Egress window calculator

What size must an egress window be?

Under the International Residential Code, the net clear opening must be at least 5.7 square feet, at least 24 inches high and at least 20 inches wide, with the bottom of the opening no more than 44 inches above the floor. (Section R310.2 in the 2021 IRC; R319.2 in the 2024 edition.)

Is there an exception for ground-floor windows?

Yes — a grade-floor opening (its sill within 44 inches above or below the ground outside) may be 5.0 square feet. Basement openings need the full 5.7.

Why doesn’t a 24 × 20 inch opening pass?

It meets both minimum dimensions but is only 480 in², or 3.33 sq ft. At 20 inches wide, the opening must be about 41 inches high to reach 5.7 sq ft.

What does “net clear opening” mean?

The actual open space when the window is opened normally — not the rough opening or the glass size. Measure the clear width and height with the sash open.

What about basement window wells?

The IRC requires a well of at least 9 sq ft, at least 36 inches in projection and width, and a permanently attached ladder or steps if it is deeper than 44 inches.

Does a double-hung window count as egress?

Only the open part counts. In a double-hung window only the lower sash opens, so the clear height is roughly half the window height, minus the frame and the sash. Many standard double-hung windows therefore fall short of the minimum height or area, while casement and sliding windows, which open further, often pass. Measure the clear opening with the sash open as far as it goes, or look for the manufacturer's stated net clear opening.

Which rooms need an egress opening?

The IRC requires an emergency escape and rescue opening in sleeping rooms and in basements (Section R310.1), with narrow exceptions for small spaces such as a basement used only for mechanical equipment. The opening must be operable from the inside without keys, tools or special knowledge. Which edition applies, and what is required in your house, is decided by your local building department.

The four checks

The calculator tests four separate requirements: the net clear opening area (at least 5.7 square feet, or 5.0 for a grade-floor opening), the net clear height (at least 24 inches), the net clear width (at least 20 inches), and the height of the bottom of the opening above the finished floor (no more than 44 inches). All four must pass, and passing three does not make up for the fourth. The area is width × height ÷ 144 for a rectangular opening, using the clear dimensions with the window open.

Worked example: four openings

A casement with a clear opening 24 inches wide and 36 inches high has an area of 6.00 square feet, and with the bottom 40 inches above the floor it passes all four checks; raise the sill to 48 inches and it fails the sill height. A 30 by 24 inch opening is 5.00 square feet: enough for a grade-floor window (5.0 square feet), but not enough for a basement window (5.7). A 20 by 24 inch opening meets both minimum dimensions but is only 3.33 square feet; at 20 inches wide it would need to be 41.0 inches high.

Common mistakes

Measuring the glass or the rough opening instead of the clear opening is the first: the frame, sash and any insect screen frame reduce the opening. The second is forgetting the sill height; a large window mounted high fails the 44 inch rule. A third is treating the 5.0 square foot grade-floor value as a general rule; it applies only where the sill is at or near the outside grade. Finally, a window well, its ladder and any cover are part of the egress path and have their own requirements, as the FAQ above notes.

Code minimums for planning only. Your local building department decides which code edition and amendments apply. How we check our calculators.

Sources: 2021 International Residential Code, Section R310 (emergency escape and rescue openings; renumbered R319 in the 2024 edition) as published by the International Code Council. Local amendments vary. Related: square footage calculator, stair calculator, rough opening calculator and ramp calculator.