr ÷ 3,412 · power draw (W) = unit BTU/hr ÷ EER
A room-by-room cooling load starts from the floor area. The quick rule uses about 25 BTU/hr per square foot for an average room — adjustable in More options — while the ENERGY STAR room air conditioner chart steps up by size band, from 5,000 BTU/hr for 100–150 sq ft to 34,000 for 2,000–2,500 sq ft; it is a little more generous for small rooms and less for large ones. Both are then adjusted for the things that change the load: taller ceilings hold more air to cool, heavy shade takes off about 10% and strong sun adds 10% (the ENERGY STAR guidance), poor insulation and old windows add heat gain, a hot climate pushes the load up, each occupant beyond two adds about 600 BTU/hr and a kitchen’s appliances add 4,000 BTU/hr. The calculator then picks the next standard unit size for a mini-split/central system or a window/portable unit, and from the unit’s EER (BTU per watt-hour) estimates its power draw, seasonal energy use and, if you enter your electricity price, the running cost. A full ACCA Manual J calculation also weighs window area and orientation, wall construction, duct losses and your local design temperatures.
Worked example
A 300 ft² sunny living room with 8 ft ceilings and 2 people: 300 × 25 = 7,500 BTU/hr, ×1.0 for ceiling height, ×1.1 for sun = 8,250 BTU/hr (0.69 tons). The nearest common mini-split is 9,000 BTU/hr (an 8,000 or 10,000 BTU/hr window unit). By the ENERGY STAR chart, 300 sq ft is 7,000 BTU/hr, ×1.1 for sun = 7,700 BTU/hr. A 9,000 BTU/hr unit with an EER of 11 draws about 820 W; at 8 hours a day for 120 days that is about 785 kWh a season.
Sources: ENERGY STAR — Room air conditioner sizing chart · US Department of Energy — Room air conditioners · ACCA — Manual J residential load calculation
Frequently asked questions
How many BTU do I need per square foot?
About 20–25 BTU/hr per square foot for a typical room is a good starting point. Very sunny, poorly insulated or high-ceilinged rooms push toward 30+, while shaded, well-insulated rooms can sit near 20. The ENERGY STAR chart works out to roughly 33 BTU/hr per sq ft for a 150 sq ft bedroom and 17 for a 1,000 sq ft open area.
Should I use the rule of thumb or the ENERGY STAR chart?
For a single room with a window or portable unit, the ENERGY STAR chart is the standard reference and what most retailers use. The per-square-foot rule is easier to adjust and suits mini-split and open-plan spaces. The chart in the results shows both across room sizes so you can see where they differ.
Is bigger always better for an AC unit?
No. An oversized unit cools the air quickly but short-cycles before it can remove humidity, leaving the room cold and clammy and wearing out the compressor. Sizing close to the calculated load gives longer, more efficient runs and better dehumidification.
What is a “ton” of cooling?
One ton equals 12,000 BTU/hr — historically the cooling once produced by melting a ton of ice in 24 hours. A 24,000 BTU/hr unit is a “2-ton” system.
How much electricity does an air conditioner use?
Divide its BTU/hr rating by its EER (or CEER) to get watts: a 9,000 BTU/hr unit with an EER of 11 draws about 820 W, or roughly 6.5 kWh in 8 hours of compressor running. Enter your price per kWh in More options to see the cost per hour and per season.
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This calculator is for educational purposes. Double-check important results before acting on them.