BTU calculator: what size air conditioner do you need?
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This BTU calculator tells you what size air conditioner your room actually needs — in BTU/h and watts, with the DOE vs ASHRAE translation portable-AC boxes hide. Fill the form; the math is printed below it.
Formula: area × 20 BTU/ft² → +1,000 BTU per ft of ceiling above 8 ft → +600 BTU per person beyond 2 → +4,000 BTU for kitchens → ±10% sun/shade → +15% poor insulation / −10% good. Watts = BTU ÷ 3.412. Estimates only — not a Manual J load calculation.
How the calculation works
Cooling load starts with floor area: the industry baseline is 20 BTU/h per square foot. Height matters because you cool a volume, so we add 1,000 BTU per foot of ceiling above 8 ft. Bodies are ~600 BTU/h heaters each, so occupants beyond two add that. Kitchens carry a flat +4,000 BTU for appliance heat. Sun-baked rooms need ~10% more; shaded ones ~10% less; leaky insulation up to 15% more. The result is expressed in BTU/h and watts (÷3.412).
The DOE vs ASHRAE trap
Portable AC boxes print two numbers. The big one (ASHRAE) predates realistic portable testing; the honest one (DOE / SACC) is usually 20–40% lower and is what should match this calculator’s output. A “14,000 BTU” ASHRAE portable is often an ~8,000–10,000 BTU DOE machine. Our result box shows both so the store label cannot fool you.
What to do with your number
Under ~4,000 BTU of true need usually means the room is small or the real job is cooling you — the under-$300 guide covers both lanes. Mid-range results fit the bedroom picks; large, hot rooms belong with the dual-hose class.
Frequently asked questions
How does this BTU calculator work?
The BTU calculator starts at 20 BTU/h per square foot, adds 1,000 BTU per foot of ceiling above 8 ft, 600 BTU per occupant beyond two and 4,000 BTU for kitchens, then adjusts ±10% for sun and up to +15% for poor insulation — the standard industry sizing logic, shown transparently below the form.
What is the difference between ASHRAE and DOE BTU?
Two rating systems on the same box. ASHRAE is the older, larger number; DOE (SACC) reflects how portable units perform in real rooms and typically lands 20–40% lower. Compare DOE to DOE — our decoder article goes deeper.
What happens if I oversize the AC?
It cools fast, shuts off before dehumidifying, and the room turns clammy — the short-cycling trap. Undersizing means all-day noise and a room that never arrives. Sizing to the calculation beats “bigger is safer”.
Do personal coolers use BTU ratings?
Rarely, and evaporative units cannot honestly use them — their effect depends on humidity, not a fixed capacity. If your result below is small, the personal-cooler lane may fit; the result box routes you.
Is this calculator exact?
It is a solid estimate, not a Manual J engineering load calculation. Unusual glazing, uninsulated lofts and open-plan spaces justify professional sizing.