What size window is required for egress?

A net clear opening of at least 5.7 square feet, with a net clear height of at least 24 inches and a net clear width of at least 20 inches, and a sill no more than 44 inches above the floor. Grade floor and below-grade openings get a slightly smaller 5 square foot minimum. Those are net clear figures, measured with the window open.

Does a replacement window have to meet egress?

That depends on your jurisdiction and it is worth asking before ordering. What is certain is the arithmetic: an insert replacement fits inside the existing frame and takes two to three inches out of the opening in each direction, which can move a compliant window below the minimum. A full-frame replacement keeps the opening.

How do I measure net clear opening?

Open the window fully and measure the actual gap, at its narrowest in each direction. Not the rough opening, not the glass, and not half the window height on a double hung, because the meeting rails overlap and the real figure is a little less than half.

Can a double hung window meet egress?

Yes, but it has to be large. Only the bottom sash opens, so at best half the height is clear. At the minimum legal 20 inch width a double hung needs about 82 inches of clear sash height to reach 5.7 square feet. A casement of the same requirement needs roughly 20 by 41 inches, because it opens fully.

Does a picture window count for egress?

No. A fixed window does not open, so it cannot be an emergency escape and rescue opening at any size. A bedroom with a large picture window and no operable window of sufficient size does not comply.

What are the window well requirements?

At least 9 square feet of horizontal area, with a projection and width of at least 36 inches each, and it must allow the escape opening to be fully opened, so a casement swinging into a tight well does not comply. Wells deeper than 44 inches need a permanently affixed ladder or steps, which may encroach up to 6 inches into the required dimensions.

Which rooms need an emergency escape opening?

Basements, habitable attics and every sleeping room, per R310.1, and each sleeping room within a basement needs its own. A room used for sleeping is a sleeping room whatever it is called on the plan, which is what catches basement and loft conversions.

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IRC R310 net clear opening, measured the way the code measures it

IRC R310.2.1 measures a bedroom window by the hole you can actually climb through, obtained by the normal operation of the window from the inside. Not the glass, not the frame. Which means a double hung needs to be about twice as tall as a casement to pass, and an insert replacement, the cheaper kind that fits inside the existing frame, can take a compliant bedroom window below the line without anybody noticing.

Net clear opening against R310

How the window opens

Clear sash width

Clear sash height

Sill height above the floor

Insert reduction per side (0 for full-frame, measure your unit)

Where the opening is

Windows in the job (for the price table below)

no.

Enter the CLEAR SASH dimensions, not the rough opening and not the glass. The code measures what normal operation leaves open.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

The sash, and the opening it actually makes

Worked example

A bedroom double hung with a 32 in clear sash width and a 60 in clear height, sill 36 in above the floor, on an upper floor, replaced full-frame so nothing is lost.

Net clear width. A double hung opens its full width: 32 in.
Net clear height. The bottom sash goes up behind the top one, so half: 30 in.
Net clear opening. 32 × 30 = 960 sq in = 6.67 sq ft.
Against R310.2.1. 6.67 over 5.7, height 30 over 24, width 32 over 20, sill 36 under 44. Passes.
Now enter a 2 in insert reduction. The opening becomes 28 × 28 = 784 sq in.
Which is 5.44 sq ft. Under the minimum. Fails.

Same window, same wall, same room. The only thing that changed is that the replacement went inside the existing frame instead of replacing it, which is the cheaper option and the one most quotes assume. Nothing in that transaction surfaces the change: the installer measures the old frame, the quote is per window, and the number that moved is one nobody wrote down. If the room is a bedroom, it is worth saying so before the order goes in.

The formula

One multiplication, and the two fractions are the whole point:

net = (w − 2r) f_w × (h − 2r) f_h / 144

w, h: CLEAR SASH width and height in in, not the rough opening and not the glass
r: insert reduction per side. Zero for a full-frame replacement
f_w, f_h: the share of width and height that actually opens. 1 and 0.5 for a double hung, 1 and 1 for a casement, 0.5 and 1 for a slider
144: square inches in a square foot

A casement and a double hung of identical size differ by a factor of two, because one of those fractions is 1 and the other is 0.5. That is the entire difference between a window that meets R310 and one that does not.

The measurement everybody takes wrong

Ask what size a bedroom window has to be and you will be told 5.7 square feet. That is right and it is not the useful half of the answer.

R310.2.1 says the dimensions are obtained by the normal operation of the opening from the inside. So the number that matters is not the window, it is the hole left when the window is open, and how big that is depends entirely on how the window opens.

A casement swings its whole sash clear. A double hung slides one sash behind the other and can never open more than half its height. A horizontal slider does the same to its width. A fixed window opens not at all and cannot be an escape opening at any size, which surprises people who have a very large picture window in a bedroom.

Which is why this page asks how the window opens before it asks how big it is, and why the answer for a double hung is a window nearly seven feet tall at the minimum legal width.

What each window type needs to pass R310.2.1
Window type	Width that opens	Height that opens	Smallest sash that passes	Its net clear opening
Casement, side hinged	1 x width	1 x height	20 x 41 in	5.70 sq ft net
Awning, top hinged	1 x width	0.55 x height	20 x 75 in	5.70 sq ft net
Double hung	1 x width	0.5 x height	20 x 82 in	5.70 sq ft net
Single hung	1 x width	0.5 x height	20 x 82 in	5.70 sq ft net
Horizontal slider	0.5 x width	1 x height	40 x 41 in	5.70 sq ft net
Fixed or picture	none	n/a	n/a	Cannot be an escape opening

Read 2026-09-08. The fractions are how much of the sash each type actually opens; the sash sizes are this page solving for the smallest one that clears every limit.

Download this table as CSV. Same cells as printed above, nothing added.

Published window replacement prices, and how old they are

Not this site's numbers. This Old House, window replacement cost, Written by Stephanie Minasian-Koncewicz, published 11 August 2026 and read here on 2026-09-08.

As published by This Old House, window replacement cost, average per installed window
Item	Low	High	Unit
Any window, average	$517	$517	installed
Single hung	$474	$474	installed
Casement	$529	$529	installed
Double hung	$551	$551	installed
Sliding	$531	$531	installed
Picture	$540	$540	installed
Aluminium frame	$453	$453	installed
Vinyl frame	$472	$472	installed
Wood frame	$544	$544	installed
Fiberglass frame	$564	$564	installed
Double-pane glass	$529	$529	installed
Triple-pane glass	$609	$609	installed

Source: This Old House, window replacement cost, published 11 August 2026, read 2026-09-08. These are AVERAGES rather than ranges, which is how the source publishes them, so they carry no spread at all and should be read as a central figure rather than a quote. The source also notes that insert replacements cost less than full-frame, which is the trade-off this page is about: the cheaper installation is the one that shrinks the opening.

This site publishes no price of its own. Why, and what that means for these figures.

What R310 requires, in full

Four short subsections govern the window in every bedroom in the country, and they are worth reading rather than summarising:

R310.1: Basements, habitable attics and every sleeping room shall have not less than one operable emergency escape and rescue opening. Where basements contain one or more sleeping rooms, an emergency escape and rescue opening shall be required in each sleeping room.
R310.2.1: Emergency escape and rescue openings shall have a net clear opening of not less than 5.7 square feet. The net clear opening dimensions required by this section shall be obtained by the normal operation of the emergency escape and rescue opening from the inside. The net clear height of the opening shall be not less than 24 inches and the net clear width shall be not less than 20 inches. Exception: Grade floor openings or below-grade openings shall have a net clear opening area of not less than 5 square feet.
R310.2.2: Where a window is provided as the emergency escape and rescue opening, it shall have a sill height of not more than 44 inches above the floor; where the sill height is below grade, it shall be provided with a window well in accordance with Section R310.2.3.
R310.2.3: The horizontal area of the window well shall be not less than 9 square feet, with a horizontal projection and width of not less than 36 inches. The area of the window well shall allow the emergency escape and rescue opening to be fully opened.

Quoted from 2018 Seattle Residential Code, Chapter 3 Building Planning. Read 2026-09-08.

The sentence that does the work is in R310.2.1, and it is easy to read past: the dimensions are obtained by the normal operation of the opening from the inside. Not the glass. Not the frame. Not the rough opening in the wall. The hole that is actually there once the window is open the way that window opens.

Why the window type decides everything

Two windows of identical size can be one compliant and one not, because they open differently.

A casement swings the whole sash clear, so its net opening is its clear size. A double hung slides the bottom sash up behind the top one, so at best half the height ever opens. A slider does the same to the width. A fixed window cannot be an escape opening at any size.

The smallest sash of each type that clears 5.7 sq ft while keeping 24 in of height and 20 in of width
Window type	Width that opens	Height that opens	Smallest sash that passes	Its net clear opening
Casement, side hinged	1 x width	1 x height	20 x 41 in	5.70 sq ft net
Awning, top hinged	1 x width	0.55 x height	20 x 75 in	5.70 sq ft net
Double hung	1 x width	0.5 x height	20 x 82 in	5.70 sq ft net
Single hung	1 x width	0.5 x height	20 x 82 in	5.70 sq ft net
Horizontal slider	0.5 x width	1 x height	40 x 41 in	5.70 sq ft net
Fixed or picture	none	n/a	n/a	Cannot be an escape opening

Compare the casement row with the double hung row. That is the same requirement, met by two very different windows, and it is why a bedroom in a house with small double hungs is the hardest room in the country to make compliant.

The awning figure is this page's own conservative estimate rather than a code value, because how far an awning opens depends on its hardware. Measure yours with the window open.

Measure the sash, not the hole in the wall

Three different measurements get called the window size and only one of them is the one R310 wants.

The rough opening is the hole framed in the wall. It is bigger than the window and it is what a builder orders against.

The glass size is what a manufacturer's brochure often leads with, and it is smaller than the sash because the sash has a frame around the glass.

The clear sash opening is what you need. Open the window fully and measure the actual gap, edge to edge, at its narrowest in each direction. That is what a person climbs through and a firefighter climbs in through, and it is what the code measures.

On a double hung, measure the height of the gap with the bottom sash raised as far as it goes, not half the window height, because the meeting rails overlap and the real figure is usually a little less than half.

Which rooms this applies to

R310.1 is specific: basements, habitable attics and every sleeping room. Where a basement contains sleeping rooms, each of those rooms needs its own opening as well.

The word doing the work is sleeping room rather than bedroom. A room marketed as a study, an office or a den that is used for sleeping is a sleeping room, and this is the point at which converting a basement or a loft stops being a decorating job and becomes a permit.

It is also why finishing a basement without an egress opening is one of the commonest ways a house fails at sale. The work is invisible until an inspector or a buyer's surveyor asks the question, and adding an opening afterwards means cutting a foundation wall and digging a window well.

The escape opening also has to lead somewhere: R310.1 wants it to open into a public way, or into a yard or court with an unobstructed path at least 36 in wide that reaches one.

What this page does not decide

Whether your jurisdiction requires a replacement window to meet current egress. This is the important one and it genuinely varies. Some places require any replacement in a sleeping room to comply. Some allow an existing non-conforming opening to be maintained at its existing size. Some only trigger on a permit. Ask before the order goes in.

Energy performance. U-factor and solar heat gain coefficient are set by the energy code against your climate zone, and that table is not reproduced on this site.

Structural openings. Making an opening bigger means a header, and that is the header calculator.

Whether a window well is needed and how big. The dimensions are on this page; whether your sill is below grade is a site question.

The replacement trap, and why it is invisible

Here is the part that matters to anybody replacing a window rather than building a wall.

An insert replacement, sometimes called a pocket replacement, fits the new window inside the existing frame. The old frame stays in the wall, and the new unit's own frame sits inside it. It is quicker, it does not disturb the siding or the trim, and it costs less. This page's price source says as much.

It also makes the opening smaller on all four sides. Two to three inches overall in each direction is ordinary, and on a marginal window that is the difference between passing and failing.

Work an example. A double hung with a 32 in clear width and a 60 in clear height gives 32 × 30 = 960 sq in, which is 6.67 sq ft and comfortably over. Lose two inches per side to an insert and it becomes 28 × 28 = 784 sq in, which is 5.44 sq ft and under the 5.7 sq ft minimum. The room was compliant on Monday and is not on Tuesday.

Nothing in the transaction surfaces this. The quote is per window, the installer measures the existing frame, and the number that changed is one nobody wrote down. So measure the net clear opening before and after, and if the room is a bedroom, say so when you get the quote.

Full-frame replacement removes the old frame and keeps the opening, which is the more expensive answer and sometimes the only compliant one.

Window wells, for the basement case

A below-grade escape opening needs a well, and the well has its own dimensions:

The horizontal area of the window well shall be not less than 9 square feet, with a horizontal projection and width of not less than 36 inches. The area of the window well shall allow the emergency escape and rescue opening to be fully opened.

Nine square feet, with a projection and width of at least 36 inches each. And the last sentence is the one people miss: the well must allow the opening to be fully opened, so a well that meets its own dimensions but fouls the casement swinging out into it does not comply.

Wells deeper than 44 in need a permanently affixed ladder or steps, which may encroach not more than 6 in into the required dimensions.

Note also that a grade floor or below-grade opening gets the smaller 5 sq ft minimum rather than 5.7, which is the one concession the section makes.

Where the code text came from

Read 2026-09-08. 2018 Seattle Residential Code, Chapter 3 Building Planning, Seattle Department of Construction and Inspections. Prints IRC Section R310 in full: where an escape opening is required, the net clear opening dimensions and how they are measured, sill height, and window wells. Used because codes.iccsafe.org returns HTTP 403.

One city's adopted code, printing the model text. Jurisdictions amend R310, and replacement windows in particular are treated differently from place to place: some require a replacement to meet current egress, some allow the existing opening to be maintained, and some only trigger on a permit. That question is for your building department and it is worth asking before the order goes in, not after.

Frequently asked questions

What size window is required for egress?: A net clear opening of at least 5.7 square feet, with a net clear height of at least 24 inches and a net clear width of at least 20 inches, and a sill no more than 44 inches above the floor. Grade floor and below-grade openings get a slightly smaller 5 square foot minimum. Those are net clear figures, measured with the window open.
Does a replacement window have to meet egress?: That depends on your jurisdiction and it is worth asking before ordering. What is certain is the arithmetic: an insert replacement fits inside the existing frame and takes two to three inches out of the opening in each direction, which can move a compliant window below the minimum. A full-frame replacement keeps the opening.
How do I measure net clear opening?: Open the window fully and measure the actual gap, at its narrowest in each direction. Not the rough opening, not the glass, and not half the window height on a double hung, because the meeting rails overlap and the real figure is a little less than half.
Can a double hung window meet egress?: Yes, but it has to be large. Only the bottom sash opens, so at best half the height is clear. At the minimum legal 20 inch width a double hung needs about 82 inches of clear sash height to reach 5.7 square feet. A casement of the same requirement needs roughly 20 by 41 inches, because it opens fully.
Does a picture window count for egress?: No. A fixed window does not open, so it cannot be an emergency escape and rescue opening at any size. A bedroom with a large picture window and no operable window of sufficient size does not comply.
What are the window well requirements?: At least 9 square feet of horizontal area, with a projection and width of at least 36 inches each, and it must allow the escape opening to be fully opened, so a casement swinging into a tight well does not comply. Wells deeper than 44 inches need a permanently affixed ladder or steps, which may encroach up to 6 inches into the required dimensions.
Which rooms need an emergency escape opening?: Basements, habitable attics and every sleeping room, per R310.1, and each sleeping room within a basement needs its own. A room used for sleeping is a sleeping room whatever it is called on the plan, which is what catches basement and loft conversions.

Check these numbers yourself

2018 Seattle Residential Code, Chapter 3 Building PlanningSeattle Department of Construction and Inspections. Prints IRC Section R310 in full: where an escape opening is required, the net clear opening dimensions and how they are measured, sill height, and window wells. Used because codes.iccsafe.org returns HTTP 403.
This Old House, window replacement costWritten by Stephanie Minasian-Koncewicz, published 11 August 2026, read 2026-09-08. These are AVERAGES rather than ranges, which is how the source publishes them, so they carry no spread at all and should be read as a central figure rather than a quote. The source also notes that insert replacements cost less than full-frame, which is the trade-off this page is about: the cheaper installation is the one that shrinks the opening.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Whether a replacement must meet current egress is a local question. Some jurisdictions require it, some allow an existing opening to be maintained, some trigger only on a permit. Ask before ordering. IRC Section R310 was read from the 2018 Seattle Residential Code on 8 September 2026 because codes.iccsafe.org returns HTTP 403. The awning opening fraction is this page's own conservative estimate rather than a code value.