BTU Calculator
Size an air conditioner or mini split for any room: enter its dimensions and conditions to get BTU and the nearest standard AC size.
- Room size & ceiling
- Sun & occupants
- Kitchen adjustment
- Standard AC sizes
Air conditioner BTU for a room
Fill in the boxes. The answer appears as you type.
How many BTU do you need?
BTU (British thermal units per hour) measures how much heat an air conditioner removes. A common sizing rule, used in ENERGY STAR guidance, is about 20 BTU per square foot of living space with 8 ft ceilings, adjusted for conditions: reduce by 10% for heavily shaded rooms, add 10% for very sunny rooms, add 600 BTU for each person over two, and add 4,000 BTU for kitchens. This calculator applies those adjustments, scales for ceiling height and recommends the nearest standard air conditioner size.
How to use BTU Calculator
- Measure the room length and width, and the ceiling height.
- Choose sun exposure and the number of people usually in the room.
- Choose Kitchen if the room has cooking appliances.
- Read the BTU needed and the recommended AC size.
AC sizing guide (8 ft ceilings)
- 100–150 sq ft: 5,000 BTU · 150–250: 6,000 · 250–300: 7,000 · 300–350: 8,000.
- 350–400 sq ft: 9,000 · 400–450: 10,000 · 450–550: 12,000 · 550–700: 14,000.
- Oversized units cycle on and off and leave rooms clammy; undersized units run constantly.
- 12,000 BTU = 1 ton of cooling.
Why use this btu calculator
- Answers as you typeNo Calculate button. Change any number and every result updates instantly.
- Shows the workingEach answer has a plain-English sentence, the steps and a formula for Excel or Google Sheets.
- Buy the right amountResults are in the units stores sell (gallons, squares, bundles, BTU sizes), with sensible allowances built in.
- PrivateYour numbers are calculated on your device and never sent anywhere.
Common uses
- Window and portable AC. Pick the right size for a bedroom or office.
- Mini splits. Size each indoor head.
- Home offices. Account for people and sun.
- Kitchens. Add capacity for cooking heat.
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Example: a sunny 15 × 12 ft bedroom
180 sq ft × 20 = 3,600 BTU, +10% for sun = 3,960 BTU → a 5,000 BTU air conditioner.
What to expect from the result
This is a room-level rule of thumb. Insulation, climate zone, windows and appliances all matter; whole-house systems need a professional Manual J load calculation.
BTU Calculator questions
How many BTU do I need per square foot?
About 20 BTU per square foot for a typical room with 8 ft ceilings, adjusted for sun, occupants and kitchens.
What size AC do I need for a 300 sq ft room?
About 6,000 BTU at 20 BTU per sq ft; choose 7,000–8,000 BTU if the room is very sunny or often has several people.
Is a bigger air conditioner better?
No. An oversized unit cools too quickly without removing humidity, wastes energy and wears out faster.
How many BTU is a ton?
12,000 BTU per hour equals one ton of cooling capacity.
Do high ceilings need more BTU?
Yes. This calculator scales the estimate by ceiling height relative to 8 ft: a 10 ft ceiling adds 25%.