Window Sizes, Rough Openings, and What Actually Passes Egress
The code gives three numbers for an escape window — 5.7 square feet, 24 inches high, 20 inches wide — and almost everyone reads them as a checklist. They are not. Meet two of them at the minimum and you fail the third by a mile, which is why so many windows that look big enough are not.
What the code actually requires of an escape window
Required in every sleeping room, every basement and every habitable attic. Note the last line especially — it is the sentence that decides whether your window qualifies, and it is the one nobody quotes.
| Requirement | Figure | Source |
|---|---|---|
| Minimum net clear opening area | 5.7 sq ft (820.8 sq in) | IRC 2021, R310.2.1 |
| Exception, grade-floor openings | 5.0 sq ft (720 sq in) | IRC 2021, R310.2.1 |
| Minimum net clear opening height | 24 in | IRC 2021, R310.2.2 |
| Minimum net clear opening width | 20 in | IRC 2021, R310.2.2 |
| Maximum sill height above the floor | 44 in to the bottom of the clear opening | IRC 2021, R310.2.3 |
| Where it is required | Every sleeping room, every basement, every habitable attic | IRC 2021, R310.1 |
| How the opening is measured | "The net clear opening dimensions shall be the result of normal operation of the opening" | IRC 2021, R310.2.2 |
Why the minimums cannot both be minimums
Take the two dimensional minimums at face value and multiply them: the result is nowhere near the required area, so at least one always has to be exceeded. This chart reads the requirement the useful way round — pick the dimension your wall fixes, and the other is decided. **Every companion figure is rounded up to the next sixteenth, never to the nearest**, because these are minimums: rounding down by a hair produces an opening that fails.
| If your clear height is | Clear width needed (5.7 sq ft) | Clear width needed (5.0 sq ft, grade floor) | Note |
|---|---|---|---|
| 24 in — the code minimum | 34-1/4 in | 30 in | The 20 in minimum width is nowhere near enough |
| 26 in | 31-5/8 in | 27-3/4 in | — |
| 28 in | 29-3/8 in | 25-3/4 in | — |
| 30 in | 27-3/8 in | 24 in | — |
| 32 in | 25-11/16 in | 22-1/2 in | — |
| 36 in | 22-13/16 in | 20 in | At 5.0 the 20 in minimum finally works here |
| 40 in | 20-9/16 in | 18 in | — |
| 28-11/16 in square | 28-11/16 in | — | The smallest square opening that works at 5.7 |
| 26-7/8 in square | — | 26-7/8 in | The smallest square at the grade-floor 5.0 |
| With only 20 in of width | 41-1/16 in tall | 36 in tall | The 20 in minimum needs this much height |
| 24 x 20, both at minimum | 3.33 sq ft | 3.33 sq ft | 58% of 5.7, 67% of 5.0. Fails either way. |
The window type matters more than the window size
A double-hung opens half its height, so half of what you paid for does not count. A casement swings clear and nearly all of it counts. Clear openings below are approximate — frame and sash allowances of about 4 in on width and 6 in on height for hung units, 5 in and 4 in for casements. Your manufacturer publishes exact clear-opening figures for every size, and those are the ones an inspector will use.
| Unit size | Type | Approximate clear opening | Area | Passes 5.7 sq ft? |
|---|---|---|---|---|
| 36 x 48 | Double-hung | 32 x 21 | 4.67 sq ft | No — and it fails the 24 in height too |
| 36 x 54 | Double-hung | 32 x 24 | 5.33 sq ft | No |
| 36 x 60 | Double-hung | 32 x 27 | 6.00 sq ft | Yes |
| 32 x 66 | Double-hung | 28 x 30 | 5.83 sq ft | Yes |
| 30 x 48 | Casement | 25 x 44 | 7.64 sq ft | Yes, comfortably |
| 36 x 48 | Casement | 31 x 44 | 9.47 sq ft | Yes, comfortably |
Standard sizes and their rough openings
Rough opening is the framed hole. The usual allowance is 1/2 inch on width and height — a quarter inch of shim space all round — but published guidance ranges from 1/2 to 1 inch and manufacturers differ. Use your unit's own instructions; the figures below are the common 1/2 inch case.
| Unit size (w x h) | Called | Rough opening | Typical use |
|---|---|---|---|
| 24 x 36 | 2-0 x 3-0 | 24-1/2 x 36-1/2 | Bath, utility |
| 24 x 48 | 2-0 x 4-0 | 24-1/2 x 48-1/2 | Secondary rooms |
| 30 x 48 | 2-6 x 4-0 | 30-1/2 x 48-1/2 | Casement egress candidate |
| 32 x 54 | 2-8 x 4-6 | 32-1/2 x 54-1/2 | Common bedroom hung |
| 36 x 48 | 3-0 x 4-0 | 36-1/2 x 48-1/2 | Common, and fails hung egress |
| 36 x 60 | 3-0 x 5-0 | 36-1/2 x 60-1/2 | Hung egress that works |
| 36 x 72 | 3-0 x 6-0 | 36-1/2 x 72-1/2 | Large bedroom |
| 48 x 48 | 4-0 x 4-0 | 48-1/2 x 48-1/2 | Picture or slider |
Three numbers that are not a checklist
Every article about escape windows gives you the same three figures: 5.7 square feet, 24 inches high, 20 inches wide. They are correct, they come straight from IRC R310, and presented as a list they are actively misleading.
Multiply the two dimensional minimums. Twenty-four by twenty is 480 square inches. The area requirement is 5.7 square feet, which is 820.8 square inches.
So a window at both stated minimums delivers 58 percent of the required area and fails. The 24 and the 20 are not a size you can build to. They are limits on shape — they exist to stop someone claiming a letterbox slot or a narrow slit as an escape route — and at least one of them always has to be exceeded, usually by a lot.
Which is why the second chart above is the one to actually use. Fix whichever dimension your wall will not let you change, and the other one is decided by arithmetic.
One thing about that chart worth saying out loud: every companion figure in it is rounded up, never to the nearest sixteenth. These are minimums, and on a minimum a rounding of a hair in the wrong direction is the difference between passing and not. Twenty-eight inches by twenty-nine and a quarter looks like it makes 5.7 square feet. It makes 5.69, and it fails.
The sentence in the code that decides everything
R310.2.2 ends with a line that almost nobody quotes:
The net clear opening dimensions shall be the result of normal operation of the opening.
Not the window size. Not the glass size. Not the hole in the wall. What is left when you open the window the way it opens.
That single clause is why the type of window matters more than its size, and it is where most of the failures come from.
A double-hung throws away half of what you bought
Open a double-hung and the lower sash slides up behind the upper one. The clear height you end up with is roughly half the height of the unit, minus frame.
Run that on a 36 by 48 double-hung — a completely ordinary bedroom window:
- clear width, after the frame: around 32 inches
- clear height, after the frame and the halving: around 21 inches
- clear area: 4.67 square feet
It fails on area, and it fails on the 24 inch minimum height before you even get to the area. Nothing about that window looks wrong. It is 3 feet by 4 feet of glass in a bedroom wall, and it is not an escape window.
To pass as a double-hung you need to get to roughly 36 by 60, which gives about 6 square feet, or a narrower 32 by 66. The height is doing the work, not the width.
A casement passes at a much smaller size
Now the same 36 by 48 unit as a casement. It swings out on hinges, so the sash clears the opening almost entirely:
- clear width: around 31 inches
- clear height: around 44 inches
- clear area: 9.47 square feet
Same hole in the wall, similar money, and it passes with room to spare. A casement as small as 30 by 48 clears the requirement comfortably.
This is the single most useful thing to know if you are putting a bedroom in a basement or an attic, where the wall you have is the wall you have. Changing the operating type is usually cheaper and easier than making the opening bigger.
The sill height catches people too
The bottom of the clear opening cannot be more than 44 inches above the finished floor (R310.2.3).
Note what that measures to. Not the sill you can see, not the stool, not the bottom of the frame — the bottom of the clear opening. On a double-hung, when the lower sash is raised, the bottom of the opening is the top rail of that raised sash, which sits well above the sill. A window whose sill measures a comfortable 40 inches can still be over 44 to the actual opening.
Rough openings: the one place the sources genuinely disagree
The fourth chart gives the common allowance of half an inch on both width and height — a quarter inch of shim space all round.
Be aware that this is not settled the way the door allowance is. Published guidance from window retailers and installers runs from half an inch up to a full inch, and manufacturers set their own. That is not the sources being sloppy; different flange details and installation methods genuinely want different gaps.
So: use the number in the instructions that came with your window. Too tight and the unit will not go in square. Too loose and you are packing out and flashing a gap you created yourself.
Where escape windows are required
Every sleeping room, every basement, and every habitable attic (R310.1). A basement needs one whether or not anyone sleeps down there, and if the basement contains bedrooms, each bedroom needs its own.
And it has to lead somewhere. The opening must give onto a public way, or onto a yard or court at least 36 inches wide that opens onto one. A window into a sealed light well with no way out is not an escape opening, however large it is.
Before you order
The manufacturers publish exact clear-opening figures for every size they make — Andersen, Pella and JELD-WEN all put them in their architectural detail manuals, and JELD-WEN publishes an egress calculator. Those figures are what an inspector will use, and they beat every chart on the internet including this one.
What this page is for is the step before that: working out what you need to be looking for. Decide which dimension is fixed by your wall, read the companion dimension off the second chart, then go and find a unit whose published clear opening beats it. Doing it the other way round — picking a window you like and hoping — is how a bedroom ends up not being a bedroom.
Common questions
Does a 36x48 window meet egress?
As a double-hung, no — twice over. Opening the lower sash gives you roughly 32 by 21 inches, which is 4.67 square feet against the 5.7 required, and the 21 inch height is already below the 24 inch minimum on its own. As a casement, comfortably yes: the same 36 by 48 unit swings almost fully open and gives around 9.5 square feet. Same hole in the wall, same money, opposite answer.
What is the smallest window that meets egress?
There is no single answer, because area and dimensions interact. The smallest square clear opening that makes 5.7 square feet is 28-11/16 by 28-11/16 inches — and note that 28-5/8, which is what rounding to the nearest sixteenth gives you, comes out at 5.69 square feet and fails. If your height is fixed at the 24 inch minimum you need 34-1/4 inches of clear width. If your width is fixed at the 20 inch minimum you need 41-1/16 inches of height. At the grade-floor 5.0 square feet the square drops to 26-7/8. Work out which dimension you cannot change, then read the other off the second chart.
Why can't I just use the 24 inch and 20 inch minimums?
Because 24 times 20 is 480 square inches, and the requirement is 820.8. You would be at 58 percent of the required area — and still only 67 percent of the reduced 5.0 square feet allowed at grade floor. The two dimensional minimums are limits on shape, meant to stop you claiming a letterbox slot, not a size you can build to. At least one of them always has to be exceeded, usually by a wide margin.
What is the rough opening for a window?
Commonly the unit size plus 1/2 inch in both directions, giving a quarter inch of shim space all round. But this is one where the sources genuinely disagree — published guidance runs from 1/2 inch up to a full inch, and manufacturers set their own. Take the figure from the instructions that came with your window rather than from any chart, including this one. Too tight and it will not go in; too loose and you are packing out a gap you have to insulate and flash.
Which rooms need an escape window?
Every sleeping room, every basement, and every habitable attic (IRC R310.1). Basements need one even without a bedroom, and if a basement contains bedrooms then each bedroom needs its own. The opening has to lead to a public way, or to a yard or court at least 36 inches wide that opens to one — an escape window into a sealed light well is not an escape.
How high can the window sill be?
The bottom of the clear opening can be no more than 44 inches above the floor (R310.2.3). Note it is measured to the clear opening, not to the frame or the stool, and it is measured from the finished floor. On a double-hung this is measured to the bottom of the opened lower sash, which sits higher than the sill you can see.
Do replacement windows have to meet egress?
This is where to stop reading a chart and ask your building department. The IRC has provisions that treat replacement openings differently from new construction, and jurisdictions amend this area heavily. What is worth knowing before you call: a replacement insert fits inside the existing frame and therefore has a smaller clear opening than the window it replaced, so a bedroom that passed can stop passing.
Where these numbers come from
- 2021 International Residential Code, Section R310: R310.1 for where escape openings are required and what they must open onto, R310.2.1 for the 5.7 square foot area and the 5.0 square foot grade-floor exception, R310.2.2 for the 24 inch height and 20 inch width minimums and the requirement that dimensions result from normal operation of the opening, and R310.2.3 for the 44 inch maximum sill height. Read from the code text.
- The second chart is arithmetic from the 5.7 square foot requirement: 5.7 square feet is 820.8 square inches, so the minimum companion dimension is 820.8 divided by the dimension you have fixed.
- The clear openings in the third chart are approximate and are this site's arithmetic, not manufacturer data: about 4 inches off the width and 6 inches off the height for a hung unit before halving, and about 5 inches off the width and 4 inches off the height for a casement. Every major manufacturer publishes exact clear-opening figures for every size in its architectural detail manuals, and Andersen, Pella and JELD-WEN all do. Those figures govern, not these.
- Rough opening allowances vary. Published guidance ranges from 1/2 inch to a full inch over the unit size, and the manufacturer's own installation instructions are the authority for the window you actually bought.
- Codes are adopted and amended locally, and escape openings — particularly in replacement work — are among the most amended provisions in the code. Where your jurisdiction differs, it governs.
Last checked 2026-08-20.