How to use the BTU calculator
- 1 Enter the room area in square feet.
- 2 Enter the ceiling height.
- 3 Choose the sunlight, and enter the people and whether it is a kitchen.
- 4 Read the recommended BTU and tons.
Worked example
A 350 square foot room with 8-foot ceilings, normal sunlight, 2 people and no kitchen. The ENERGY STAR chart gives 8,000 BTU for 300 to 350 square feet, with no adjustments, so a unit of about 8,000 BTU per hour (0.67 tons, 2.34 kW) fits.
Choosing an air conditioner
- Measure the room you want to cool.
- Do not oversize: it leaves the room damp.
- Add for sun, people and kitchens.
- Check the energy rating (CEER) before buying.
Measure the room with the Square Footage Calculator, convert BTU and kilowatts with the Power Converter, and convert temperatures with the Temperature Converter. Outdoors in winter, check the Wind Chill Calculator.
How it works: The room area is matched to the ENERGY STAR sizing chart. Ten percent is taken off for a heavily shaded room or added for a very sunny one, the size is scaled up for ceilings over 8 feet, and 600 BTU per extra person and 4,000 BTU for a kitchen are added.
Features
ENERGY STAR chart
The standard room air conditioner sizing table.
Real-room adjustments
Sunlight, ceiling height, people and kitchens.
BTU and tons
Plus kilowatts and BTU per square foot.
When to use it
Buying a window unit
Choose the right size for a bedroom or living room. Measure it with the Square Footage Calculator.
Running costs
Estimate electricity use with the Energy Converter.
Mini-splits
Size a single zone.
Comparing units
Convert kilowatts and BTU with the Power Converter.
Related Construction tasks
Frequently asked questions
How many BTU do I need per square foot?
About 20 BTU per square foot for a typical room. A 350 square foot room needs about 8,000 BTU.
What size AC do I need for a 350 square foot room?
About 8,000 BTU per hour under normal conditions, according to the ENERGY STAR chart.
How many square feet does a 12,000 BTU unit cool?
About 450 to 550 square feet under normal conditions. 12,000 BTU is one ton of cooling.
What is a BTU?
A British thermal unit: the heat needed to raise one pound of water by 1°F. Air conditioners are rated in BTU per hour of heat removed.
How do sunlight and shade change the size?
ENERGY STAR suggests adding 10% for a very sunny room and taking off 10% for a heavily shaded one.
Do more people need more BTU?
Yes. Add 600 BTU for each person beyond two who regularly uses the room.
Why add BTU for a kitchen?
Cooking adds heat. ENERGY STAR suggests adding 4,000 BTU for a kitchen.
Is a bigger air conditioner better?
No. An oversized unit cycles on and off quickly, does not remove humidity well and wastes energy.
What is a ton of air conditioning?
12,000 BTU per hour, the cooling from melting a ton of ice over 24 hours.
Can I use this for central air?
Only as a rough guide. Whole-house systems should be sized with a Manual J load calculation by a contractor.
What size AC do I need for 500 square feet?
About 12,000 BTU (one ton) under normal conditions.
Do higher ceilings need more BTU?
Yes. The calculator scales the size for ceilings above 8 feet, because there is more air to cool.
Is BTU the same for heating?
Heating needs depend more on climate and insulation; a heating load is often higher than the cooling size in cold regions.
Built and reviewed by Hamza Farooq, founder and lead developer
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