What size AC do I need?
A home needs roughly 20 to 25 BTU per square foot to cool, adjusted for climate, ceilings, sun, and the people in the room. Size a central system in tons or a window unit in BTU, and see why a bigger AC is not a better one.
AC size calculator
BTU · tonsNothing you enter here leaves your browser. The math runs on your device. Figures round to sensible increments; your home's real load depends on details a rule of thumb cannot see.
Where the number comes from
The base of a cooling estimate is a rule of thumb: about 20 to 25 BTU per hour for each square foot of living space, the figure used across the major HVAC manufacturers' sizing guides. A hot, humid climate sits at the top of that band; a cool northern one sits at the bottom. The calculator starts from your climate zone and adjusts from there.
Then come the corrections. Tall ceilings mean more air to cool, so the load scales with ceiling height. A sunny room runs about 10% higher and a shaded one about 10% lower. Each person past two adds roughly 600 BTU, and a kitchen adds about 4,000 for the heat of cooking. These are the ENERGY STAR room-unit adjustments, applied consistently.
Central systems come in half-ton steps, where one ton is 12,000 BTU per hour, so the raw load rounds to the nearest nominal size. Window and portable units use the ENERGY STAR area chart directly.
Why bigger is not better
It is tempting to round up and buy more cooling than you need. With air conditioning that backfires. An oversized unit blasts the air cold in a few minutes, hits the thermostat target, and shuts off, all before it has run long enough to wring humidity out of the air. The result is a house that feels cold and damp at the same time.
The constant stopping and starting, called short-cycling, also wears the compressor and uses more energy, not less. A right-sized system runs longer, gentler cycles that actually dehumidify. When a contractor's Manual J calculation comes in lower than a round number, that lower number is usually the better buy.
Common questions
What size AC do I need for a 1,500 sq ft house?
As a starting point, a home uses roughly 20 to 25 BTU per square foot, so 1,500 square feet is in the range of 30,000 to 36,000 BTU, or about 2.5 to 3 tons of central cooling. Climate, ceiling height, sun, and insulation move that number, which is why the calculator asks for them. A contractor's Manual J is the real answer before you buy.
How many square feet does 1 ton of AC cool?
One ton equals 12,000 BTU per hour. The common 20 to 25 BTU per square foot rule works out to roughly 500 to 600 square feet per ton, which is what this calculator uses. Be aware it runs generous: ACCA, the HVAC design body, found real Manual J load calculations often land well above 600 square feet per ton for a well-built home, because good insulation and tight construction cut the load. Treat the rule as a shopping estimate, and trust a contractor's Manual J when it comes in lower.
Is it bad to oversize an air conditioner?
Yes, and this surprises people. An oversized AC cools the air quickly and then shuts off before it has run long enough to pull humidity out. You get a house that is cold but clammy, plus more wear from the constant stopping and starting, called short-cycling. Matching the load beats padding it. Bigger is not better with air conditioning.
What size window AC do I need for a 12x12 room?
A 12 by 12 room is 144 square feet, which the ENERGY STAR chart puts at about 5,000 BTU. Add roughly 10% if the room is very sunny, subtract 10% if it is well shaded, add 600 BTU for each regular occupant over two, and add 4,000 BTU if it is a kitchen. Switch the calculator to window / room mode to size it.
Should I size a central system or a window unit?
Window and portable units are rated by BTU and sized off the ENERGY STAR area chart, one room at a time. Central systems are rated in tons and sized to the whole cooled area with climate and other factors folded in. The calculator does both: pick central for a whole-home system, or window / room for a single unit.