BTU Calculator for Room AC Size
Right air conditioner size in BTU and tons from room area and conditions.
What it does
Air conditioners are sized by heat removal, measured in BTU per hour. Too small and the room never cools on hot days. Too big and the unit short cycles: it blasts cold air, shuts off before wringing out humidity, and leaves the room cool but damp.
The tool above sizes from room area with the trade rule of 20 BTU per square foot, adjusted for ceiling height, sun load, kitchen appliances, and extra occupants. It returns BTU, tons, and the nearest standard unit size.
The 20 per foot rule assumes normal insulation and windows. A sun blasted room with west glass runs hotter, while a shaded basement room runs cooler, which is why exposure moves the answer by ten percent either way.
Whole houses need a proper load calculation with windows, insulation, and climate, since single room rules miss duct and leakage losses. For one room with a window or portable unit, this math lands well.
How to use
- Switch the unit system first. Metric rooms use square meters, and every result also shows kilowatts.
- Measure room area in square feet.
- Enter ceiling height. Tall rooms hold more hot air.
- Mark sun exposure honestly, counting big west windows.
- Add the kitchen flag and extra occupants, then shop the suggested size.
Formula
- area x 20
- Base cooling: 20 BTU per hour for each square foot.
- ceiling / 8
- Volume correction against the standard 8 ft ceiling.
- sun
- 0.9 shaded, 1.0 average, 1.1 very sunny.
- kitchen, occupants
- 4,000 BTU for cooking heat plus 600 per extra person.
- 12,000
- BTU per hour in one ton of cooling, a holdover from ice melting.
A ton of cooling melts one ton of ice per day in the old definition, which froze the 12,000 number into the trade. Standard room units step through 5,000, 6,000, 8,000, 10,000, 12,000, and upward.
Worked example
Example: 400 ft2 room, 8 ft ceiling, average sun
Living room 400 ft2, standard ceiling, average sun, not a kitchen, two people.
- Base: 400 x 20 = 8,000.
- Ceiling: 8 / 8 = 1, so no change.
- Sun: x 1.0, still 8,000. Kitchen 0, extra people 0.
- Need: 8,000 BTU, which is 8,000 / 12,000 = 0.67 tons.
- Shop: nearest standard size up is an 8,000 BTU unit.
Reference table
| Room | BTU | Common unit |
|---|---|---|
| 150 ft2 bedroom | 3,000 | 5,000 BTU |
| 250 ft2 bedroom | 5,000 | 5,000 BTU |
| 400 ft2 living room | 8,000 | 8,000 BTU |
| 550 ft2 open space | 11,000 | 12,000 BTU |
| 800 ft2 apartment | 16,000 | 18,000 BTU |
Limits
- Single rooms only. Whole houses need window and insulation detail.
- Rule of thumb accuracy runs about plus or minus 15%.
- Poor insulation, leaky ducts, and desert heat push past the estimate.
- Heating mode (heat pumps) follows different math.
Common mistakes
- Buying double the need for faster cooling, then living with humidity.
- Ignoring west glass, the largest single heat source in most rooms.
- Sizing off floor area while forgetting twelve foot ceilings.
- Blocking airflow with curtains over the unit.
- Skipping the kitchen adder for open plan cooking spaces.
Try it yourself
Quick checks. Answers sit tucked under each question.
A 300 ft2 room, standard conditions. BTU?
300 x 20 = 6,000 BTU. Shop a 6,000 unit.
Same room but very sunny with 4 occupants. BTU?
300 x 20 x 1.1 = 6,600, plus 2 x 600 = 1,200, total 7,800. Shop an 8,000 unit.
Questions
What does BTU mean for AC?
BTU per hour of heat removed. Higher numbers cool bigger or hotter rooms.
What is a ton of cooling?
12,000 BTU per hour. A 24,000 BTU system is a 2 ton system.
Can a unit be too big?
Yes. Oversized units cool quickly without dehumidifying, leaving cold damp air and higher bills.
Do high ceilings matter?
Yes. A 10 ft ceiling holds 25% more air than an 8 ft one, so size scales with the ceiling ratio.
What about portable vs window units?
Same BTU math, but single hose portables cool less than rated. Size them a step up or pick dual hose.
How do I cool a whole house?
Get a Manual J load calculation from a contractor. Room rules miss ducts, infiltration, and orientation.