Air Conditioner Size (BTU) Calculator
Find the right cooling capacity for a room in BTU and tons, adjusted for ceiling height, sun exposure, occupancy and kitchen heat.
How it works
BTU = area × 20 × (ceiling ÷ 8) × sun factor · + 600 per person over 2 · + 4,000 if a kitchen · Tons = BTU ÷ 12,000
Cooling capacity starts from a baseline of about 20 BTU per hour for each square foot of floor at a standard 8-foot ceiling, the rule of thumb in ENERGY STAR sizing guidance. Taller ceilings mean more air volume, sunnier rooms gain heat through windows, and extra bodies and kitchen appliances each add load, so the base is nudged up or down for those. One "ton" of cooling equals 12,000 BTU/hr, the unit central systems are sold in.
Worked example
A sunny 300 sq ft living room with 8-foot ceilings and two occupants: 300 × 20 = 6,000 BTU, × 1.10 for sun = 6,600, no occupancy or kitchen additions. Rounded to 6,500 BTU/hr, that is about 0.54 tons — a typical mid-size window or portable unit.
Frequently asked questions
What happens if I oversize the AC?
An oversized unit cools the air fast but short-cycles before it can remove humidity, leaving the room cold and clammy and wearing the compressor. Right-sizing keeps runs longer, humidity lower and comfort more even.
How do BTU and tons relate?
One ton of air conditioning equals 12,000 BTU per hour, a legacy of the heat needed to melt a ton of ice in a day. Window units are rated in BTU; central and mini-split systems are usually sold in tons.
Is this a substitute for a Manual J load calculation?
No. This is a quick rule-of-thumb estimate for a single room. Whole-home or high-performance installs should use a professional Manual J calculation that accounts for insulation, window types, orientation and local climate.
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This calculator is for educational purposes. Double-check important results before acting on them.