Egress Window Size Calculator
Check a bedroom or basement window against the 2021 IRC R310 emergency-escape minimums: net clear opening, clear width, clear height and sill height. Enter the actual clear dimensions and see, criterion by criterion, whether the opening passes or falls short.
Calculator
A 34 × 36 in clear opening is 8.50 sq ft — meets the 2021 IRC R310 minimums (≥ 5.7 sq ft, width ≥ 20 in, height ≥ 24 in, sill ≤ 44 in). The 20×24 in minimums alone give only 3.33 sq ft — they do not add up to 5.7. Local amendments prevail — confirm with your building official.
Every sleeping room, and most basements, needs at least one window (or door) big enough to climb out of in a fire and for a firefighter in gear to climb in. The 2021 IRC spells this out in Section R310: the opening must give a net clear opening of at least 5.7 square feet (5.0 for an opening at grade), with a clear width of at least 20 inches, a clear height of at least 24 inches, and a sill no higher than 44 inches above the finished floor. This tool checks all four at once.
The catch that trips people up is that these are four separate minimums, not a menu. An opening can clear the width and height minimums and still fail on area — because the 20-inch and 24-inch minimums do not multiply up to 5.7 square feet. Enter your real clear dimensions (measured with the window fully open, not the rough or nominal size) and read each criterion on its own line.
Formula
clear_opening_sq_ft = width_in × height_in ÷ 144
The opening passes only when every one of these is true:
clear_opening ≥ 5.7 sq ft(or ≥ 5.0 sq ft at grade)width_in ≥ 20height_in ≥ 24sill_in ≤ 44
The 144 converts square inches to square feet (12 × 12). Width and height are the clear dimensions of the opening when the window is fully operated — not the glass size and not the frame’s outside dimensions.
Worked example
A double-hung bedroom window gives a clear opening of 34 in wide by 36 in high, with the sill 40 in above the floor, above grade:
34 × 36 ÷ 144 = 8.5 sq ft
8.5 sq ft clears the 5.7 minimum; 34 in beats the 20-in width; 36 in beats the 24-in height; and the 40-in sill is under the 44-in cap. All four pass — the opening meets R310.
Now the classic trap: an opening at exactly the 20 in by 24 in minimums gives 20 × 24 ÷ 144 = 3.33 sq ft — well under 5.7. It satisfies width and height yet fails on area. To reach 5.7 sq ft you must exceed at least one of the linear minimums, which is why real egress windows are noticeably larger than 20 by 24.
Clear opening, window wells and local amendments
Measure the clear opening, not the window you bought. On a double-hung window the sash only rises so far, so the clear height is roughly half the glass height; on a casement the operable width is limited by the hardware and how far the sash swings. That is why casements are popular for egress in narrow basement openings: a single sash can crank fully open and give more clear width than a slider or double-hung of the same size. Always test the real opening with the window fully operated.
A basement bedroom usually needs a window well, and R310 has rules for that too: the well must give room to open the window and, if it is deep, a permanently attached ladder or steps. Bars, grilles and insect screens are allowed only if they release from the inside without tools, keys or special knowledge. These minimums come from the 2021 IRC; earlier and later editions differ slightly, and your local jurisdiction may amend them. Treat this check as a planning tool and confirm the requirement with your local building official before you order a window or cut an opening.
Frequently asked questions
What size window meets egress?
Under the 2021 IRC R310 the net clear opening must be at least 5.7 square feet (5.0 at grade), with a clear width of at least 20 inches, a clear height of at least 24 inches, and a sill no more than 44 inches above the floor. All four must be met at the same time, measured with the window fully open.
Is a 20 by 24 inch window big enough for egress?
No. A 20 by 24 inch opening is only 3.33 square feet (20 × 24 ÷ 144), far short of the 5.7 square foot minimum. The 20-inch and 24-inch figures are separate minimums that do not multiply up to the required area — a real egress opening has to be larger than the bare linear minimums on at least one dimension.
How do I calculate net clear opening?
Multiply the clear width by the clear height in inches and divide by 144 to get square feet. Use the unobstructed opening you can actually pass through with the window fully operated, not the glass size or the frame’s outside dimensions.
What is the maximum sill height for an egress window?
44 inches above the finished floor under R310, so the opening is reachable in an emergency. If your sill sits higher than that, the window does not qualify for egress no matter how large the opening is.
Does every bedroom need an egress window?
Generally yes — the IRC requires an emergency escape and rescue opening in every sleeping room and in basements with habitable space, unless the room opens directly to the outside through a door. Local amendments vary, so confirm with your building official.