About this tool
BTU / AC & Heater Sizing Calculator is a free, in-browser tool that estimates the cooling and heating capacity a room needs, in BTU per hour, and translates the cooling load into air-conditioner tons. All figures are computed locally as you type — nothing is uploaded.
Cooling starts from a base of 20 BTU per square foot, then is scaled by ceiling height ÷ 8 for taller or shorter rooms. Each occupant beyond two adds 600 BTU, the result is adjusted for sun exposure (+10% very sunny, 0 average, −10% heavily shaded), and a kitchen adds 4,000 BTU for appliance heat. Cooling tons are the cooling BTU ÷ 12,000, and the recommended AC size rounds that up to the next half-ton. Heating is estimated separately as area × a climate factor in BTU/ft² (30 hot, 40 moderate, 50 cold, 60 very cold).
Use it to right-size a window unit, mini-split or furnace: an oversized unit short-cycles and an undersized one runs constantly, so match the tonnage to the estimate and adjust for your local climate and insulation.
Frequently asked questions
How is the cooling BTU calculated?
Base load is area × 20 BTU/ft², scaled by ceiling height ÷ 8, plus 600 BTU for each occupant over two, then multiplied by the sun-exposure factor and, for kitchens, plus 4,000 BTU. This mirrors the widely used rule-of-thumb HVAC sizing method.
How do BTU relate to AC tons?
One ton of air conditioning equals 12,000 BTU/hr, so cooling tons are cooling BTU ÷ 12,000. The recommended size rounds up to the next half-ton because units are sold in standard 0.5-ton increments.
Why is heating figured separately from cooling?
Heating demand depends heavily on climate, so it uses area × a BTU-per-square-foot factor you pick by climate band (30 to 60). Cooling depends more on internal gains — people, sun and appliances — which the cooling formula adds explicitly.
Is this a substitute for a Manual J load calculation?
No. It is a quick estimate; a professional Manual J accounts for insulation, windows, orientation and air sealing. Use this for planning, then confirm with an HVAC contractor. All math stays in your browser.
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