Enter your room’s size and sun exposure to find the cooling power you need — in BTUs and in AC tons.
What Is a BTU Calculator?
A BTU (British Thermal Unit) calculator sizes an air conditioner for a room: cooling load in BTUs per hour. The rule of thumb is about 20 BTU per square foot for average conditions — 400 sq ft needs roughly 8,000 BTU. Sunny rooms need about 25 per sq ft; shaded rooms about 15. Add 600 BTU per extra person beyond two, since bodies are heaters. AC capacity is also sold in “tons”: 12,000 BTU = 1 ton, so 8,000 BTU is 0.67 tons — you would buy a 0.75-ton (9,000 BTU) unit.
How to Use the BTU Calculator
- Enter the room area in square feet.
- Pick the sun exposure.
- Enter extra people beyond 2.
- Click “Calculate BTUs” and shop for the nearest AC size at or above the result.
BTU Formula
BTU = area × factor (15/20/25) + 600 × extra people. Tons = BTU ÷ 12,000.
Worked Example
400 sq ft, average exposure, 2 people: 400 × 20 = 8,000 BTU = 0.67 tons — a 9,000 BTU (0.75 ton) unit is the right buy.
How It Works, In Plain Words
The estimate is built from three inputs. The room area sets the base load: every square foot needs a certain amount of cooling, and the sun-exposure choice sets how much — 15 BTU per square foot for a shaded, cool room, 20 for an average room and 25 for a sunny, hot one. The tool multiplies area by that factor, so the worked example’s 400 sq ft at the average factor gives 400 × 20 = 8,000 BTU before anything else is added. People come next: the first two occupants are already allowed for, and each extra person adds 600 BTU, because — as the introduction puts it — bodies are heaters. Finally the BTU total is divided by 12,000 to express the same capacity in tons, the unit many ACs are sold in: 8,000 ÷ 12,000 = 0.67 tons. The cooling figure is shown as a whole number with thousands separators, and the tons to two decimal places.
Real-World Uses
Sizing a unit for one room before you shop. The worked example is the everyday case: a 400 sq ft room with average exposure needs 8,000 BTU, and the right purchase is the 9,000 BTU (0.75 ton) unit — the nearest size at or above the result. Comparing quotes given in different units. One seller quotes BTUs and another quotes tons; because the tool shows both, a 0.75-ton offer and a 9,000 BTU offer can be recognised as the same capacity instead of two different machines. Checking a sunny room honestly. The mistakes list gives the trap: the same 400 sq ft room in full sun needs 10,000 BTU, not 8,000, so running the sunny setting before buying prevents an undersized unit that runs constantly and never cools.
Benefits of the BTU Calculator
- Sun exposure built in — the biggest load variable
- People heat counted (600 BTU each)
- Tons conversion included for shopping
- Undersizing warning in the text below
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device.
Common Mistakes
- Buying by price alone: an undersized AC runs constantly and never cools
- Ignoring sun: a sunny 400 sq ft room needs 10,000 BTU, not 8,000
- Oversizing hugely: too-big units short-cycle, wasting energy and leaving humidity
Common Mistakes to Avoid
The list above covers buying by price, ignoring the sun and oversizing. Three input habits also skew the result. The first is counting everyone in the people box: it asks for extra people beyond 2, so a room used by four people takes 2 in that box, not 4 — entering 4 adds 600 BTU twice over. The second is entering the whole home’s area when the unit will cool one room: the area must be the space the AC actually serves. The third is treating the estimate as exact for unusual rooms: as the FAQ explains, tall ceilings and kitchens add load the three inputs do not model, and sunny kitchens should use the 25 factor.
FAQ
How many BTUs for a 400 sq ft room?
About 8,000 BTU (0.67 tons) for average exposure.
How many BTUs per square foot?
Roughly 20 for average rooms, 25 for sunny, 15 for shaded.
What does 1 ton of AC mean?
12,000 BTU/hour of cooling capacity.
Can I use a bigger AC than needed?
Slightly bigger is fine; much bigger short-cycles — cooling fast but leaving the air humid and wasting energy.
Do ceiling height and kitchens matter?
Yes — tall ceilings and kitchens add load; the occupancy adder covers people, and sunny kitchens should use the 25 factor.
What happens if I leave the extra people box empty?
It is treated as zero extra people, so only the first two occupants — already allowed for in the base figure — are counted. A negative entry is treated as zero too, so the people adder can never reduce the cooling load.
Why does the worked example buy a bigger unit than the result?
Because air conditioners are sold in fixed sizes. The example room needs 8,000 BTU (0.67 tons), and the nearest size at or above that figure is a 9,000 BTU (0.75 ton) unit — which is why the How to Use steps say to shop for the nearest size at or above the result, not below it.