BTU Calculator
Find how many BTUs per hour of cooling each room needs, and the total in BTU/h, tons, and kW. Enter the size of each room, its ceiling height, how sunny it is, how many people use it, and whether it's a kitchen. Choose your climate zone to also see a heating estimate.
- Choose US or Metric. This sets the units of the fields and which unit the results show first: BTU/h for US, kW for Metric.
- Enter the Length and Width of the room. For a room that isn't a rectangle, choose Known area and enter its floor area. You can work out the area with the Square Footage Calculator.
- If the ceiling is higher than 8 ft (2.44 m), enter the Ceiling height. For a sloped ceiling, enter the average height.
- Choose the Sunlight the room gets, enter the number of People usually in it, and check Kitchen for a kitchen.
- Use Add room for each room that gets its own air conditioner or heater. Enter an L-shaped room as one room with its total area, since each room is looked up in the chart once.
- Under Heating, choose your Climate zone to see how much heat each room needs.
The fields accept whole numbers and decimals, such as 12 or
10.5. The length fields also accept fractions and mixed
numbers, such as 12 1/2.
How the cooling BTUs are worked out
The calculator looks up each room's floor area in the room air conditioner sizing chart from ENERGY STAR, the U.S. efficiency program for appliances. The chart is calculated for an 8 ft ceiling.
| Room area | Cooling capacity |
|---|---|
| 100 to 150 ft² | 5,000 BTU/h |
| 150 to 250 ft² | 6,000 BTU/h |
| 250 to 300 ft² | 7,000 BTU/h |
| 300 to 350 ft² | 8,000 BTU/h |
| 350 to 400 ft² | 9,000 BTU/h |
| 400 to 450 ft² | 10,000 BTU/h |
| 450 to 550 ft² | 12,000 BTU/h |
| 550 to 700 ft² | 14,000 BTU/h |
| 700 to 1,000 ft² | 18,000 BTU/h |
| 1,000 to 1,200 ft² | 21,000 BTU/h |
| 1,200 to 1,400 ft² | 23,000 BTU/h |
| 1,400 to 1,500 ft² | 24,000 BTU/h |
| 1,500 to 2,000 ft² | 30,000 BTU/h |
| 2,000 to 2,500 ft² | 34,000 BTU/h |
Each row includes its upper end, so a 150 ft² room uses 5,000 BTU/h. The calculator doesn't blend between rows. Rooms under 100 ft² use the smallest size, 5,000 BTU/h, and rooms over 2,500 ft² scale the top row at 13.6 BTU/h per ft² (34,000 ÷ 2,500).
Then it adjusts the chart value the way ENERGY STAR describes:
Cooling = chart value × ceiling factor × sunlight factor + 600 × (people − 2) + 4,000 for a kitchen
- Sunlight: 10% less for a heavily shaded room, 10% more for a very sunny room.
- People: 600 BTU/h for each person over 2 who usually uses the room. Leaving the field blank counts as 2 or fewer.
- Kitchen: 4,000 BTU/h more for a kitchen.
- Ceiling height: for a ceiling higher than 8 ft, the calculator multiplies by ceiling height ÷ 8 ft, up to 20 ft.
ENERGY STAR doesn't give a formula for higher ceilings. Scaling with the ceiling height follows the volume of air in the room. Heat through the floor and ceiling doesn't grow with the height, so for a tall room this tends to run high. It's a rough adjustment.
The total in tons is BTU/h ÷ 12,000, since a ton of cooling is 12,000 BTU/h. In watts, 1 BTU/h is about 0.293 W (1,055.05585262 J per BTU ÷ 3,600 seconds), so 12,000 BTU/h is about 3.52 kW. The calculator doesn't round the result up to a unit size. Air conditioners are sold in steps such as 5,000, 6,000, and 8,000 BTU/h, or half-ton steps for central systems.
Heating estimate
The heating estimate uses a rule of thumb from Bryant, a furnace maker, for the heat a home needs per square foot in each U.S. climate region. The calculator uses the middle of each range, for a home with average insulation:
Heating = floor area (ft²) × ceiling factor × BTU/h per ft² for your zone
| Climate zone | Rule of thumb |
|---|---|
| Zone 1 – Hot-humid | 32.5 BTU/h per ft² |
| Zone 2 – Hot-humid or hot-dry | 32.5 BTU/h per ft² |
| Zone 3 – Hot, mixed, or marine | 32.5 BTU/h per ft² |
| Zone 4 – Mixed or marine | 45 BTU/h per ft² |
| Zone 5 – Cold | 55 BTU/h per ft² |
| Zone 6 – Cold | 55 BTU/h per ft² |
| Zone 7 – Very cold | No rule of thumb |
| Zone 8 – Subarctic | No rule of thumb |
Bryant lists 30 to 35 BTU/h per ft² for warm regions, 40 to 50 for moderate regions, and 50 to 60 for cold regions, with colder or poorly insulated homes toward the high end. Better or worse insulation changes the estimate. For insulation and furnace efficiency, use the Furnace Size Calculator. The zones are the IECC climate zones, named with the Building America climate regions. Find yours by county on the DOE climate zone map. Bryant doesn't give a value for the very cold and subarctic zones 7 and 8, so the calculator shows no heating estimate there. Use the Furnace Size Calculator to enter your own heating factor, or add up the heat lost through each wall and window with the Heat Loss Calculator. Outside the U.S., choose the zone with the most similar winters.
The sunlight, people, and kitchen adjustments apply only to cooling. Sun, people, and cooking add heat, which raises the cooling load and lowers the heating load.
Examples
A sunny kitchen
A 16 ft by 14 ft kitchen (224 ft²) that gets a lot of sun, with 4 people usually in it, needs about 11,800 BTU/h (1.0 tons, 3.46 kW): 6,000 BTU/h from the chart × 1.1 for the sun, plus 1,200 BTU/h for 2 extra people and 4,000 BTU/h for the kitchen.
Three rooms in zone 4
A 12 ft by 12 ft bedroom, a 10 ft by 12 ft kitchen, and a 20 ft by 25 ft living room need 5,000, 9,000, and 12,000 BTU/h of cooling, for a total of 26,000 BTU/h (2.2 tons). In zone 4, their 764 ft² need about 34,380 BTU/h of heat (10.08 kW) at 45 BTU/h per ft².
A room with a 10 ft ceiling
A 15 ft by 20 ft room (300 ft²) uses 7,000 BTU/h from the chart. With a 10 ft ceiling, that's × 1.25, or 8,750 BTU/h. In zone 1, the heating estimate is 300 × 1.25 × 32.5 = 12,188 BTU/h.
Metric
A 5 m by 4 m room (20 m², about 215 ft²) needs 1.76 kW (6,000 BTU/h) of cooling. In zone 5, it needs about 3.47 kW of heat (173.5 W/m²).
FAQ
About 5,000 BTU/h. A 12 ft by 12 ft room is 144 ft², in the 100 to 150 ft² row of the ENERGY STAR chart. Add 10% for a very sunny room, 600 BTU/h for each person over 2, or 4,000 BTU/h for a kitchen.
About 12,000 BTU/h, or 1 ton, for an average room with an 8 ft ceiling. 500 ft² is in the 450 to 550 ft² row of the ENERGY STAR chart.
12,000 BTU/h, about 3.52 kW. To get tons from BTU/h, divide by 12,000: a 24,000 BTU/h air conditioner is 2 tons.
About 32,500 BTU/h in zones 1 to 3, 45,000 BTU/h in zone 4, and 55,000 BTU/h in zones 5 and 6, for a home with average insulation and 8 ft ceilings. Better insulation lowers it, and poor insulation raises it.
No. An air conditioner that's too big cools the room quickly and shuts off before it removes the humidity, so the room feels cold and clammy. Pick a unit close to the size you need.
Related calculators
- Furnace Size Calculator
Furnace size in BTU/h from floor area, climate zone, and insulation, with the input rating for the AFUE.
- Heat Loss Calculator
Heat loss in BTU/h and watts through walls, windows, doors, ceilings, and floors, plus air leakage.
- Pool Shock Calculator
How much cal-hypo, dichlor, or liquid chlorine to add to shock a pool, from its volume and the free chlorine you need.
- Pool Salt Calculator
How much salt to add to a saltwater pool from its volume and salt level, and how many bags to buy.
- Pool Volume Calculator
How many gallons or liters your pool holds, from its shape, size, and shallow and deep end depths.
- Voltage Drop Calculator
Voltage drop in volts and percent for a wire size, length, and current, with the voltage at the load.