Egress window requirements (IRC R310): does your window qualify?

A bedroom escape window must clear 5.7 sq ft, be at least 20 in wide and 24 in tall, with a sill no higher than 44 in — and all four apply at once. Here is how to screen an opening, and the area trap that catches the unwary.

By Francesco Zinghinì

An egress window is the window a person can climb out of — or a firefighter can climb in through — in an emergency. Building codes require them in sleeping rooms and finished basements for exactly that reason, and the requirement trips up more remodels than almost any other window rule, because a window can look plenty big and still fail. This guide explains the numbers behind IRC R310 and how to screen an opening before you order.

The four requirements

The International Residential Code, section R310, sets minimums for an emergency escape and rescue opening. There are four, and all four must be met at once:

Minimum clear opening area: 5.7 square feet (or 5.0 square feet for an opening at grade-floor level).
Minimum clear opening width: 20 inches.
Minimum clear opening height: 24 inches.
Maximum sill height above the floor: 44 inches.

The word doing the heavy lifting is clear. These are the dimensions of the actual hole you can pass through when the window is fully open — not the frame size, not the glass size, and not the nominal size on the order. On a double-hung window, only half the height is ever open at once, so the clear opening is roughly the sash height by the sash width. On a casement, the whole opening clears when the crank is run out. That difference is why casements so often pass egress where a same-size double-hung fails. The egress window checker takes your clear width, clear height and sill height and returns a pass or fail against each of the four rules.

Why area is the sneaky one

Here is the trap. The minimum width is 20 inches and the minimum height is 24 inches. Multiply those and you get 20 × 24 ÷ 144 = 3.33 square feet — well short of the 5.7 square foot area minimum. So a window can satisfy the width rule and the height rule and still fail on area. You cannot hit the minimum width and minimum height together and pass; at least one dimension has to be generously larger. Work an example the other way: a casement with a 30-inch clear width and a 36-inch clear height gives 30 × 36 ÷ 144 = 7.5 square feet, comfortably over 5.7, and with a 40-inch sill it passes all four. Change it to a 20 × 24 slider at a 42-inch sill and every individual minimum is met except area — 3.33 square feet — so it fails. The window area calculator is handy for turning clear dimensions into square feet, and the egress requirements table lists the labeled minimums.

Sill height and window wells

The sill can be no more than 44 inches above the finished floor, so the opening is reachable in a panic and in the dark. In a basement, the window is often below grade, which brings in a second requirement most people forget: the window well. A below-grade egress window needs a well large enough to open the window and climb out — codes call for a minimum well area and, where the well is deep, a permanently attached ladder or steps. Screening the window itself does not screen the well; treat them as two checks.

Enlarging an opening

If your existing opening is too small for an egress unit — common when finishing a basement bedroom — you are into structural territory. Making the hole bigger means cutting framing and usually installing a proper header, which is engineering work, not a window swap. That also almost always triggers a permit. The timeline and permit reference covers when a permit is typical, and the rough-opening calculator helps you size the new framed hole — but the header itself is for a structural engineer or a licensed contractor, not for this site.

Screen, then confirm

Everything here is a screen against the labeled IRC R310 minimums, not a compliance sign-off. Your local jurisdiction may amend the code, and the authority having jurisdiction (the AHJ) makes the final call — some areas apply different area minimums or specific well rules. Use the checker to plan, to compare products, and to catch the 20 × 24 trap before you order, then confirm the exact requirement with your building department and confirm the clear opening on the manufacturer's spec sheet, because two windows of the same nominal size can have different clear openings depending on frame and hardware.

Frequently asked questions

What are the IRC R310 egress minimums?

A clear opening of at least 5.7 square feet (5.0 at grade-floor level), a minimum clear width of 20 inches, a minimum clear height of 24 inches, and a sill no more than 44 inches above the floor. All four must be met together.

Can a window meet the width and height and still fail egress?

Yes. A 20 by 24 inch clear opening is only 20 times 24 divided by 144 = 3.33 square feet, well short of the 5.7 square foot area minimum. At least one dimension must be generously larger than its own minimum to reach the required area.

Why do casement windows pass egress more easily than double-hungs?

A casement swings its whole sash clear, so its clear opening is close to the full window size. A double-hung opens only half at a time, so its clear opening is much smaller than the unit — often failing where a same-size casement passes.

Is the egress checker a code sign-off?

No. It screens an opening against the labeled IRC R310 minimums for planning. Local jurisdictions may amend the code and the authority having jurisdiction (AHJ) makes the final call, including window-well rules for below-grade basements. Confirm with your building department.