Match the room’s square footage to an ENERGY STAR BTU range first, then adjust 10% for shade, 600 BTUs per extra person, and check the unit’s voltage.
A 350-square-foot bedroom and that same room baking in full afternoon sun need different machines, and buying by BTU alone is how people end up with a unit that never quite catches up. The U.S. Department of Energy’s ENERGY STAR program publishes a straightforward sizing method: measure, match, then adjust for the conditions in your home. This guide walks through each step, including the voltage detail most shoppers skip until the box is already in the trunk.
How Do You Measure A Room For A Window AC?
Multiply the room’s length by its width to get square footage, then match that number to the sizing chart below. ENERGY STAR’s method starts there and adds a few measurement rules for spaces that aren’t simple rectangles.
- Triangular areas: multiply length by width, then divide by two.
- Connected rooms without doors: add both rooms together, since one unit has to cool the full space.
- Open floor plans: treat the whole uninterrupted area as one room rather than sizing to a single zone.
Getting this number right matters more than any other step. A unit sized for half the actual area will run constantly and still leave the far corner warm.
What Size Window Air Conditioner Do You Need?
Use ENERGY STAR’s chart to translate square footage into BTUs per hour, then adjust for the four conditions that change the real cooling load.
| Room Size (sq. ft.) | BTUs Per Hour |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
Those four adjustments, straight from ENERGY STAR’s guidance, are where most sizing mistakes happen:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two regular occupants: add 600 BTUs for each additional person.
- Kitchen use: increase capacity by 4,000 BTUs to handle cooking heat.
A sunny 320-square-foot living room with three regular occupants, for example, starts at 8,000 BTUs, climbs to roughly 8,800 for the sun, then adds 600 for the third person — pointing toward a 9,000 or 10,000 BTU unit rather than the 8,000 the raw square footage suggested. If you’d rather start from a shortlist, this roundup of tested 120-volt window air conditioners covers models that plug into a standard outlet.
Does A Bigger BTU Rating Always Cool Better?
No. Oversizing is a real problem, and ENERGY STAR frames its chart as a matching guide for comfort and savings rather than a case for maximum power. A unit with too much capacity cools the air fast, shuts off before it has pulled much humidity out, and leaves the room feeling clammy. It also cycles on and off more often, which wears the compressor and wastes electricity compared with a properly matched unit running steadily. The right BTU number is the one that matches your adjusted room load — not the biggest one that fits the window.
What Voltage And Circuit Does Your Unit Need?
Standard household receptacles in the U.S. run on 115-volt branch circuits, per ENERGY STAR’s room AC fact sheet — and that detail decides which units are even installable in your home.
- Smaller 115-volt units: plug into a normal outlet.
- Larger 115-volt units: may need a dedicated circuit to avoid tripping a shared one.
- 230-volt units: may require a special circuit, and higher-capacity models use 230 volts rather than a standard 120-volt plug.
Check the circuit before you buy, not after. An electrician’s visit to add a dedicated line can cost more than the difference between two units, and a 230-volt model simply won’t run on a standard bedroom outlet.
Run through this sequence once and the decision is settled: measure length times width, match the total to the chart, apply the shade, occupant, and kitchen adjustments, then confirm the voltage your outlet can actually support. Buy the unit that matches your adjusted number. ENERGY STAR’s room air conditioner guidance is worth a look before you commit, since it’s the source of the sizing chart and the voltage rules above.
FAQs
Can I cool two connected rooms with one window unit?
Only if you size for both spaces combined. ENERGY STAR says to add together the size of rooms that aren’t separated by doors, because the unit has to cool the whole open area. Sizing for one room and expecting the air to drift into the next usually leaves both spaces underwhelming.
Do I need an electrician for a 230-volt window air conditioner?
Likely yes. A 230-volt unit may require a special circuit, and these models don’t use the standard 120-volt plug found in most homes. Confirm the outlet type and circuit capacity before purchase so installation doesn’t stall after the box arrives.
Why does a sunny room need a bigger unit?
Sunlight through windows adds heat that the unit has to remove, so ENERGY STAR recommends increasing capacity by 10% for very sunny rooms. Heavily shaded rooms get the opposite treatment — reduce capacity by 10%, since they carry a lighter cooling load.
References & Sources
- ENERGY STAR. “Room Air Conditioners.” Source of the BTU sizing chart, adjustment rules, and voltage guidance.
- ENERGY STAR. “Room AC Fact Sheet.” Confirms 115-volt standard receptacles and 230-volt requirements for larger units.
- Consumer Reports. “How to Size a Window Air Conditioner.” Independent guidance on matching capacity to room conditions.
