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
in
in
in
in
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.
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. 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.
The size table works out to about $75 per sq ft of window at the low end and $108 to $117 at the high end, at every size. This Old House, in the table above, averages a double-pane replacement window at $529.
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) fw × (h − 2r) fh / 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
fw, fh
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.
Four short subsections govern the window in every bedroom in the country, and they are worth reading rather than summarizing:
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.
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.
What does double glazing cost?
Double glazing means two panes of glass with a sealed gap between them, so a double glazed window is sold in the US as double-pane. This Old House averages a double-pane replacement window at $529 installed and a triple-pane window at $609. The glazing does not change the egress arithmetic: the net clear opening is measured with the sash open, whatever glass is in it.
What does it cost to replace a broken window or a failed seal?
Where the frame is sound, the glass can be replaced on its own. HomeAdvisor gives $181 to $414 per window for glass replacement, $200 to $600 for tempered glass, and $50 to $75 per hour of labor for a glass contractor. A fogged double-pane window has a failed seal between the panes, and the sealed glass is what gets replaced. New glass leaves the sash and the clear opening unchanged, so it does not affect egress.
Does a bay or bow window count for egress?
Only through a unit that opens. A bay window usually projects as three units set at angles and a bow as four or more in a curve, and a fixed center unit cannot be an escape opening at any size. An operable unit has to meet the net clear minimums on its own. HomeAdvisor prices a new bow window at $1,500 to $15,000 installed, with installer labor at $300 to $1,900.
How much does it cost to put in a new window?
This Old House averages a replacement window at $517 installed, from $474 for a single hung to $564 for a fiberglass frame. Those figures are for a window going into an existing opening. Putting a window into a wall that has none means cutting a new rough opening with a header over it, which the header calculator sizes, and no source on this page prices that framing work.
How much does it cost to rescreen a window?
HomeAdvisor gives $147 to $522 for a typical screen replacement job. By mesh, fiberglass runs $60 to $260, aluminum $80 to $390 and stainless steel $80 to $460, and a pet-proof screen is $150 to $560 per window installed.
Does a dormer window have to meet egress?
If it is the escape opening for a habitable attic or a sleeping room, yes. R310.1 names habitable attics and every sleeping room, so that dormer window needs at least 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. No source on this page prices building the dormer itself.
Does the sliding window price cover a sliding glass patio door?
No. This Old House's sliding figure, $531 installed, is a sliding window. Sliding glass patio doors and French doors are priced on the door calculator, including the cost of replacing one with the other.
How much does it cost to install shutters?
HomeAdvisor's June 2026 guide gives $1,033 to $4,028 for a typical shutter job, $2,508 on average, and $20 to $45 per square foot of window installed. By window size it gives $50 to $200 for a 16 by 20 in window, $300 to $600 for 36 by 60 in and $400 to $800 for 60 by 72 in. Per panel, faux wood is $70 to $300, vinyl $100 to $300 and hardwood $200 to $350, and a hurricane shutter $100 to $400.
How much do double pane windows cost?
HomeAdvisor's June 2026 guide gives $300 to $2,100 per window installed, $1,000 on average. By size it gives $450 to $700 for a 2 by 3 ft window, $900 to $1,300 for 3 by 4 ft and $1,200 to $1,800 for 4 by 4 ft, which works out to about $75 per square foot of window at the low end and $108 to $117 at the high end. By style, double-hung is $250 to $1,100 and casement $250 to $2,000. This Old House averages a double-pane window at $529.
How much do double glazed windows cost?
In the US double glazing is sold as double-pane. HomeAdvisor's June 2026 guide gives $300 to $2,100 per window installed, $900 to $1,300 for a 3 by 4 ft window and $250 to $1,100 for a double-hung window. This Old House's August 2026 article averages a double-pane replacement window at $529.
How much does it cost to replace a window seal or a broken window?
HomeAdvisor's June 2026 guide gives $181 to $414 per window, $150 to $400 for float glass, $200 to $600 for tempered glass and $400 to $900 for an argon-filled unit, with a glass contractor at $50 to $75 an hour.
How much does an energy efficient window cost?
HomeAdvisor's June 2026 guide gives $300 to $2,100 per double-pane window and $500 to $1,400 for a window with a low-E coating. HomeAdvisor's June 2026 guide gives $300 to $1,000 to replace the glass with low-e glass.
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: Lucas Pradella, and the method is on 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.
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.