Key takeaways
- Measuring the glass instead of the opening. The unit sits in the open frame, so jamb-to-jamb width is what counts.
- Forgetting the storm window. An exterior storm sash blocks the condenser and must come off.
- Ignoring maximum width. Side panels extend only so far, usually to 36 or 38 inches. A 42 inch wide opening needs plywood filler or a wider kit.
- Buying too many BTU. A 12,000 BTU unit in a 150 square foot room cools fast, shuts off before dehumidifying, and leaves the room clammy. Size to the chart.
- Skipping the weight check. A 70 pound unit on a 60-year-old softwood sill without a bracket is a hazard.
- Assuming any outlet works. 5,000 to 8,000 BTU units draw 4 to 7 amps and are fine on a shared 15 amp circuit; 12,000 BTU and above should have a dedicated circuit.
- Not checking the sash lock. A sash that will not stay up needs the unit’s top rail and possibly an L-bracket to keep it from dropping onto the unit.
What's inside
The Short Answer
Open the window fully and measure three things: the clear width of the opening from jamb to jamb, the clear height from the sill to the bottom of the raised sash, and the depth of the sill from the inside stop to the outside edge. Most double-hung windows offer a 23 to 36 inch wide opening and at least 14 to 16 inches of height with the sash raised; most window air conditioners need a minimum opening of 22 to 27 inches wide and 13 to 16 inches tall. Compare your numbers against the unit’s listed minimum and maximum window width, and confirm the sill can carry 40 to 80 pounds. If the window is a slider or casement, you need a different mounting kit or a different unit entirely.
Sizing the cooling capacity is the second step: 5,000 BTU covers up to 150 square feet, 6,000 BTU up to 250, and 8,000 BTU up to 350. Both measurements matter, and the window one is the one people skip.
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Standard Sizes
| Unit class | Room coverage | Typical unit width x height x depth (in) | Typical unit size (cm) | Min window width | Min window height | Weight | Price range |
|---|---|---|---|---|---|---|---|
| 5,000 BTU | Up to 150 sq ft | 16 x 12 x 15 | 41 x 30 x 38 | 22 to 23 in (56 to 58 cm) | 13 in (33 cm) | 36 to 45 lb | $130 to $170 |
| 6,000 BTU | Up to 250 sq ft | 18 x 13 x 16 | 46 x 33 x 41 | 23 to 25 in (58 to 64 cm) | 13.5 to 14 in (34 to 36 cm) | 40 to 55 lb | $160 to $230 |
| 8,000 BTU | Up to 350 sq ft | 19 x 14 x 20 | 48 x 36 x 51 | 25 to 27 in (64 to 69 cm) | 14 to 16 in (36 to 41 cm) | 50 to 70 lb | $250 to $350 |
| 10,000 to 12,000 BTU | Up to 550 sq ft | 21 x 15 x 22 | 53 x 38 x 56 | 27 to 29 in (69 to 74 cm) | 15 to 17 in (38 to 43 cm) | 65 to 85 lb | $350 to $500 |
| U-shaped inverter (8,000 to 12,000 BTU) | Up to 550 sq ft | 22 x 14 x 20 | 56 x 36 x 51 | 22 to 36 in (56 to 91 cm) with bracket | 13.75 in (35 cm) | 55 to 75 lb | $300 to $500 |
Current examples: the GE 5,000 BTU at about $138 and the Frigidaire 5,000 BTU at about $161 both fall in the smallest class for rooms up to 150 square feet; the Midea 6,000 BTU at about $170 covers up to 250 square feet; and the Midea 8,000 BTU inverter with heat at about $310 handles up to 350 square feet. Always read the exact minimum and maximum window width in the listing, since the side panels set the range.
How to Measure Your Space
- Open the window all the way. Raise the lower sash as high as it goes. If it does not lock in place, note that; you will need the unit’s top rail or a sash lock to hold it.
- Measure clear width. Run a tape from the inside face of the left jamb to the inside face of the right jamb at the sill level. Do not include trim. Write it down in inches.
- Measure clear height. From the top of the sill to the bottom of the raised sash. This must exceed the unit’s height plus about 1 inch for the top rail and gasket.
- Measure sill depth. From the inside window stop to the outer edge of the sill. Units need roughly 2 to 4 inches of interior sill for the front to sit on and enough exterior overhang or a support bracket for the rest.
- Check sill condition and slope. Press on the sill. Soft or rotted wood will not hold 50 pounds. Note whether the sill slopes outward, which is normal and helps drainage.
- Check the storm window or screen. If a storm window or screen frame sits outside the sash, it must be removable or it will block the unit’s rear.
- Find the outlet. Window units use a 6 foot cord and should plug directly into a grounded outlet, never an extension cord. Measure from the window to the nearest outlet.
- Measure the room. Length times width in feet. Add 10 percent if the room gets direct afternoon sun, subtract 10 percent if it is heavily shaded, and add 600 BTU for each additional person who regularly occupies the room beyond two.
By Room or Use Case
Small bedroom or office, up to 150 square feet
A 5,000 BTU unit like the GE or Frigidaire examples fits windows 22 to 36 inches wide and needs 13 inches of height. These weigh 36 to 45 pounds and can be lifted into place by one person. Check for a 12 by 12 foot room or smaller; a 12 by 14 foot room at 168 square feet is already past the comfortable range.
Medium bedroom or study, 150 to 250 square feet
A 6,000 BTU unit such as the Midea Easycool at about $170 needs a window at least 23 to 25 inches wide and about 14 inches tall. At 40 to 55 pounds it is still a one-person lift for most adults, though a second set of hands makes the outdoor tilt safer.
Living room or large bedroom, 250 to 350 square feet
An 8,000 BTU unit like the Midea inverter at about $310 needs 25 to 27 inches of width and 14 to 16 inches of height, and weighs 50 to 70 pounds. This is a two-person installation. Inverter models run quieter and hold temperature more evenly, which matters in a room you sit in for hours.
Sliding or casement windows
Standard window units are designed for double-hung windows that open vertically. Horizontal sliders and crank-out casements need a unit specifically labeled for slider or casement windows, which are taller and narrower (typically 14 to 16 inches wide and 20 to 22 inches tall), or a portable air conditioner with a slider window kit. Measure the opening the same way but note that the tall dimension is the height of the slider opening.
Upper-floor windows and rentals
Anything above the first floor should use an exterior support bracket rated for the unit’s weight, and many cities and landlords require one. Brackets add 2 to 3 inches to the required sill depth. U-shaped units, which straddle the sash and let the window close partially, are a good fit for renters because they leave the window usable and install with a bracket that needs no exterior fasteners.
Common Sizing Mistakes
- Measuring the glass instead of the opening. The unit sits in the open frame, so jamb-to-jamb width is what counts.
- Forgetting the storm window. An exterior storm sash blocks the condenser and must come off.
- Ignoring maximum width. Side panels extend only so far, usually to 36 or 38 inches. A 42 inch wide opening needs plywood filler or a wider kit.
- Buying too many BTU. A 12,000 BTU unit in a 150 square foot room cools fast, shuts off before dehumidifying, and leaves the room clammy. Size to the chart.
- Skipping the weight check. A 70 pound unit on a 60-year-old softwood sill without a bracket is a hazard.
- Assuming any outlet works. 5,000 to 8,000 BTU units draw 4 to 7 amps and are fine on a shared 15 amp circuit; 12,000 BTU and above should have a dedicated circuit.
- Not checking the sash lock. A sash that will not stay up needs the unit’s top rail and possibly an L-bracket to keep it from dropping onto the unit.
FAQ
What is the minimum window size for a window air conditioner?
For a 5,000 BTU unit, about 22 to 23 inches of clear width and 13 inches of clear height. Larger units need 25 to 29 inches wide and 14 to 17 inches tall. Check the specific listing, since the number varies by a few inches between brands.
Can a window AC fit in a window that is too wide?
Yes, up to the limit of the side panels, typically 36 to 38 inches. Beyond that, cut a piece of plywood or rigid foam to fill the gap and seal it with weatherstripping. The unit itself just needs a solid sill to rest on.
How much of the unit should hang outside?
Most units are designed to sit with a slight outward tilt of about a quarter inch per foot so condensate drains outside, with roughly one-half to two-thirds of the depth outdoors. A support bracket carries that overhang on anything over 50 pounds or above the first floor.
How many BTU per square foot do I need?
Roughly 20 BTU per square foot for a room with 8 foot ceilings and average sun. A 12 by 12 foot room at 144 square feet needs about 2,900 BTU by that math, so a 5,000 BTU unit has margin; a 15 by 20 foot room at 300 square feet needs 6,000 to 8,000. Manufacturer coverage figures in 2026 listings are generous, so lean toward the lower end of their stated range.
