🪟What size window air conditioner does this room need?

✓ Tested & verified Updated: Aug 8, 2026 How we calculate this

Window AC size calculator

Room length (ft)

Room width (ft)

Ceiling height (ft)

Sun on this room

Insulation

People in the room

It is a kitchen (+4,000 BTU)

5,000 BTU/h≈ 1.5 kW

The room works out at 4,160 BTU/h — 5,000 is the nearest size actually sold.

Floor area: 208 sq ft
In tons of refrigeration: 0.4
Working rate: 20 BTU/h per sq ft

Above 15,000 BTU most window units need a dedicated 240 V circuit — worth checking before you buy, not after.

Try also

What size window air conditioner do you need?

Twenty BTU per hour for every square foot of floor, then corrections. A 16 × 13 ft bedroom is 208 sq ft and lands at 4,160 BTU/h, so the 5,000 BTU unit that sits at the bottom of every shop’s range is the right one — and the two people sleeping in it do not change that.

Window units are the machine this rate was written for. They sit in the wall of the room they cool, with the hot side outside and nothing between the cold side and you, so almost nothing is lost on the way. That is why a window unit and a portable rated at the same BTU do not cool the same room.

Two limits decide the buy as often as the BTU does. Check the window opening: most 5,000 to 12,000 BTU units need 22 to 36 inches of width and a sash that closes onto the top of the chassis. Check the circuit: above about 15,000 BTU you are usually into a dedicated 240-volt outlet, which is an electrician rather than a delivery.

Window units are bought in a hurry, usually during the first hot week of the year, and the decision is made standing in an aisle in front of four boxes. These are the four numbers worth having on your phone before you get there.

Unit	Cools about	Window width needed	Typical circuit
5,000 BTU/h	up to 250 sq ft	22–36 in	115 V, shared
8,000 BTU/h	up to 400 sq ft	24–39 in	115 V, shared
10,000 BTU/h	up to 500 sq ft	26–41 in	115 V, own circuit
12,000 BTU/h	up to 600 sq ft	27–42 in	115 V, own circuit
15,000 BTU/h	up to 750 sq ft	28–44 in	115 V or 230 V
18,000 BTU/h	up to 900 sq ft	29–45 in	230 V, dedicated

The “cools about” column is the base rate with no corrections. A sunny top-floor room with four people in it is one line further down the table than its floor area suggests.

Frequently asked questions

What size window air conditioner for a 300 sq ft room?

6,000 BTU/h before corrections, which is a stocked size. Add a fourth and fifth person and it becomes 7,800 and you move to the 8,000. Give it a west-facing window with no shade and 6,600 also fits the 8,000. The corrections matter more than the base rate at this size, because the steps on the ladder are close together down here.

Can a window unit be too big for the room?

Yes, and it shows up faster than with any other type. The unit is in the room, so you hear every start; oversized means short, loud cycles instead of a long quiet one. It also leaves the room cold and damp, because dehumidification only happens while the compressor runs and an oversized unit hits the setpoint before it has removed much moisture.

Does the window itself change the size I need?

The glass does, not the opening. A room with a large single-glazed west-facing window gains heat all afternoon, which is the 10% sun correction, and single glazing is also part of the insulation correction. The opening size only decides whether the unit physically fits — check the minimum and maximum sash width in the specification before ordering.

What circuit does a window air conditioner need?

Up to about 12,000 BTU most units run on an ordinary 115 V outlet drawing under 12 amps, though they should not share the circuit with anything else that has a motor. Between 12,000 and 15,000 the amp draw gets close to the limit of a general-purpose circuit, and above 15,000 units are commonly 230 V with a plug that will not fit a normal socket.

How does a window unit compare to a mini-split for the same room?

Same cooling, very different install and running cost. The window unit costs a few hundred, goes in yourself in an afternoon, blocks a window and is noticeably louder. A mini-split costs several times more with an installer, is close to silent indoors, and its inverter compressor modulates instead of cycling — so it holds the temperature more evenly and uses less electricity over a season.

Should I match the size of the unit I am replacing?

Only after checking it was right. Replacing like for like copies a decision somebody made years ago, possibly for a room that has since been insulated or re-glazed. Work out the number first; if it lands within one step of the old unit, buy that size with confidence rather than by inheritance.

How we calculate this

Sizing uses the ENERGY STAR room air conditioner rate of 20 BTU per hour per square foot of floor at an eight-foot ceiling, scaled by ceiling height ÷ 8 and corrected for sun (±10%), insulation (+15% poor, −10% good), occupancy (+600 BTU/h per person past two) and kitchens (+4,000 BTU/h). The result rounds up to the capacities window units are built in: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 21,000 and 24,000 BTU/h. No derating is applied — unlike a portable, a window unit rejects its heat outdoors through the wall it is mounted in, so its label capacity is what reaches the room. Electrical and window-opening notes are typical manufacturer specifications rather than calculated figures.

Worked example

With Room length (ft) 16, Room width (ft) 13, Ceiling height (ft) 8, Sun on this room Average light, Insulation Average — typical home, People in the room 2, this page works out 5,000 BTU/h ≈ 1.5 kW. The room works out at 4,160 BTU/h — 5,000 is the nearest size actually sold.

Floor area208 sq ft
In tons of refrigeration0.4
Working rate20 BTU/h per sq ft

Sources: