Egress Window Calculator

The International Residential Code Section R310 sets four net clear opening limits for an emergency escape window: a minimum opening area of 5.7 square feet (5.0 square feet for a grade-floor or below-grade opening), a minimum opening width of 20 inches, a minimum opening height of 24 inches, and a maximum sill height of 44 inches above the floor. The calculator checks all four and flags any that fall short. Every sleeping room and basement with habitable space generally needs at least one opening that meets these limits.

The minimum width and minimum height are separate requirements from the minimum area, and a window at exactly 20 by 24 inches gives only 480 square inches, or 3.33 square feet, far below the 5.7 square foot minimum. To pass on area you have to open wider, taller, or both, since area equals clear width times clear height. This mismatch is the single most common reason a window misses egress.

The net clear opening is the actual unobstructed space you can climb through when the window is fully open, measured from the inside. Measure the open width and open height of the gap itself, not the glass size or the rough frame. On a single-hung or double-hung window only one sash moves, so the clear height is roughly half the window height, which is why many tall double-hung units fail.

The 5.0 square foot minimum applies to grade-floor and below-grade openings, which the code defines as openings whose sill sits no more than 44 inches above or below the adjacent finished grade. Typical basement windows served by a window well and many ground-floor windows qualify. Choose the at or below grade option in the calculator to apply the 5.0 square foot threshold, since upper-floor bedroom windows use the full 5.7 square feet.

The window sill can be no more than 44 inches above the finished floor so an occupant can reach and use the opening during an emergency. If the opening sits below the adjacent ground, it must be served by a window well that meets the code's size and ladder requirements. The calculator flags any sill above 44 inches as a fail.

No. This tool applies the model IRC R310 limits, but your local jurisdiction may adopt amendments and the final call rests with your building inspector. Use the result to size or shop for a window, then confirm the specific opening, window well, and installation details with your local building department.