How many BTU do I need? (cooling)
ENERGY STAR publishes a room air conditioner sizing chart by area, with adjustments for sun exposure, occupants and a kitchen:
- Base for a 500 sq ft room: 12,000 BTU/h (the 450-550 sq ft bracket)
- Heavily shaded room: -10%. Very sunny room: +10%.
- More than 2 occupants regularly in the room: +600 BTU per extra person
- Kitchen installation: +4,000 BTU (appliance heat)
Example: a 1,000 sq ft, very sunny room with 4 regular occupants and a kitchen: 18,000 BTU/h base × 1.1 (sun) + 1,200 BTU (2 extra occupants) + 4,000 BTU (kitchen) = 25,000 BTU/h -- round up to the nearest standard size, 28,000 BTU/h. ENERGY STAR's chart assumes an 8-foot ceiling; for a taller room, this calculator scales the base BTU by ceiling height ÷ 8 as a simple volume-based adjustment (ENERGY STAR itself only says to "select a higher capacity" without giving a formula).
ENERGY STAR room-AC sizing chart
| Area to cool | BTU/h | Tons |
|---|---|---|
| 150 sq ft | 5,000 | 0.42 |
| 250 sq ft | 6,000 | 0.5 |
| 350 sq ft | 8,000 | 0.67 |
| 450 sq ft | 10,000 | 0.83 |
| 550 sq ft | 12,000 | 1 |
| 700 sq ft | 14,000 | 1.17 |
| 1,000 sq ft | 18,000 | 1.5 |
| 1,200 sq ft | 21,000 | 1.75 |
| 1,500 sq ft | 24,000 | 2 |
| 2,000 sq ft | 30,000 | 2.5 |
How many BTU in a ton?
12,000 BTU/h = 1 ton of air conditioning capacity. "Ton" is a legacy HVAC unit (the cooling effect of melting a ton of ice over 24 hours), unrelated to the equipment's physical weight. A 2-ton unit is 24,000 BTU/h; a 3-ton unit is 36,000 BTU/h.
How to size a heater or furnace
A common rule of thumb multiplies square footage by a BTU-per-square-foot factor that depends on climate:
| Zone | BTU/sq ft (published range) |
|---|---|
| Zone 1 -- hot (S. Florida, Hawaii, S. Texas) | 30-35 BTU/sq ft |
| Zone 2 -- warm (much of the South, low-elevation AZ/NV) | 35-40 BTU/sq ft |
| Zone 3 -- moderate (mid-latitude states, e.g. the Carolinas) | 40-45 BTU/sq ft |
| Zone 4 -- cool (upper Midwest/Northeast) | 45-50 BTU/sq ft |
| Zone 5 -- cold (northern tier, e.g. MN, ME) | 50-60 BTU/sq ft |
Example: a 2,000 sq ft home in a moderate climate (zone 3, 42.5 BTU/sq ft): 2,000 × 42.5 = 85,000 BTU/h (7.08 tons equivalent). This is a rule-of-thumb estimate; a whole-house furnace or boiler should be sized with a professional Manual J load calculation, which factors in insulation, windows, air sealing and your area's design temperature.
BTU to watts, kW and tons
- 1 watt = 3.412142 BTU/h (1 kWh = 3,412.14 BTU)
- 12,000 BTU/h = 1 ton
- 12,000 BTU/h = 3,517 W = 3.52 kW = 1 ton (cooling capacity, not electrical input power)
Mini splits and standard unit sizes
Mini-split heat pumps use the same BTU/h and ton sizing as window and central units -- the ENERGY STAR area-based chart above is a reasonable starting point. Standard sizes are commonly sold in steps like 9,000, 12,000, 18,000, 24,000 and 36,000 BTU/h; the calculator rounds your result up to the nearest common size so you don't undersize.
More electrical & energy tools
- Electricity cost calculator: estimate what running that AC or heater actually costs.
- Watts to amps calculator: check the current draw for a unit's wattage and voltage.
- Square footage calculator: work out a room's area first if you don't already know it.
Frequently asked questions
How many BTU do I need for a room?
It depends mostly on the room's area: ENERGY STAR's room-AC sizing chart calls for 12,000 BTU/h for a 500-sq-ft room (its 450-550 sq ft bracket), before adjusting for sun exposure, extra occupants or a kitchen. Use the calculator above with your own room's area and conditions.
How many BTU in a ton?
12,000 BTU per hour equals 1 ton of air conditioning -- "ton" is just a traditional HVAC unit (originally the cooling from melting a ton of ice in 24 hours), not a unit of weight for the equipment. A 12,000-BTU/h unit is 1 ton.
How many BTU per square foot?
For cooling, ENERGY STAR's chart works out to roughly 18-33 BTU/sq ft depending on room size (smaller rooms need more BTU per sq ft because of a fixed baseline load). For heating, a common rule of thumb is 30-60 BTU/sq ft depending on climate zone (see the table below) -- a 1,000 sq ft home in a moderate climate (zone 3, 42.5 BTU/sq ft) needs about 42,500 BTU/h.
How many BTU for 1,000 sq ft?
For cooling, ENERGY STAR's chart puts 700-1,000 sq ft at 18,000 BTU/h. For heating, it depends heavily on climate: a moderate climate (zone 3) needs about 42,500 BTU/h for 1,000 sq ft, while a cold climate (zone 5, 55 BTU/sq ft) needs about 55,000 BTU/h.
How does room size, sun and occupants change the BTU I need?
Using the ENERGY STAR chart: a 1,000 sq ft room normally needs 18,000 BTU/h. Make it very sunny (+10%), add 4 regular occupants (2 over the 2-person baseline, +1,200 BTU) and a kitchen (+4,000 BTU), and it rises to about 25,000 BTU/h -- the nearest standard unit size would be 28,000 BTU/h.
What size mini split do I need?
The same ENERGY STAR area-based sizing above is a reasonable starting point for a mini-split, since it's fundamentally the same cooling-load question. Mini splits are sized in the same BTU/h and ton units as window and central units, so use the calculator's cooling mode with your room's area, sun exposure and occupancy, then check that figure against the specific mini-split models available at that capacity.
How do I size a furnace?
A common rule of thumb multiplies your home's square footage by a BTU-per-square-foot factor for your climate zone (30-60 BTU/sq ft, see the table below): a 2,000 sq ft home in a moderate climate (zone 3, 42.5 BTU/sq ft) needs about 85,000 BTU/h. This is a ballpark for a single space or a simple estimate -- a whole-house furnace or boiler should be sized with a professional Manual J load calculation, which accounts for insulation, windows, air leakage and your area's design temperature.
How do I convert BTU to watts or kWh?
1 watt = 3.412142 BTU/h (the standard "International Table" BTU conversion; 1 kWh = 3,412.14 BTU). A 12,000-BTU/h air conditioner is about 3,517 watts (3.52 kW) of cooling capacity -- note this is COOLING capacity, not the electrical power the unit draws, which is lower (a unit's efficiency, e.g. its EER/SEER rating, determines how many input watts it needs per BTU/h of cooling).
How many BTU is an air conditioner for an air conditioner tonnage calculator?
"Tons" and "BTU/h" describe the same thing at a fixed ratio (1 ton = 12,000 BTU/h), so an AC tonnage calculator and a BTU calculator solve the same sizing problem. A 500 sq ft room's 12,000-BTU/h ENERGY STAR figure is exactly 1 ton.
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