How BTUs are estimated
Air conditioners are rated in BTUs (British Thermal Units) per hour — the amount of heat they can remove. The baseline rule is about 20 BTU per square foot of floor area, then adjustments are layered on:
BTU = Area × 20 × height factor × sun factor + occupants + kitchen
- Ceiling height: scaled against a standard 8-ft ceiling (taller rooms hold more air to cool).
- Sun: −10% if heavily shaded, +10% if very sunny.
- Occupants: +600 BTU for each person beyond two.
- Kitchen: +4,000 BTU for the heat from cooking appliances.
Matching a unit to the room
| Room size | Approx. BTU |
|---|---|
| Up to 150 sq ft | 5,000 |
| 150–250 sq ft | 6,000 |
| 250–350 sq ft | 8,000 |
| 350–450 sq ft | 10,000 |
| 450–550 sq ft | 12,000 |
Bigger isn't better
An oversized air conditioner cools the air quickly, then shuts off before it has pulled out enough humidity — leaving the room cold but damp and clammy, and wearing the compressor with frequent cycling. Sizing close to the actual load keeps the room comfortable and efficient.
Frequently asked questions
How many BTUs do I need to cool a room?
A common rule of thumb is about 20 BTU per square foot of floor space. A 15 × 15 room (225 sq ft) needs roughly 4,500 BTU as a baseline, then adjustments are added for sunny rooms, high ceilings, extra occupants, and kitchens. This calculator applies those adjustments for you.
What size air conditioner do I need?
Match the BTU rating to your room. As a guide: up to 150 sq ft needs about 5,000 BTU, 250 sq ft about 6,000, 350 sq ft about 8,000, 450 sq ft about 10,000, and 550 sq ft about 12,000 BTU. Oversizing is not better — an oversized unit cools fast but cycles off before removing humidity, leaving the room cold and clammy.
What is a ton of cooling?
In HVAC, one ton of cooling equals 12,000 BTU per hour. So a 24,000 BTU central or mini-split system is a 2-ton unit. The term dates back to the cooling power of a ton of melting ice.
What adjustments affect the BTUs needed?
Add roughly 10% for a very sunny room and subtract 10% for a heavily shaded one, add about 600 BTU for each occupant beyond two, add 4,000 BTU for a kitchen, and scale up for ceilings higher than 8 feet. Climate, insulation, and window quality matter too — for a precise figure, an HVAC pro runs a Manual J load calculation.
Disclaimer: This is a rule-of-thumb estimate for cooling and does not replace a professional Manual J load calculation, which accounts for climate, insulation, windows, and air sealing. Use it as a starting point.