Planning Guide

Egress Window Size and Opening Guide

The clear-opening numbers that decide whether a basement window counts as a legal emergency escape, and how they shape the well, the cut and the finished room.

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An egress window is defined by the size of the hole a person can actually climb through, not the size of the glass or the frame. Before you compare quotes or choose a product, it helps to know the numbers an inspector will check.

The Five Numbers That Matter

Requirement	Minimum	Why it exists
Net clear opening area	5.7 sq ft (5.0 sq ft for openings at or below grade)	Room for an adult, and a firefighter in gear, to pass through
Clear opening height	24 inches	Measured through the fully open sash
Clear opening width	20 inches	Measured through the fully open sash
Sill height above finished floor	No more than 44 inches	Reachable without a permanent ladder inside
Window well horizontal area	9 sq ft, at least 36 inches in every direction	Room to stand and open the window outward

Net clear opening is the actual gap when the window is fully open, not the rough opening and not the glass size. A unit can be physically large and still fail if only part of it opens.

Why 5.7 Square Feet Is Harder Than It Sounds

Meeting 5.7 square feet while also holding 24 inches of height and 20 inches of width usually takes a window wider and taller than a standard basement slider. Two things make it tricky:

Only the operable part counts. On a slider or a double-hung, half the unit is fixed glass. The clear opening is the moving half.
Hardware eats into the gap. Sash frames, screens and stops all reduce the measured opening below the catalog rough-opening number.

A casement window, hinged on the side and cranked fully open, gives the largest clear opening for its size, which is why it is the most common choice for a true egress retrofit.

How the Opening Drives the Rest of the Project

The foundation cut

A larger clear opening means a wider, taller cut in poured concrete or block, which changes the header, the method and the cost.

The window well

The well has to clear the sash as it swings and still leave 36 inches of standing room, so a bigger window needs a bigger well.

The sill height

Dropping the sill to stay under 44 inches from the finished floor often means cutting below grade, which brings drainage into the plan.

The finished room

Framing, furring and trim all reference the rough opening, so the final dimensions should be settled before drywall.

Measuring an Existing Window

Open it as far as it will go.
Measure the actual clear height and clear width of the gap, inside the sash, not the frame.
Multiply height by width and divide by 144 to get square feet.
Measure from the finished basement floor to the sill.

If any number falls short, the opening has to be enlarged, which is a structural change and needs a permit. See the Michigan code and permit guide.

Window Wells and Ladders

When the sill sits below grade, a window well is required. If the well is deeper than 44 inches, it needs a permanently attached ladder or step system that stays usable even with a cover in place. The well also needs drainage so it does not fill with water, covered in the drainage guide.

Egress Window Size FAQs

Does a bigger window always mean it is egress-compliant?

No. Compliance depends on the clear opening when the window is fully open, plus the sill height and well size. A large fixed unit can still fail.

What size window well do I need?

At least 9 square feet of floor area and at least 36 inches in every direction, with enough room for the sash to open fully inside it.

Can a slider be an egress window?

Sometimes, if the operable half on its own meets 5.7 square feet, 24 inches of height and 20 inches of width. Many standard sliders do not.

How high can the bottom of the window be?

No more than 44 inches above the finished floor. Lower is often better for daily use and for a legal bedroom.

Related Information

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