BTU Calculator
A BTU calculator multiplies room area by a BTU-per-sq-ft factor and ceiling height ÷ 8: a 12 × 15 ft room at 20 BTU/sq ft needs 3,600 BTU/hr (0.30 tons).
How many BTUs do I need?
Energy Star’s chart sizes a window unit by floor area: 5,000 BTU/hr for 100 to 150 sq ft, and 9,000 BTU/hr for 350 to 400 sq ft. The chart assumes an 8 ft ceiling.
This BTU calculator estimates the heating or cooling load of one room. Multiply the floor area by a BTU-per-square-foot factor, then scale by the ceiling height divided by 8 ft. Divide the result by 12,000 to get tons. A 12 × 15 ft bedroom with an 8 ft ceiling at the default 20 BTU per square foot needs 3,600 BTU per hour, or 0.30 tons.
Enter the room’s length, width, and ceiling height, then set the factor for your climate and exposure. The output gives BTU per hour, tons, and floor area, ready to match against the rating on a window unit, mini-split head, or space heater.
The formula in plain words
- Area = length × width
- BTU/hr = area × BTU per sq ft × (ceiling height ÷ 8)
- Tons = BTU/hr ÷ 12,000
The factor bundles climate, sun, insulation, and how many people use the room into one number. The ceiling term corrects for rooms taller than the standard 8 ft the factor assumes.
Worked examples
All three match the calculator.
- Bedroom, 12 × 15 ft, 8 ft ceiling, 20 BTU/sq ft
- Area: 12 × 15 = 180 sq ft
- Load: 180 × 20 × (8 ÷ 8) = 3,600 BTU/hr
- Tons: 3,600 ÷ 12,000 = 0.30 tons
The smallest common window unit is about 5,000 BTU, so this room would take that size.
- Den, 14 × 16 ft, 9 ft ceiling, 20 BTU/sq ft
- Area: 14 × 16 = 224 sq ft
- Load: 224 × 20 × (9 ÷ 8) = 5,040 BTU/hr
- Tons: 0.42 tons
- Sunny great room, 20 × 25 ft, 10 ft ceiling, 25 BTU/sq ft in a hot climate
- Area: 20 × 25 = 500 sq ft
- Load: 500 × 25 × (10 ÷ 8) = 15,625 BTU/hr
- Tons: 1.30 tons
A tall ceiling plus a hotter factor turns 500 sq ft into a load that needs a 1.5-ton (18,000 BTU) mini-split, not a 1-ton one.
Choosing a factor
| Situation | Factor (BTU/sq ft) |
|---|---|
| Shaded, well insulated, mild climate | 15–18 |
| Typical room, average climate | 20 |
| Sunny exposure or warm climate | 25 |
| Top floor under a hot attic, or very hot climate | 30 |
| Sunroom or room with large west-facing glass | 30–35 |
These are planning ranges. Energy Star’s room air conditioner guidance also suggests cutting capacity about 10% for a heavily shaded room and adding about 10% for a very sunny one.
Matching the result to equipment
| Load (BTU/hr) | Tons | Typical equipment |
|---|---|---|
| Up to 6,000 | 0.5 | Small window unit |
| 9,000 | 0.75 | Small mini-split head |
| 12,000 | 1.0 | Mid-size mini-split or large window unit |
| 18,000 | 1.5 | Large mini-split head |
| 24,000 | 2.0 | Small central system |
| 36,000 | 3.0 | Central system for a mid-size house |
Round up to the next size you can buy, but only by one step. Jumping two sizes brings back the short-cycling problem.
How to measure the room
- Measure length and width at floor level in feet. For an L-shaped room, split it into two rectangles and add the areas.
- Measure ceiling height. For a sloped ceiling, use the average of the low and high sides.
- Count large openings. A wide archway into another room means both rooms share the air, so include both areas.
Common mistakes
- Using one factor for every room. A north bedroom and a west sunroom in the same house can need factors 10 to 15 apart.
- Ignoring the kitchen. Cooking adds a lot of heat. Energy Star suggests about 4,000 extra BTU when a room unit cools a kitchen.
- Sizing a whole house by adding room estimates. Rooms do not all peak at the same hour. A Manual J will usually come in lower than the sum.
- Forgetting heating. In cold climates the winter load can be larger than the summer load. Check it with the heat loss calculator.
Related calculators
The mini-split sizing calculator picks a head size from the load, and the furnace size calculator handles whole-house heating. For new equipment, have a licensed HVAC contractor run a full load calculation before you buy.
Frequently asked questions
How many BTUs do I need per square foot?
About 20 BTU per square foot is the common starting point for cooling a room with an 8 ft ceiling, which is the calculator's default. Raise it to 25 to 30 for a hot climate, a sunny west-facing room, or a top-floor room under a hot attic, and lower it toward 15 for a shaded, well-insulated space.
What is a ton of cooling?
One ton is 12,000 BTU per hour, the heat needed to melt a ton of ice over 24 hours. Central air and heat pumps are sold in half-ton steps. A 30,000 BTU/hr load is 30,000 ÷ 12,000 = 2.5 tons.
Does ceiling height matter?
Yes, because you are cooling the air in the room, not the floor. The calculator multiplies the load by ceiling height ÷ 8. A 9 ft ceiling adds 12.5%, a 10 ft ceiling adds 25%, and a 12 ft ceiling adds 50% to the same floor area.
What size window air conditioner do I need for my room?
Energy Star's chart suggests about 5,000 BTU for 100 to 150 sq ft, 6,000 for 150 to 250, 7,000 for 250 to 300, 8,000 for 300 to 350, and 9,000 for 350 to 400 sq ft, assuming an 8 ft ceiling. Add about 600 BTU for each regular occupant beyond two and about 4,000 BTU if the room is a kitchen. Above 400 sq ft, check the chart on energystar.gov.
Why does an oversized unit cause problems?
A unit that is too big cools the air quickly and shuts off before it has run long enough to pull moisture out. The room ends up cold and clammy, and the compressor wears from short on-off cycles. Pick the nearest size at or just above your load, not the biggest one.
Is this a full Manual J load calculation?
No. It is a fast square-foot estimate for one room. Whole-house systems should be sized with an ACCA Manual J calculation, which accounts for insulation, windows, orientation, air leakage, and local design temperatures. Use this to sanity-check a room unit or a quote.
Estimates only. Verify quantities with your supplier before purchasing.