AC Sizing Calculator: BTU and Tons for Your Home
A fast rule-of-thumb estimate of the cooling capacity in BTU/hr and tons your home needs. This is a starting point for a conversation with a contractor, not a replacement for an ACCA Manual J load calculation.
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How the estimate works
Base load = area × BTU per sq ft for the climate (20 to 35). That is scaled by ceiling height (height ÷ 8), insulation and sun factors, then 600 BTU per person above two and 4,000 BTU for an open kitchen are added. The total is divided by 12,000 BTU per ton and rounded to the nearest half ton.
Oversizing is as harmful as undersizing: a unit that is too big short-cycles, removes less moisture and wears out sooner. A Manual J calculation accounts for windows by direction, duct location, infiltration and local design temperatures.
Worked example
1,800 sq ft, moderate climate, 8 ft ceilings, three people, average insulation: 1,800 × 25 = 45,000, plus 600 for the third person, is 45,600 BTU/hr, or about 3.8 tons. Rounded, a 4 ton system.
Frequently asked questions
Is this the same as a Manual J?
No. Manual J is the industry standard load calculation and is more accurate. This is a quick sanity check.
Is bigger better?
No. An oversized system short-cycles and dehumidifies poorly. Aim for a unit that matches the calculated load.
How many square feet does a ton cool?
Roughly 400 to 600 sq ft per ton depending on climate and construction.
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This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.