How many BTU do I need?
Measure the floor of the room in feet and multiply the length by the width. Find that size in ENERGY STAR's chart below, then adjust it for sun, shade, people and kitchens. ENERGY STAR's chart assumes an 8-foot ceiling.
Bigger is not always better. ENERGY STAR says an oversized unit cools the room before it removes the humidity, so the room feels damp and clammy, and it costs more and wastes energy.
How to calculate BTU
Here it is with your numbers:
- Multiply the length by the width: 12 × 16 = 192 sq ft.
- 192 sq ft is in ENERGY STAR's 150 – 250 sq ft row: 6,000 BTU.
It means looking for a room air conditioner close to 6,000 BTU per hour. ENERGY STAR says bigger is not always better.
BTU chart for room air conditioners
ENERGY STAR's sizing chart. The chart writes each row as "from … up to", so a size like 150 sq ft appears in two rows. A room exactly on a dividing line gets the smaller size, following ENERGY STAR's advice that bigger is not always better. The calculator picks the row from the floor area rounded to 2 decimals, the number it shows you.
| Floor area | BTU per hour |
|---|---|
| 100 – 150 sq ft | 5,000 |
| 150 – 250 sq ft | 6,000 |
| 250 – 300 sq ft | 7,000 |
| 300 – 350 sq ft | 8,000 |
| 350 – 400 sq ft | 9,000 |
| 400 – 450 sq ft | 10,000 |
| 450 – 550 sq ft | 12,000 |
| 550 – 700 sq ft | 14,000 |
| 700 – 1,000 sq ft | 18,000 |
| 1,000 – 1,200 sq ft | 21,000 |
| 1,200 – 1,400 sq ft | 23,000 |
| 1,400 – 1,500 sq ft | 24,000 |
| 1,500 – 2,000 sq ft | 30,000 |
| 2,000 – 2,500 sq ft | 34,000 |
Under 100 sq ft, the calculator uses the chart's smallest size, 5,000 BTU. The chart stops at 2,500 sq ft, so the calculator gives no size above that.
Adjusting for sun, shade, people and kitchens
- Heavily shaded room: take off 10% of the chart size.
- Very sunny room: add 10% of the chart size.
- More than two people usually in the room: add 600 BTU for each extra person.
- In a kitchen: add 4,000 BTU.
ENERGY STAR lists these together. The calculator works the 10% out from the chart size, then adds the people and the kitchen.
What is a British thermal unit (BTU)?
A British thermal unit is a measure of heat: the heat needed to warm one pound of water by 1 °F, about as much as burning a match gives off. An air conditioner's size is the heat it can take out of a room in an hour, so a 6,000 BTU unit removes 6,000 BTU of heat an hour.
This calculator is for cooling only. ENERGY STAR's chart is for room air conditioners, and we haven't found an official chart like it for sizing a heater.
Source U.S. Energy Information Administration · British thermal units (Btu) ↗
FAQs
How do I calculate air conditioner BTU for room size?
Multiply the room's length by its width to get square feet, find that size in ENERGY STAR's chart, then adjust: take off 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU for each person over two and 4,000 for a kitchen. A 12 × 16 ft room is 192 sq ft, which needs 6,000 BTU.
What size room will a 12000 BTU air conditioner cool?
ENERGY STAR's chart gives 12,000 BTU for 450 – 550 sq ft. That assumes an 8-foot ceiling and no adjustments: a sunny room, more people or a kitchen need more.
How many rooms will 10,000 BTU cool?
ENERGY STAR sizes a room air conditioner by the floor area of the space you want to cool, not the number of rooms. Its chart gives 10,000 BTU for 400 – 450 sq ft.
What size room will a 30000 BTU cool?
ENERGY STAR's chart gives 30,000 BTU for 1,500 – 2,000 sq ft, before any adjustments.
What happens if BTU is too high?
A unit that is too big for the room cools it before it has a chance to remove the humidity, so the room can feel damp and clammy. ENERGY STAR adds that an oversized unit costs more to buy, wastes energy and does not cool any better.
How many BTUs to cool a 20x20 room?
A 20 × 20 ft room is 400 sq ft, right on the line between ENERGY STAR's 9,000 and 10,000 BTU rows. We use the smaller size, 9,000 BTU, because the chart says "up to 400". If the room is very sunny, it's 9,900 BTU.
Sources
- ENERGY STAR: Room air conditioners: properly sized room air conditioners (sizing chart and adjustments)
- U.S. Energy Information Administration: British thermal units (Btu)
- NIST Handbook 44 (2026): Appendix C, General Tables of Units of Measurement (feet, inches, meters)