AC size calculator: how many BTU do I need?
Measure the room, adjust for sun, people and kitchen, and get the BTU per hour your window unit, portable AC or mini-split needs. Based on the ENERGY STAR sizing table.
Area: 180 sq ft
6,000
BTU per hour
6,000 BTU per hour
About 0.5 tons (1 ton = 12,000 BTU). Table base: 6,000 BTU.
Pick the unit closest to this number. A bigger one does not cool better: it leaves the room clammy and wastes energy.
The ENERGY STAR table
For a room with an 8-foot ceiling. Higher ceilings mean more air to cool.
| Area (sq ft) | BTU per hour |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
Adjustments
- Heavily shaded room: 10% less.
- Very sunny room: 10% more.
- More than two people regularly: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU.
Whole house: Manual J load calculation
Central AC or a multi-zone mini-split system should not be sized by square feet alone. The standard method is ACCA Manual J: it accounts for insulation, windows, which way the house faces, air leaks and your local climate. Ask an AC technician for that calculation before you replace a system.
Frequently asked questions
How many BTU do I need for a 12 × 12 room?
That is 144 square feet: the ENERGY STAR table gives 5,000 BTU per hour. Add 10% if the room is very sunny and 600 BTU for each person beyond two.
How many square feet does a 12,000 BTU unit cool?
In the ENERGY STAR table, 12,000 BTU matches a 450 to 550 square foot area with an 8-foot ceiling and normal sun.
Is a bigger air conditioner better?
No. ENERGY STAR notes that an oversized unit cools the room before it removes the humidity, so it feels damp and clammy, and it wastes energy.
Can I use this to size central AC for my house?
No. The table is for a room or open area up to 2,500 square feet. For a whole house, a technician should run a Manual J load calculation, which accounts for insulation, windows, orientation and climate.
How many BTU is one ton?
One ton of cooling is 12,000 BTU per hour. A 24,000 BTU unit is a 2-ton unit.
Sources
Checked October 21, 2026.