The calculator gives an estimate based on a standard sizing chart. Check the maker’s guidance and your electrical limits before you buy.
Picking an air conditioner by guesswork leads to a cold, damp room or a unit that never catches up. Use the calculator below for the BTU your room needs.
Add 600 BTU for each person beyond 2 regularly using the room.
Minimum BTU you need
Floor area
144 sq ft
Base BTU
2,880
Adjustments
+0
Table of Contents
Real air conditioners that clear this BTU
Ceiling Height ÷ 8, then ±10% for sun exposure, +600 BTU per occupant beyond 2, and +4,000 BTU if the room is a kitchen. This mirrors the sizing guidance most AC manufacturers and energy-efficiency guides publish.
How This BTU Calculator Works
BTU is the amount of heat an air conditioner can remove in an hour. The calculator starts with a base BTU from a common sizing chart for your floor area, scales it by ceiling height, then adjusts for sun and people.
Formula: BTU = (base BTU for area × ceiling height ÷ 8 × sun factor) + extras
- Sun factor: shaded rooms × 0.90, normal × 1.00, very sunny × 1.10.
- Extras: 600 BTU for each person beyond two, plus 4,000 BTU if the room is a kitchen.
The answer rounds up to the next 500 BTU. Above 2,500 sq ft, it switches to a whole-house rule of about 600 sq ft per ton.
Here is where room cooling is heading: inverter window units that run quieter and use less power, plus ductless and heat pump models that cool and heat. More units also ship with Wi-Fi, so you can start the cooling before you get home.
Worked Examples
- 12 × 12 ft bedroom (144 sq ft), normal sun, two people. The chart says 5,000 BTU, so a 5,000 BTU unit fits.
- 15 × 20 ft living room (300 sq ft), very sunny, four people. Base 7,000 × 1.10 = 7,700, plus 1,200 for two extra people, rounded up to 9,000 BTU.
- 14 × 16 ft kitchen (224 sq ft), normal sun, two people. About 5,740 plus 4,000 for the kitchen, rounded up to 10,000 BTU.
A kitchen can need almost double the BTU of a bedroom of the same size.
Window vs Portable: Read the BTU Label Closely
Window units are the most efficient way to cool one room. Portable units are easier to move but lose some of their cooling to the hose and warm air that leaks back in, so many people size up. Federal test rules for portable units now use a stricter measure, which gives lower numbers than older labels. Compare like with like. See the best window air conditioners and the best portable air conditioners.
When to Size Up or Down
- Size up: top-floor rooms, west-facing windows, poor insulation, lots of electronics or a hot climate.
- Size down: basement rooms, well-shaded rooms and rooms with thick walls.
Do not jump more than one size up. A unit that is far too big short-cycles and leaves the air damp.
Watch the Outlet
Check the unit’s amp draw and your circuit. Many 115-volt window units are fine on a standard 15-amp circuit, but bigger ones may need their own circuit or a 230-volt outlet. Do not use an extension cord with an air conditioner.
The downside is bad sizing and bad numbers. A unit that is too big cools fast and leaves the room clammy, and one that is too small runs nonstop and still falls behind. Portable units often lose real cooling power through the hose and warm air leaks, so the number on the box can mislead. Heat waves are also getting longer, so a size picked for a mild summer may fall short. Expect more units sold on BTU numbers alone. Size for the room, and look at how the BTU rating was measured.
Beyond One Room
Cooling a whole floor or house takes a different approach. Try the AC tonnage calculator for whole-house sizing, or read how to choose the right size air conditioner. The same room-volume math is used for air cleaners too, as explained in how to choose the right size air purifier.
FAQ
How many BTU do you need for a 300 sq ft room?
About 7,000 BTU in normal conditions, more for a sunny room, extra people or a kitchen.
Does ceiling height change the BTU?
Yes. The calculator scales the base number by ceiling height compared with 8 feet.
Can an air conditioner be too big?
Yes. It shuts off too soon and leaves the air damp and clammy.
Is a portable AC as strong as a window AC at the same BTU?
Usually not. Portable units lose some cooling through the hose and leaks, so many people size up.
Final Word
Measure the room, run the calculator and pick a unit within one size of the answer. Check the circuit before you plug in.
What comes next: more inverter models, more heat pump window units and more rules about how BTU is measured. Run the numbers below, then pick a unit with a little room to spare.
Related Guides
Keep going with how to choose the right size air conditioner, best window air conditioners, best portable air conditioners, AC tonnage calculator and how to choose the right size air purifier.
