BTU Calculator — What Size AC Do I Need?

Enter your room size and a few details about sun, people, and use — and get the air conditioner size that actually fits, in BTU, tons, and kW.

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Standard is 8 ft (2.4 m). Taller rooms need more cooling.

Recommended cooling capacity

0 BTU

buy the nearest standard size

Tons: 0 Cooling power: 0 kW Room area: 0 sq ft

Rule-of-thumb sizing for a single room. For whole-house systems, poorly insulated rooms, or extreme climates, get a Manual J load calculation from an HVAC professional.

Now check what it costs to run

AC / Heater Running Cost → What this AC will add to your monthly bill. Watts to Amps Check the circuit can handle the AC's draw. Heat Pump Savings Heating too? See if a heat pump pays off.

How this BTU calculator works

Air conditioner capacity is measured in BTU per hour — the amount of heat the unit can remove. The classic sizing rule is 20 BTU per square foot of floor area for a room with 8 ft ceilings. This calculator starts there, then adjusts for the real world:

  • Ceiling height — the estimate scales by height ÷ 8 ft, since taller rooms hold more air.
  • Sun exposure — very sunny rooms need about 10% more; shaded rooms about 10% less.
  • People — each person beyond two adds roughly 600 BTU of body heat.
  • Kitchens — cooking appliances add about 4,000 BTU of heat load.

The result is rounded up to the nearest standard unit size (5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 14,000 / 18,000 / 24,000 BTU and up), because a slightly bigger unit beats one that can't keep up — but avoid going more than one size over.

BTU & AC sizing FAQ

How many BTU do I need per square foot?

About 20 BTU per square foot with standard 8 ft ceilings. A 300 sq ft room needs roughly 6,000 BTU. Then adjust: +10% for very sunny rooms, −10% for shaded rooms, +600 BTU per person beyond two, and +4,000 BTU for kitchens.

What happens if my AC is too big or too small?

Undersized: it runs non-stop and never quite cools. Oversized: it short-cycles — cooling the air fast but shutting off before dehumidifying, leaving the room cold and clammy while wasting electricity and wearing the compressor. Pick the closest standard size.

How many BTU is a ton of cooling?

One ton = 12,000 BTU/h. An 18,000 BTU unit is 1.5 tons; a 3-ton central system is 36,000 BTU/h.

Does ceiling height change the BTU I need?

Yes — the 20 BTU/sq ft rule assumes 8 ft ceilings. Scale by height ÷ 8: a room with 10 ft ceilings needs about 25% more BTU than the same floor area at 8 ft.