Our free BTU calculator tells you exactly how many BTUs you need. It factors in windows, insulation, sun exposure, and room use so you get the perfect fit — not too big, not too small.
Modify the values and click the calculate button to use
| Area (sq ft) | BTU/hr |
|---|---|
| 100-150 | 5,000 |
| 150-250 | 6,000 |
| 250-300 | 7,000 |
| 300-350 | 8,000 |
| 350-400 | 9,000 |
| 400-450 | 10,000 |
| 450-550 | 12,000 |
| 550-700 | 14,000 |
| 700-1,000 | 18,000 |
| 1,000-1,200 | 21,000 |
| 1,200-1,400 | 23,000 |
| 1,400-1,500 | 24,000 |
| 1,500-2,000 | 30,000 |
| 2,000-2,500 | 34,000 |
Source: EnergyStar.gov — recommended cooling capacity based on room size
BTU stands for British Thermal Unit. Sounds fancy, but it's simple. One BTU is the amount of energy needed to heat one pound of water by one degree Fahrenheit.
Think of it like this: imagine you have a cup of water. To make it one degree warmer, you need one BTU of energy. That's it.
For air conditioners and heaters, BTUs tell you how much cooling or heating power they have. A higher BTU number means more power. A 12,000 BTU AC can cool a bigger room than a 5,000 BTU one.
Our calculator uses a formula that's way smarter than just "square footage times 20." It considers:
Room size (length × width)
Ceiling height (most people forget this!)
Insulation quality
Number of windows and their direction
Sun exposure
How many people are usually in the room
What the room is used for (kitchen, bedroom, garage, etc.)
Sounds like a lot? That's why we built the calculator. You just answer a few questions, and it does the rest.
Okay, for those of you who like math, here's the basic formula. But remember, this is the simplified version. Our calculator is way more accurate.
Measure your room. Length × Width = Square footage.
Multiply by 20. This gives you a rough BTU number. So a 200 sq ft bedroom: 200 × 20 = 4,000 BTUs.
Adjust for ceiling height. If your ceiling is higher than 8 feet, add 10% for every extra foot.
Adjust for windows. South-facing windows? Add 10%. Lots of windows? Add another 10%.
Adjust for people. Each person beyond two adds 600 BTUs.
Adjust for the room type. Kitchens need 4,000 extra BTUs. Basements can subtract 10%.
See why our calculator is easier?
Sarah's 300 sq ft living room, 9-ft ceilings, two west-facing windows, 3 people. Basic: 6,000 BTUs. +Ceiling: 6,600. +Windows: 7,260. +People: 7,860. She needs about 8,000 BTUs.
Mike's 200 sq ft kitchen, standard ceiling, one window, cooks a lot. Basic: 4,000 BTUs. +Kitchen: 8,000 BTUs. Why so much? Ovens and fridges generate tons of heat.
Jenna's 500 sq ft basement, partially underground, no windows, good insulation. Basic: 10,000 BTUs. -Basement: 9,000 BTUs. Basements are naturally cooler so you need less power.
Here's the thing that drives HVAC pros crazy: people buy the biggest AC they can find, thinking more power = better cooling. Wrong.
Oversized AC
Cools too fast, doesn't remove humidity. You get a cold, clammy room. Cycles on/off constantly — wears out the compressor.
Undersized AC
Runs nonstop trying to cool. Never reaches target temperature. Electric bill goes through the roof.
The Sweet Spot
A unit that runs 15-20 minutes at a time, then takes a break. That's what our calculator helps you find.
Window units, portable ACs, and central air all use BTUs. For window units, stick close to the calculator's number. For central air, get a professional Manual J calculation.
Space heaters, garage heaters, and furnaces have BTU ratings. For a garage, add 20-30% because garages are usually poorly insulated. For well-insulated rooms, subtract 10%.
Gas fireplaces and wood stoves are rated in BTUs. A typical gas fireplace puts out 20,000-40,000 BTUs — way more than a room AC because they're heating, not cooling.
You're cooling equipment, not just the room. Add 3,400 BTUs for every kilowatt of equipment power. A 2 kW server rack needs about 6,800 BTUs just for equipment, plus the room.
Grow lights generate massive heat. A 1,000 watt HPS light = about 3,400 BTUs. Four lights = 13,600 BTUs. Start at 20 BTUs per sq ft, then add 3,400 BTUs per light.
You might hear HVAC people talk about "tons" instead of BTUs. Here's the conversion:
1 ton = 12,000 BTUs
So a 2-ton AC is 24,000 BTUs. A 3-ton is 36,000 BTUs.
Where did "ton" come from? Back in the day, ice was used for cooling. One ton of ice melting over 24 hours absorbs 12,000 BTUs of heat. The name stuck.
The BTU was invented in the 1800s by British engineers who needed a way to measure heat. It's still used today in the US and UK, but most of the world uses joules or watts.
One BTU equals about 1,055 joules. Or 0.293 watt-hours. So a 10,000 BTU AC uses about 2,930 watts of power.
Fun fact: the average human body gives off about 400 BTUs per hour. So if you have 10 people in a room, that's 4,000 BTUs of heat you need to remove. That's why crowded rooms feel so hot.
Measure accurately. Don't guess the room size. Use a tape measure.
Be honest about insulation. If your walls are thin, say so.
Count all windows. Even small ones let in heat.
Think about the sun. A room that bakes in afternoon sun needs more cooling.
Consider your climate. Phoenix needs more cooling than Seattle.
And remember: our calculator gives you a recommendation, not a law. If you're between sizes, go with the bigger one. But don't go too big.
Our calculator is great for window units, portable ACs, and small heaters. But for central air, whole-house heating, or commercial spaces, you should get a professional to do a Manual J calculation.
That's the gold standard. It considers everything: insulation values, window types, ductwork, climate zone, and more. It's complicated, but it's the most accurate way to size an HVAC system. For everything else? Our calculator has you covered.
BTU stands for British Thermal Unit. It's a measure of energy. One BTU is the amount of energy needed to heat one pound of water by one degree Fahrenheit. For air conditioners and heaters, it tells you how much cooling or heating power they have.
For a basic 200 sq ft room with 8-foot ceilings and average conditions, you need about 4,000-5,000 BTUs. But that number changes based on windows, insulation, sun exposure, and how many people are in the room. Use our calculator for the exact number.
Neither is good, but oversizing is actually worse. An oversized AC cools too fast, doesn't remove humidity, and cycles on/off too much — wearing out the compressor. Undersized units run constantly and never catch up. Aim for the sweet spot.
High ceilings mean more volume to cool. For every foot above 8 feet, add 10% to your BTU calculation. So a room with 12-foot ceilings needs 40% more BTUs. Our calculator handles this automatically.
One ton equals 12,000 BTUs. The term comes from when ice was used for cooling — one ton of ice melting over 24 hours absorbs 12,000 BTUs of heat. HVAC pros still use tons today.
A rough rule is 20 BTUs per square foot for cooling. But this varies based on ceiling height, windows, insulation, sun exposure, room use, and climate. Our calculator factors all of these in.
This usually means your AC is oversized. It cools the air too fast without running long enough to remove humidity. A properly sized unit runs 15-20 minute cycles to dehumidify properly.
Add 20-30% more BTUs than a standard room of the same size. Garages are typically poorly insulated with thin walls and large doors that leak air. For heating a 400 sq ft garage, you might need 10,000-12,000 BTUs.
Yes, but the numbers may differ slightly. Heating often requires more BTUs than cooling because you're fighting against cold outdoor temperatures. Our calculator handles both.
1 BTU = 0.293 watt-hours. So a 10,000 BTU AC uses about 2,930 watts. To convert: multiply BTUs by 0.293 to get watt-hours.
Manual J is the industry standard for sizing HVAC systems. It's a detailed calculation that considers insulation, windows, ductwork, climate zone, and more. Required for central air installations — our calculator is fine for window units.
A 12,000 BTU unit uses about 1,200-1,500 watts per hour. At $0.14/kWh, that's about $0.17-$0.21 per hour. Running 8 hours a day = roughly $40-$50 per month. Efficiency ratings (EER/SEER) affect actual costs.
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