BTU Calculator
Size the room, not the catalogue.
BTU Calculator
Estimate the cooling load of a room and translate it into BTU, tons, and a common air-conditioner size.
Above two, each adds 600 BTU/h
A planning estimate, not a Manual J load calculation. Whole-house systems, vaulted ceilings, large glass areas, and duct losses need a professional room-by-room assessment.
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What This Calculator Does
The BTU Calculator estimates the cooling load of a room from its floor area, ceiling height, insulation, sun exposure, occupants, and whether a kitchen is included. It returns BTU per hour, tons of cooling, kilowatts, and the nearest common air-conditioner size, so a showroom label can be compared with the room rather than with the last unit that happened to be on sale.
How It Works
The model starts at 20 BTU per hour per square foot for an 8-foot ceiling with average insulation and average sun. Ceiling height scales that base proportionally. Insulation and sun apply the stated multipliers, each additional person above two adds 600 BTU per hour, and a kitchen adds 4,000. The total is rounded to the nearest 100 BTU per hour. One ton of cooling is 12,000 BTU per hour and one BTU per hour is about 0.293 watts.
Example
A 300 sq ft room with an 8-foot ceiling, average insulation, sunny exposure, and two occupants has a base load of 300 x 20 = 6,000 BTU/h. The sunny factor raises it to 6,600 BTU/h, or 0.55 tons. The nearest common size at or above that is an 8,000 BTU/h unit. Adding a kitchen raises the load to 10,600 BTU/h, which points to 12,000 BTU/h.
Formula
area x 20 BTU/h per sq ftbase x (ceiling height / 8)base x insulation factor x sun factor+600 BTU/h for each person above two+4,000 BTU/h when a kitchen is includedBTU/h / 12,000Reading the Results
BTU per hour
The estimated heat the room gains on a design day, and therefore the cooling capacity the unit must remove. It is a snapshot of one room, not a whole-house figure.
Recommended unit size
The smallest common nominal size at or above the estimate. Slightly larger is normal because units are sold in fixed steps; much larger is not better.
Tons
A convenience unit in the trade: one ton is 12,000 BTU per hour. A 0.55-ton load is a small room; a 3-ton unit is a large living area.
Sources and References
ASHRAE Handbook - Fundamentals
Cooling load principles and heat gain components
The professional reference behind the simplified factors used here. A full calculation accounts for each surface, window, and internal load.
ACCA Manual J - Residential Load Calculation
Room-by-room residential heating and cooling load method
The standard method for equipment sizing. Use it when the result matters, especially for a whole-house system.
U.S. Department of Energy - Energy Saver: Room Air Conditioners
Sizing and efficiency guidance
Confirms that an oversized unit cools quickly, cycles on and off, and removes less humidity.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
Is 20 BTU per square foot a reliable rule?
It is a starting point for a typical room with an 8-foot ceiling, average insulation, and average sun. Large windows, a west-facing wall, a top-floor flat, or a poorly insulated room can push the real load well above it. The adjustments here are still simpler than a Manual J calculation.
Why not buy a bigger unit for faster cooling?
An oversized unit reaches the thermostat setpoint quickly and shuts off before it has removed much humidity. The room feels cold and clammy, and the compressor cycles more often. Matching capacity to the load is more comfortable and usually uses less energy.
What is a ton of cooling?
One ton is 12,000 BTU per hour, a historical reference to the cooling effect of melting a ton of ice in a day. It is a rate of heat removal, not the weight of the equipment.
Can I use this for heating as well?
No. Heating load depends on the outdoor design temperature, the building envelope, and the heating system, and the multipliers are different. Use a heating load calculation or a professional assessment for a heat pump or furnace.
Does the calculator handle open-plan or vaulted spaces?
Only approximately. It scales for ceiling height, but it does not model air stratification, a connected stairwell, or heat moving in from an open kitchen. For a vaulted or open-plan space, treat the result as a lower bound and get a room-by-room assessment.