BTU Calculator
Estimate the air conditioner capacity your room needs based on its size, ceiling height, and how sunny it is.
Example
A 300 sq ft room with standard 8 ft ceilings and average sun: 300 × 20 = 6,000 BTU base × 1.0 (height) × 1.0 (sun) = 6,000 BTU, or about 0.50 tons of cooling.
How it works
Start from the rule of thumb of ~20 BTU per square foot, then multiply by a ceiling-height factor (height ÷ 8 ft standard) and a sun-exposure factor (heavily shaded 0.9, average 1.0, very sunny 1.1). Tonnage is BTU ÷ 12,000.
Good to know
The BTU Calculator estimates how much cooling capacity an air conditioner needs to keep a single room comfortable. You enter the room's floor area, ceiling height, and how much direct sun it gets, and it returns a BTU-per-hour figure plus the equivalent AC size in tons. It is aimed at homeowners and renters sizing a window unit, portable AC, or mini-split before they buy, rather than at HVAC pros doing a full Manual J load calculation.
Reach for it when you are shopping for a room air conditioner and want a quick sanity check on the capacity printed on the box, or when an existing unit struggles to keep a sunroom or top-floor bedroom cool and you suspect it is undersized. Because it works one room at a time, run it separately for each space you want to cool rather than entering whole-home square footage.
To read the result, the large BTU number is your target hourly cooling output, the "AC size (tons)" converts that to the unit most central and mini-split equipment is rated in, and the "Base (20/sqft)" line shows the unadjusted starting point before the height and sun multipliers are applied. Comparing the base to the final number tells you how much your ceiling height and sun exposure are adding to the load.
One practical caveat: this is a square-footage shortcut, so it does not see insulation quality, window count, climate, or appliances. Oversizing by a wide margin is counterproductive too: an AC that is too large short-cycles, cools the air fast but never runs long enough to pull out humidity, leaving the room cold and clammy. Use the note's add-ons for extra people and kitchens, then aim for a unit close to the result rather than rounding far above it.
Frequently asked questions
How many BTU do I need per square foot?
A common rule of thumb is about 20 BTU per square foot of living space for cooling. This calculator uses that baseline, then adjusts for taller ceilings and sun exposure since both change the actual heat load.
How do BTU convert to AC tons?
One ton of air conditioning equals 12,000 BTU per hour. To get tonnage, divide the required BTU by 12,000, so a 6,000 BTU need is 0.5 tons and 24,000 BTU is 2 tons.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
What size air conditioner do I need for a 12x12 room?
A 12x12 room is 144 square feet, which at roughly 20 BTU per square foot works out to about 5,000 BTU with standard 8-foot ceilings and average sun. A common 5,000 BTU window unit is typically sufficient for a space that size.
Is it better to oversize or undersize an AC unit?
Neither extreme is ideal; matching capacity to the load is best. An undersized unit runs constantly and never reaches the set temperature, while an oversized one short-cycles and removes too little humidity, leaving the room cool but damp.
What is a BTU and what does it measure?
BTU stands for British Thermal Unit, a measure of heat energy. For air conditioners it is usually expressed as BTU per hour, meaning how much heat the unit can remove from a room in one hour.
How many square feet will a 12,000 BTU air conditioner cool?
At the common 20 BTU per square foot baseline, 12,000 BTU corresponds to roughly 600 square feet under average conditions. Higher ceilings, strong sun exposure, or heat-generating appliances reduce the effective area it can handle.
Does ceiling height affect how many BTU I need?
Yes, because cooling capacity is really about the volume of air, not just the floor area. A taller ceiling means more air to cool, so this calculator scales the BTU figure by ceiling height divided by the 8-foot standard.
How many watts does an air conditioner use per BTU?
Power draw depends on the unit's efficiency rather than a fixed ratio, but a rough guide is that wattage is roughly the BTU rating divided by the EER (Energy Efficiency Ratio). For example, a 12,000 BTU unit with an EER of 10 draws about 1,200 watts.
Should I add BTU for a kitchen or extra people in the room?
Yes; heat sources raise the cooling load beyond what square footage alone suggests. A common adjustment is about 600 BTU for each occupant beyond two people and roughly 4,000 BTU if the space includes a kitchen.
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