Room AC BTU calculator

How many BTUs does your room need?

A room AC BTU calculator answers a size question, not a shopping question. The number on this page is British thermal units per hour, the rate heat has to leave the room to hold a comfortable temperature. Divide by 12,000 if a salesperson insists on tons. A 400-square-foot bedroom is not a 3-ton central system, and this calculator will not round you up to one.

Start with the square feet in the bar at the top. The sketch multiplies that area by 22 Btu per square foot, which is a zone-4 baseline for a normal room, not a sunroom and not a warehouse. Your city then applies a climate factor. Zone 1, the hottest, multiplies by 1.28. Zone 2 is 1.18, zone 3 is 1.08, zone 4 is 1.00, zone 5 is 0.90, zone 6 is 0.82, and zone 7 is 0.75. If you have not picked a city, the calculator uses zone 4 so the number is still usable.

Windows, insulation, and the people in the room

Glass direction matters because the sun does not hit every wall the same way. North-facing glass multiplies by 0.92, east by 1.00, south by 1.06, and west by 1.14. West is the default because late-day sun is the usual complaint. Insulation multiplies again: poor by 1.18, typical by 1.00, good by 0.84. A ceiling taller than 8 feet adds 4 percent per extra foot. Each person after the second adds 400 Btu/h. A kitchen in the same room adds 3,500 Btu/h for the cooking heat. The result is rounded to the nearest 100, and it will not drop below 4,000.

What to buy with the number

Match a window unit or a single zone to the room, not to the whole house. If the sketch says 8,000 Btu/h, a 12,000 Btu window unit is already the next size, not the starting point. Oversizing a room unit short-cycles it: the air gets cold, the humidity stays, and the machine wears the compressor. Undersizing runs all afternoon and never catches up. Use the low end of the manufacturer's range when the room is shaded and well insulated. Use the high end when the glass faces west and the insulation is poor.

This is not a Manual J. A real load calculation measures the windows, the attic, the ducts, and the leaks. Use this number to walk into that conversation with a size in mind, and to check a quote that jumps two sizes without saying why. The city pages for mini-split cost use the same sketch on an example home so you can see what the equipment costs after the size is chosen.

If you rent

A window unit is often yours to choose. A sleeve through the wall, or a mini-split line set, usually is not. The BTU number does not change with the lease. Permission does. If the circuit you were given is already near its limit, open the circuit load calculator before you add a 1,200-watt unit to a fridge and a microwave.

Reviewed as a method on 2026-09-01. The city you pick supplies the climate zone. The price of the machine is on the mini-split page, not here. Size first. Then look at the dollars.

If the number feels high, check the inputs before you buy a larger unit. West glass and poor insulation are doing a lot of the work. A shaded room with good insulation in a mild city should come down. Then open the mini-split cost calculator and keep the size you just calculated. Do not let a showroom tonnage replace it.