This BTU calculator tells you what size room air conditioner you need. It uses the ENERGY STAR sizing chart for room size, then adjusts for sun, the number of people and kitchens, and converts the answer to tons and kilowatts.
ENERGY STAR air conditioner sizing chart
| Area to cool | Capacity |
|---|---|
| 100–150 sq ft | 5,000 BTU/hr |
| 150–250 sq ft | 6,000 BTU/hr |
| 250–300 sq ft | 7,000 BTU/hr |
| 300–350 sq ft | 8,000 BTU/hr |
| 350–400 sq ft | 9,000 BTU/hr |
| 400–450 sq ft | 10,000 BTU/hr |
| 450–550 sq ft | 12,000 BTU/hr |
| 550–700 sq ft | 14,000 BTU/hr |
| 700–1,000 sq ft | 18,000 BTU/hr |
Adjustments
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people regularly in the room: add 600 BTU/hr for each extra person.
- Kitchen: add 4,000 BTU/hr.
Worked example
A 15 × 20 ft living room is 300 sq ft, which needs 7,000 BTU/hr on the chart. If it is very sunny (+10%) and three people usually use it (+600), you need about 8,300 BTU/hr, so a unit of 8,300 BTU or slightly larger.
Why the right size matters
A unit that is too small runs constantly and never reaches a comfortable temperature. One that is too large cools the air quickly and switches off before it has removed enough humidity, leaving the room cold and clammy. Ceilings higher than about 8 ft, poor insulation and heat-producing equipment also add to the load.
Frequently asked questions
How many BTUs do I need per square foot?
Around 20 BTU per square foot is a common shortcut, but the ENERGY STAR chart is more precise and often lower. For example, it puts a 12,000 BTU unit at 450–550 sq ft rather than 600.
What is a ton of air conditioning?
One ton of cooling equals 12,000 BTU per hour, the heat needed to melt a ton of ice in a day.
Is a bigger air conditioner better?
No. An oversized unit cycles on and off too often, removes less humidity and wears faster. Choose the size the chart suggests.
Can I use this for central air or a mini split?
For a single room or zone, yes, as a starting point. For a whole house, a professional Manual J load calculation is recommended.
How many watts does a BTU equal?
One watt equals about 3.412 BTU per hour, so 12,000 BTU/hr is about 3.5 kW of cooling.
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Sources
Last reviewed: October 2026.