Egress window requirements: does your basement window pass?

Your basement window meets egress code if, opened the normal way, it gives at least 5.7 square feet of clear space, at least 24 inches tall and 20 inches wide, with the bottom of the opening no higher than 44 inches above the floor.¹ Those are the model numbers in section R310 of the 2021 International Residential Code. Your city or county can add to them, so your building department has the final word.²

TL;DR

• All four numbers must pass at once: 5.7 square feet of net clear opening, 24 inches of clear height, 20 inches of clear width and a sill no more than 44 inches above the floor.¹
• Measure the hole the open window leaves, not the frame. A casement opens across nearly its whole sash. A slider or hung window opens one sash, so it needs a much bigger unit to pass.
• A 48 x 48 double-hung and a 36 x 48 double-hung both fail on height. A 48 x 48 slider and a 36 x 60 (3x5) single-hung can pass on paper, but only the maker's listed net clear opening settles it.
• Under the IRC, each basement bedroom needs its own egress window unless the home has a qualifying sprinkler system, and a well deeper than 44 inches needs a fixed ladder or steps.¹
• Local rules can be tighter or looser than the IRC. Get your building department's rule before anyone orders a window.³

What does the 2021 IRC require for an egress window?

Section R310 of the 2021 International Residential Code sets five rules for a basement egress window: at least 5.7 square feet of net clear opening, at least 24 inches of clear height, at least 20 inches of clear width, a sill no more than 44 inches above the floor, and no keys or tools needed to open it.¹

The code's own name for it is an "emergency escape and rescue opening."¹ It's a way out in an emergency such as a fire, and a way in for emergency crews.²

Here is the full set of numbers in one chart, all from the 2021 IRC.¹ The square inch figures come from multiplying by 144, which is how you'll check a real window.⁴³

The grade-floor exception drops the area to 5 square feet.¹ Use 5.7 for a basement window, because that's the number that applies unless your opening counts as grade-floor. If you think yours does, ask your building department before you size anything around the smaller figure.

For the bigger picture on adding one, see the guide to basement egress windows.

How do you measure the net clear opening?

Open the window the way it normally opens. Measure the clear gap from edge to edge, height and width, in inches. Multiply the two and divide by 144 to get square feet.³ The result must reach 820.8 square inches, with the height at least 24 inches and the width at least 20 inches.¹⁴

The code says the net clear opening dimensions "shall be the result of normal operation of the opening."¹ Maine's fire marshal puts it the same way for existing buildings: the opening is measured with the window in its natural open position.³ So a sash you'd have to lift out, or a panel you'd have to unclip, doesn't add to your number.

Two worked examples from that Maine sheet show why the area rule matters on its own. A 20 by 24 inch opening meets both minimum dimensions and still fails, because 20 x 24 is only 480 square inches.³ A 32 by 26 inch opening passes: 832 square inches, which works out to about 5.78 square feet.³

Measure the sill from the floor to the bottom of the clear opening, not to the bottom of the frame.¹ HomeGuide's summary of the IRC words it as no more than 44 inches from the finished floor.⁵ If your finishing plan adds a subfloor and flooring, the floor comes up and the sill measurement gets a little shorter, which helps.

One more trap. A big window can still fail if it only opens partway.² The frame size on the label tells you very little until you know how much of it actually opens.

What is the smallest window that meets egress requirements?

The minimum is 820.8 square inches, with at least 24 inches of height and 20 inches of width.¹⁴ Dividing 820.8 by each of those two minimums works out to the smallest openings that pass: 24 inches tall by 34.2 inches wide, or 20 inches wide by 41.04 inches tall. The window unit itself has to be bigger than that, because its frame and sash take up part of the hole.

Every other passing shape sits between those two. Pick a clear height, divide 820.8 by it, and you get the clear width you need. This chart is our own arithmetic for common heights, rounded up to the next tenth of an inch so you never land just short.

Any width in that chart is still subject to the 20 inch floor, and any height to the 24 inch floor.¹ Eaton County, Michigan publishes a similar grid of opening heights and widths with the products in square inches.⁴ A few of its cells carry typos, so multiply out any size you plan to rely on.

What the chart can't tell you is the unit size. Two casements with the same outside dimensions can have different clear openings depending on the frame and hinges. Ask for the net clear opening figure for the exact model and size, and check it against the chart.

Worked calculation: casement, slider and double-hung windows

A casement's opening can come close to its full sash, because it swings clear. A slider or hung window opens one sash over another, so its opening is at most about half the unit. Work out that best case first. If it misses 24 inches tall, 20 inches wide or 820.8 square inches, the window fails.¹⁴

Here's how each type moves. A casement is hinged on one side and swings outward. A slider opens sideways along a track.⁵ Many sliders have one fixed pane and one moving pane. A single-hung window has one sash that slides up from the bottom while the top stays fixed. A double-hung window has two sashes that slide up and down.⁶ Either way, the sash that moves has to park over the one that doesn't, which is why only about half the unit can ever open.

The best case is a ceiling, not a pass. The frame, the sash rails, the meeting rail in the middle of a hung window and the hardware all take inches. How many depends on the product. So a best case that fails is a firm no, and a best case that passes means "now get the listed number."

This table reads every size as width by height, in inches. Every opening and area in it, and in the three answers below it, is our own arithmetic from the code minimums, before the frame takes its share.

Every "passes" or "can pass" above still needs the sill at 44 inches or less from the floor.¹ Check the sill on every candidate, because a window with a perfect opening still fails if it sits too high on the wall.

Does a 48x48 window meet egress?

It depends on how it opens. A 48 x 48 casement should pass easily, since its best case works out to nearly three times the minimum area. A 48 x 48 slider can pass, since half its width is 24 inches and its best-case area works out to 1,152 square inches. A 48 x 48 double-hung or single-hung fails, because half its height is exactly 24 inches before the frame and meeting rail take their share.

For the slider, the frame has 331.2 square inches to use up before the window fails on area, and 4 inches of width before it drops under 20. That's a workable margin, but the listed net clear opening is still the number to check.

Does a 3x5 single-hung window meet egress?

A 3x5 single-hung can pass if it stands 36 inches wide and 60 inches tall. Its best-case opening is 36 by 30 inches, which works out to 1,080 square inches and clears 820.8 with 259.2 to spare. Turned on its side at 60 wide by 36 tall, the same size fails, because the opening is only 18 inches tall.

The upright version has a catch in a basement. With its sill at 44 inches, a 60 inch tall unit would put its top about 104 inches off the floor. That's a lot of wall for a basement, so the window may need to sit lower, which means a deeper cut and often a deeper well. Check the listed opening before you plan the cut.

Does a 36x48 window meet egress?

Only as a casement. A 36 wide by 48 tall casement opens up to 1,728 square inches and passes. As a slider it opens 18 inches wide, under the 20 inch minimum. As a double-hung or single-hung it opens 24 inches tall at best, and the frame takes it under. Turned to 48 wide by 36 tall, a slider has just 43.2 square inches of room.

On paper, 24 by 36 inches works out to 864 square inches against a minimum of 820.8.⁴ The 43.2 square inch margin, spread along a 36 inch tall opening, works out to about 1.2 inches of width. That's a thin margin for a frame and sash to fit inside, so treat it as a likely fail until the maker's listed opening says otherwise.

Can a double-hung window be used for egress?

Yes. A double-hung window qualifies when its normal opening, with one sash raised, reaches 5.7 square feet, 24 inches of clear height and 20 inches of clear width, with the sill no more than 44 inches above the floor.¹ Because only about half the unit opens, it needs a tall unit to get there.

The code sets sizes and operation rules. It doesn't list approved window styles, and egress windows come as casements, sliders, single-hung and double-hung units.² Casements are the popular basement choice because they open fully.²

The height problem is what trips up double-hung windows. At 48 inches tall, half the unit works out to exactly 24 inches, which fails once the frame is counted. So a passing double-hung is usually well over 48 inches tall. In a basement that pushes the sill down the wall and deeper below the ground outside. A deeper well brings its own rule: past 44 inches deep, it needs a ladder or steps.¹

Window opening control devices and fall prevention devices are allowed on an egress window if they comply with ASTM F2090 and sit not more than 70 inches above the finished floor.¹ The window still has to open from inside without keys, tools or special knowledge.¹

Does an egress window need a window well and a ladder?

A basement egress window needs an area well when the bottom of its opening sits below the ground outside.¹ The well must be at least 9 square feet, with at least 36 inches of projection from the wall and 36 inches of width.¹ It needs a permanently attached ladder or steps only when it's more than 44 inches deep.¹

The well also has to let the window open fully.¹ That matters for a casement, which swings out into the well. A well that fits a slider may not fit a casement of the same size.

When a ladder is required, the 2021 IRC sets its shape.¹

• Ladders: inside width of at least 12 inches, projecting at least 3 inches from the wall, with rungs no more than 18 inches apart on center for the full height of the well.¹
• Steps: inside width of at least 12 inches, tread depth of at least 5 inches and risers no taller than 18 inches.¹
• Clearance: the open window can't block the ladder or steps. The ladder or steps may take up to 6 inches of the well's required size.¹

Two more rules catch people out. The well has to drain into the foundation drainage system, unless the house sits on well-drained soil of the kind the code lists as Group I.¹ And any grate, cover, bar or screen over the window or the well has to come off from inside without a key, a tool or more force than it takes to open the window. It also can't shrink the opening below the minimums.¹

A clear cover keeps out rain and snow while letting light through. A metal grate adds security but blocks more light.⁵ Either works for code as long as it releases from inside and keeps the opening at full size.

Which basement rooms need an egress window?

Under the 2021 IRC, a basement needs at least one egress window, and every sleeping room in it needs its own.¹ The exceptions are narrow. A basement used only for mechanical equipment, up to 200 square feet, is exempt.¹ Basement bedrooms are exempt only in a home with an automatic sprinkler system and the exits that exception requires.¹

The sprinkler exception needs the basement to have either one code-compliant means of egress plus one egress window, or two code-compliant means of egress.¹

Finishing an existing basement has its own rule, R310.7. New sleeping rooms created in an existing basement need an egress window. Other alterations or repairs to an existing basement don't trigger one.¹ That's narrower than you'll read in some guides. HomeGuide, for example, says codes require an egress window in each basement bedroom, office, TV room or other living space.⁵ The model code text is the baseline, and your local code may be stricter, so settle it with your building department before framing.

R310.7 also has an exception for existing windows. An existing operable window can serve as the egress opening if it gives at least 4 square feet of net clear opening, at least 22 inches of clear height and at least 20 inches of clear width.¹ Whether your jurisdiction adopted that exception is a question for the permit office.

If you're planning a basement apartment or in-law suite, count egress windows bedroom by bedroom. An office that someone might later sleep in is worth planning with an egress window now, while the walls are open.

Do replacement windows have to meet the full egress size?

Not always. Under R310.5 of the 2021 IRC, a replacement window is exempt from the size and sill rules and the cover rule if it's the maker's largest standard size that fits the existing frame or rough opening, opens in the same style or one with an equal or larger opening, and isn't part of a change of occupancy.¹

This matters when you're swapping an old window in a basement that's already finished. It can let you replace a small window without enlarging the opening, as long as the new unit meets those conditions.¹ If the project turns a room into a bedroom, you're back under the rules for new sleeping rooms.

If you're reworking a finished basement rather than building out a bare one, the basement remodeling page covers that kind of job.

How do local amendments change egress rules?

Local rules sit on top of the IRC. States, counties and cities adopt a code and can add their own requirements, and some write separate policies for older homes. Before anyone orders a window, get your building department's current egress rule, or have your contractor confirm it when pulling the permit.²³

Three examples show how far the local layer can move things.

Maine, older buildings. A 2013 policy from Maine's State Fire Marshal requires 5.7 square feet for buildings constructed after 1976.³ For buildings constructed before 1976, it allows a total net clear opening of 3.3 square feet if the window is wood or vinyl, the opening still meets 20 inches wide and 24 inches tall, and the overall sash is at least 5.0 square feet.³ The same policy says local jurisdictions can adopt something more restrictive.³

Eaton County, Michigan. The county's egress sheet lists the same core numbers but under older section numbers. It puts the sill limit under R310.1 and window wells under R310.2, where the 2021 code has them at R310.2.3 and R310.4. It also lists a window sill rule: if the sill is more than 72 inches above the ground outside and less than 24 inches above the room's floor, the window needs a device that limits it to a 4 inch opening in normal operation.⁴ Section numbers on a local handout tell you which code edition your inspector is reading.

Homeowner associations. Code isn't the only approval. The Muirfield Association requires written approval from its design committee before an egress window goes in, with a $25.00 review fee, and wants the above-grade parts of the well painted to match the house.⁷ Check your HOA rules before the dig.

Each permit office applies the code its own jurisdiction adopted. If you're finishing a basement in Denver or finishing a basement in metro Atlanta, your contractor pulls the permit and books the inspections for that jurisdiction.

What should you ask your contractor about an egress window?

Ask for the listed net clear opening of the exact unit, the planned sill height above the finished floor, the well's size and depth, whether it needs a ladder, how the well drains, and who pulls the permit. Those six answers tell you whether the window will pass inspection before anyone cuts the foundation. For the wider vetting questions, see how to choose a basement contractor.

A good quote for an egress window shows these line items:

• The window: make, model, size, operating style and its listed net clear opening in square inches.
• The sill: height from the finished floor to the bottom of the clear opening, 44 inches or less.¹
• The well: material, size (at least 9 square feet, 36 inches out and 36 inches wide) and depth.¹
• The ladder or steps: included if the well is deeper than 44 inches.¹
• Drainage: how the well ties into the foundation drain.¹
• The cover: type, and confirmation it releases from inside.
• The permit and inspection: who applies, and what the fee is.

Published cost figures vary, so here are two. HomeGuide puts an egress window at $6,000 to $8,000 on average with installation, and says complex jobs can exceed $10,000.⁵ Rent.com's 2024 glossary page puts it at $2,500 to $5,000 including labor and materials.² Budget from the HomeGuide range. It's newer, dated October 2025.⁵ HomeGuide also spells out what's inside that price: the window, a window well, excavation and installation labor.⁶ HomeGuide adds that permits typically run $200 to $500+, and a drainage system costs $1,000 to $4,000 where the well needs one.⁵ Anything beyond that is priced at your estimate.

Basement Remodeling Co. matches you with one vetted, licensed basement contractor for your metro. That contractor gives you a free on-site estimate and a written, itemized quote, pulls the permit and books the inspections. If egress is part of a bigger build-out, get a written quote for finishing your basement, or find the contractor who covers your ZIP code.