October 7, 2026 · Climate & Energy · 5 min read

Cooling Load Basics: How Many BTUs Does Your Room Need?

Buy an air conditioner that's too small and it will run forever without cooling the room. Buy one that's too big and it will cool fast, switch off, leave the air clammy, and waste electricity. Getting the size right — the cooling load, measured in BTU per hour — is worth the ten minutes it takes to estimate properly.

The rule of thumb

A quick starting point for a moderately insulated room in a warm climate is about 20 BTU/h per square foot of floor area — roughly 215 BTU/h per square meter.

• 120 sq ft bedroom → ~2,400 BTU/h
• 250 sq ft living room → ~5,000 BTU/h
• 400 sq ft open space → ~8,000 BTU/h

Air conditioners are often labeled in tons: one ton equals 12,000 BTU/h, so a 1-ton unit suits a room of roughly 550–600 sq ft under average conditions.

The adjustments that actually matter

• Sun exposure: add ~10% for a room that gets strong direct sun, subtract ~10% for a heavily shaded room.
• Occupants: each person beyond the first adds roughly 600 BTU/h.
• Kitchens: cooking heat is significant — add about 4,000 BTU/h for a room containing a kitchen.
• Ceiling height: the rule of thumb assumes a standard ~8 ft (2.4 m) ceiling. Tall ceilings hold more air volume, so scale up proportionally.
• Insulation: poor insulation or old single-glazed windows push the number up; excellent insulation brings it down.
• Climate: very hot or humid regions need more capacity than mild ones.

Why oversizing is a real problem

An oversized unit reaches the thermostat setpoint quickly, then shuts off before it has dehumidified the air. The result is a cold-but-clammy room, short cycling that wears the compressor, and a higher bill. Aim for the calculated load — or slightly above — not "as big as I can afford."

Humidity is the hidden half of comfort — the Wet Bulb Calculator shows how much moisture your air actually carries, which is exactly what an oversized unit fails to remove.

From BTU to electricity

Cooling capacity in BTU/h is not the same as the power the unit draws. Efficiency (EER or SEER) tells you how much cooling you get per watt. Two units with the same BTU rating can differ by 20–30% in running cost, so check the efficiency label too.

Comparing a heat pump instead of a dedicated air conditioner? The Heat Pump Calculator sizes both the heating and the cooling side in one place.

Related reading: Solar Panel Sizing Explained shows how to pair a high-consumption appliance like an air conditioner with a solar array and battery bank.