BTU Calculator
Size an air conditioner properly — BTUs and tonnage from your room's square footage, adjusted for ceilings, insulation, sun, and how many people share the space.
Cooling needed
8,000
BTU/hr
AC tonnage
0.67
12,000 BTU = 1 ton
Buy (standard size)
0.75 ton
Rule-of-thumb sizing (~20 BTU per sq ft) for window units and rough planning. Whole-home systems should get a contractor's Manual J load calculation — climate zone, windows, and ductwork shift the real number. Oversizing cools fast but short-cycles and leaves the air clammy; closest-size-up is the right call only when the estimate is honest.
How AC Sizing Works
A BTU (British thermal unit) is the heat needed to raise a pound of water 1°F; an air conditioner's rating is how many BTUs of heat it can move out per hour. The ~20-BTU-per-square-foot rule covers a typical room, with adjustments for everything that adds heat — sun, bodies, cooking — and bigger isn't better: an oversized unit cools the air before dehumidifying it, then shuts off, leaving the room cold but clammy.
Quick sizing reference
BTU ≈ sq ft × 20 × (ceiling/8) × insulation × sun
+ 600 per person beyond two + 4,000 for kitchens
150–250 sq ft → ~5,000–6,000 BTU window unit
340–450 sq ft → ~8,000–10,000 BTU
550–700 sq ft → ~12,000–14,000 BTU (≈1 ton+)
1 ton of cooling = 12,000 BTU/hrFrequently Asked Questions
How many BTUs do I need for a 12×12 bedroom?
144 sq ft × 20 BTU ≈ 2,900 BTU on paper, but the smallest window units sold are 5,000 BTU — which is the right buy for any bedroom up to about 150 sq ft. The rule-of-thumb numbers matter more as rooms grow: a 350 sq ft living room genuinely needs the 8,000 BTU class.
What does a 'ton' of air conditioning mean?
12,000 BTU per hour — the heat absorbed by melting one ton of ice over 24 hours, a unit left over from when buildings literally cooled with ice deliveries. A typical US home's central AC runs 2–5 tons; the number on the unit is capacity, not weight.
Should I round up and oversize my AC to be safe?
Round up to the next standard size, but never more — an oversized unit hits the thermostat target before it's dehumidified the air, then shuts off. The result is a cold, clammy room, more on/off cycling wear, and worse efficiency. Right-sized units run longer, steadier, and drier.
Do the same BTU numbers work for heating?
No — heating loads depend heavily on climate and run higher: roughly 30–60 BTU per sq ft from the warm South to the cold North, versus ~20 for cooling. Furnaces also list input BTUs with an efficiency rating, so a 80,000-BTU furnace at 95% delivers 76,000 to the house.