Understanding your result
The headline is the window unit size to buy, in BTU per hour, from the list of sizes actually sold (5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 15,000, 18,000, 21,000, 24,000 and 25,000). The sub-line gives the cooling the room needs before rounding; the difference between the two is the unit’s spare capacity, and the “size below” row in the table tells you how far short the next smaller unit would fall, so you can judge whether it is worth the saving.
The electrical lines are estimates from a typical efficiency, and they are there to answer the practical question of what outlet and circuit the unit needs. Anything that needs 230 V, or more than about 10 A at 115 V, is flagged in the notes with a reminder that circuit work is a job for a licensed electrician. Read the actual amps from the unit’s label before buying; a high-efficiency ENERGY STAR unit draws a tenth less than the estimate.
If the room is more than 1,000 sq ft or needs more than 25,000 BTU/hr, the notes say so: a window unit cannot cool a space that large evenly, and the mini-split size calculator or the central AC size calculator is the right tool. For a room where a window unit cannot be fitted, the portable AC size calculator applies the same chart with the extra correction portable units need.
How we calculate this
The chart is the one ENERGY STAR publishes for room air conditioners, from 5,000 BTU/hr for rooms up to 150 sq ft to 34,000 for rooms up to 2,500 sq ft, with its four adjustments: reduce by 10% for a heavily shaded room, increase by 10% for a very sunny one, add 600 BTU/hr for each person beyond two who regularly occupies the room, and add 4,000 BTU/hr if the unit is for a kitchen. The chart assumes 8 ft ceilings, so the calculator scales it by the room’s air volume for taller ones. The BTU per square foot table reproduces the chart next to the whole-house figures.
In metric mode the area and ceiling are converted to square feet and feet before the lookup, and the results are shown in kilowatts; the unit sizes stay in BTU/hr because that is how they are sold in North America, while elsewhere the same units are labelled 1.5, 1.8, 2.3, 2.6 and 3.5 kW. The unit conversions page has the factors.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Sizing chart | ENERGY STAR room air conditioner chart | US EPA / DOE ENERGY STAR, “Properly sized room air conditioners” |
| Shade and sun | −10% / +10% | Same ENERGY STAR guidance |
| People and kitchen | +600 BTU/hr per person beyond two; +4,000 BTU/hr kitchen | Same ENERGY STAR guidance |
| Ceiling | +10% per foot above 8 ft | Air volume; chart assumes 8 ft |
| Open plan | +20% | Judgement: cool air lost to an adjoining room through an open doorway |
| Efficiency for the electrical estimate | CEER 11 | Federal minimum for most window units is about 11; ENERGY STAR models are 12 or more |
| Voltage | 115 V to 15,000 BTU/hr; 230 V above | Typical US window unit electrical requirements |
Assumptions last reviewed October 7, 2026.
The calculator does not know the room’s orientation, how well the window seals around the unit, or how much heat comes through an uninsulated ceiling under a hot roof, which can add a third to a top-floor room’s load. It treats the chart’s area bands as the chart does, with a step at each boundary. The guide to window and portable AC sizing covers the cases where the chart under- or over-estimates.
Two worked examples
A bedroom of 300 sq ft
A 15 × 20 ft bedroom with 8 ft ceilings, average sun, two occupants, no kitchen, a door that closes.
- Chart value for 300 sq ft: 7,000 BTU/hr
- Adjustments: sun 1.0, ceiling 1.0, no people or kitchen additions
- Cooling needed: 7,000 BTU/hr
- Unit: 8,000 BTU/hr; the 6,000 BTU/hr size below is 1,000 short
- Draw: 8,000 ÷ 11 = 727 W, 6.3 A at 115 V
The 8,000 BTU unit has 14% in hand. If the room faces north or is shaded by trees, the shaded adjustment brings the need to 6,300 BTU/hr and the 6,000 unit becomes the better buy: cheaper, quieter, and running long enough to keep the room dry.
A sunny open kitchen and dining room
A 550 sq ft kitchen-dining space with large south and west windows, three people, 8 ft ceilings.
- Chart value for 550 sq ft: 12,000 BTU/hr
- Sun: 12,000 × 1.1 = 13,200 BTU/hr
- People beyond two: 600 BTU/hr; kitchen: 4,000 BTU/hr
- Cooling needed: 17,800 BTU/hr
- Unit: 18,000 BTU/hr at 230 V, about 1,640 W and 7.1 A
The kitchen allowance alone is a third of the load, which is why kitchen units are so much larger than bedroom units of the same floor area. At 18,000 BTU/hr the unit needs a 230 V outlet; if there is none near the window, the installation cost of the circuit is part of the price, and a 230 V unit should be compared with a single-zone mini-split.
Where to find your inputs
Floor area. Length times width of the room the unit will actually cool. For an L-shaped room, add the two rectangles. Do not include a closet with a closed door.
Sun exposure. A room with windows facing north, or shaded by a porch, trees or a neighbouring building, is “heavily shaded”. A room with large west- or south-facing windows and no shade is “very sunny”. Most rooms are average.
People. The number regularly in the room when the unit runs: two for a bedroom, however many sit down to dinner for a kitchen.
CEER. The efficiency used for the electrical estimate is printed on the yellow EnergyGuide label of any unit you are considering; the amps are printed on the unit’s nameplate and in the manual.
Common mistakes
- Buying the biggest unit the window will hold. Oversizing is the most common fault with room units; it leaves the room clammy and the unit cycling.
- Forgetting the kitchen allowance. The stove, the refrigerator and the dishwasher add about 4,000 BTU/hr, two-thirds of a small unit’s capacity.
- Ignoring the ceiling. A 10 ft ceiling adds 20% to the air to be cooled; older houses and lofts often have them.
- Plugging a 12,000 BTU unit into a shared circuit. It will trip the breaker the first time a vacuum cleaner or microwave starts on the same circuit.
- Sizing a room that has no door as if it were closed. Cool air leaks into the next room; use the open-plan option or combine the areas.
- Comparing by BTU alone. Two units of the same BTU rating can differ by 20% in electricity use; compare the CEER on the label as well.
Questions people ask
- What size window AC do I need for a 12 by 12 room?
- A 144 sq ft room falls in the "up to 150 sq ft" row of the ENERGY STAR chart, which calls for 5,000 BTU per hour. A heavily shaded room can go 10% lower, which still rounds to a 5,000 BTU unit; a very sunny room needs 5,500, which means a 6,000 BTU unit. If more than two people sleep or work there, add 600 BTU each. A 5,000 or 6,000 BTU unit is the answer for almost every 12 by 12 room.
- What size window unit do I need for 300 square feet?
- 7,000 BTU per hour according to the chart, which no one sells, so the choice is between a 6,000 BTU unit (about 14% short) and an 8,000 BTU unit (14% over). For a bedroom with average sun, the 8,000 is the usual pick; for a shaded room or a room used mainly at night, the 6,000 will run longer, dehumidify better and cost less.
- Is a bigger window air conditioner better?
- No. An oversized unit cools the air to the setpoint in a few minutes and switches off before the coil has run long enough to wring moisture out of the air, so the room feels cold and damp and the compressor cycles constantly. The right size runs for 15 to 20 minutes at a time on a hot afternoon. Go up a size only if the room is sunnier, more crowded or more open than the inputs describe.
- How many amps does a window air conditioner use?
- Divide the BTU rating by the unit's combined energy efficiency ratio (CEER, printed on the yellow label) to get watts, then divide by the voltage. A 8,000 BTU unit at CEER 11 draws about 730 W, or 6.3 A at 115 V; a 12,000 BTU unit about 1,090 W, or 9.5 A; an 18,000 BTU unit about 1,640 W and needs a 230 V circuit. Starting current is briefly two to three times higher.
- Can I run a window AC on a regular outlet?
- Units up to about 12,000 BTU/hr run on a standard 115 V, 15 A circuit, but they should not share it with much else; a 10 A air conditioner and a 1,500 W space heater or hair dryer on the same circuit will trip the breaker. Units of 12,000 to 15,000 BTU/hr often want a dedicated 20 A circuit, and anything larger needs 230 V. Have a licensed electrician check the circuit before installing a large unit.
- What does "open to an adjoining room" change?
- Cool air spills through an open doorway or archway into the next room and warm air comes back, so the unit is cooling more than the room it sits in. The calculator adds 20% for an open plan. If the second room is as big as the first, treat the two as one room and enter their combined area instead; if it is much bigger, a window unit will struggle and a mini-split head in each space works better.