BTU Calculator
Find how many BTUs you need to cool a room — enter the size, ceiling height, sun exposure and climate to get the cooling BTUs and a recommended air conditioner size in BTU and tons.
This is a quick rule-of-thumb estimate to get you in the right size range. For a permanent installation, a contractor’s Manual J load calculation is the accurate way to size equipment.
How many BTUs do I need to cool a room?
The starting point for cooling is roughly 20–25 BTUs per square foot of floor area. From there you adjust for the things that change the heat load: tall ceilings, strong sun, a hot climate, the number of people, and kitchen appliances. The calculator above applies these factors and rounds the result up to the nearest standard air-conditioner size.
BTU ≈ area × 22 × (ceiling ÷ 8) × sun × climate + people + kitchen
AC size by room area
| Room area | Cooling BTUs | Tons |
|---|---|---|
| 150 sq ft | 5,000–6,000 | 0.5 |
| 300 sq ft | 7,000–8,000 | 0.6 |
| 500 sq ft | 10,000–12,000 | 1.0 |
| 700 sq ft | 14,000–18,000 | 1.5 |
| 1,000 sq ft | 18,000–24,000 | 2.0 |
Ranges for average conditions — sun, high ceilings and hot climates push toward the top.
Looking at a ductless system? The mini split sizing calculator rounds to standard mini-split sizes. New to the unit? See what a BTU is and convert to kW or tons.
Frequently asked questions
How many BTUs do I need to cool a room?
As a rule of thumb, plan on about 20–25 BTUs per square foot of room area for cooling, then adjust for ceiling height, sun, climate, occupants and any kitchen heat. A 300 sq ft room needs roughly 6,000–7,500 BTUs. The calculator above applies these adjustments and rounds to a standard unit size.
What size air conditioner do I need for a 12×12 room?
A 12×12 room is 144 square feet, which needs about 5,000–6,000 BTUs to cool under average conditions. A sunny room, high ceilings or a hot climate push it toward the higher end. Enter your details above for a tailored size.
How many BTUs are in a ton of cooling?
One ton of air-conditioning equals 12,000 BTUs per hour. So a 24,000 BTU system is 2 tons, and 36,000 BTUs is 3 tons. The calculator shows both the BTU figure and the equivalent in tons.
Is it bad to oversize an air conditioner?
Yes. An oversized AC cools the air quickly but short-cycles — turning on and off before it removes humidity — which leaves the room cold and clammy and wears the equipment. Right-sizing matters more than simply buying the biggest unit, which is why the adjustments in this calculator are worth getting right.