How to measure an egress opening
Open the window the way a person would in an emergency, as far as it goes by normal operation. Then measure the hole you could actually climb through. Measure between the sash, stops and hardware that stay in the way, not the frame and not the rough opening in the wall. The size printed on the unit tells you almost nothing.
A double-hung window is the usual trap. Only one sash moves, so a window that looks large can give you barely half its height. If you are still choosing a window, ask for the clear opening on the maker's data sheet, because we found no reliable factor for working it out from the unit size. That is why this calculator asks you to measure.
Measure the sill from the finished floor. A thick new floor raises the floor and helps you, while taking a floor up does the reverse. Then measure the sill against the ground outside, because that decides which area applies. A sill within 44 inches of the ground counts as grade floor, and a basement sill deeper than that does not.
A worked example
Here is a basement casement with a clear opening of 28 by 30 inches, with its sill 40 inches above the floor and 60 inches below the ground outside. These lines come from the same formula the calculator above runs.
28 by 30 in clear opening in a deep basement
- Clear area: 28 × 30 ÷ 144 = 5.83 sq ft
- Sill 60 in below ground, over 44, so 5.7 sq ft is needed
- Height needed at 28 in: 29.31 in
- Well: 36 × 40 ÷ 144 = 10.0 sq ft, against 9 needed
Now try only the two named minimums, 20 by 24 inches. That gives 3.33 square feet, which fails. The minimums for height and width are there to stop a very narrow or very low opening, and the area does the rest. If you are planning the room around this, the basement finishing cost calculator gives a price range for the whole job, and the window replacement cost calculator prices the unit.
Clear height needed at each width
Choose your clear width and read across for the clear height it takes. Below a width of about 34 inches, the area is what decides. Past that, the 24 inch minimum height takes over.
| Clear width | Height for 5.7 sq ft | Height for 5 sq ft, grade floor |
|---|---|---|
| 20 in | 41.04 | 36.00 |
| 22 in | 37.31 | 32.73 |
| 24 in | 34.20 | 30.00 |
| 26 in | 31.57 | 27.69 |
| 28 in | 29.31 | 25.71 |
| 30 in | 27.36 | 24.00 |
| 32 in | 25.65 | 24.00 |
| 34 in | 24.14 | 24.00 |
| 36 in | 24.00 | 24.00 |
Mistakes that fail an inspection
- Measuring the unit. The net clear opening is what the code asks for. Frames, sashes and stops all come out of it.
- Trusting the two minimums. 24 by 20 inches is only 3.33 square feet, so the area still has to be checked.
- Applying 5 square feet to any basement. Older handouts say grade floor or below grade. The 2021 definition limits it to a sill within 44 inches of the ground.
- A well the window cannot open into. A casement needs room to swing fully. The 36 inch minimum may not be enough for a big sash.
- Bars or covers that lock. Anything over the opening must work from inside with no key, tool or special knowledge.
Questions
What size window counts as egress?
An egress window needs a net clear opening of at least 5.7 square feet, at least 24 inches high and 20 inches wide, with the sill no more than 44 inches above the floor, under the 2021 IRC. A grade-floor opening needs 5 square feet.
Does a basement bedroom need an egress window?
Yes, a basement bedroom generally needs one, since the code requires an emergency escape opening in every sleeping room and in basements. There are exceptions, such as some sprinklered houses, and your town may have amended the rule. Ask your building department before you build.
How big does a window well need to be?
A window well needs at least 9 square feet of horizontal area, with both the projection and the width at least 36 inches, under 2021 IRC R310.4.1. It must also let the window open fully. A well deeper than 44 inches needs a permanent ladder or steps.
Is meeting 24 by 20 inches enough?
No, those two minimums alone give only 3.33 square feet, well under the 5.7 required. At a width of 20 inches you need about 41 inches of clear height, and at 24 inches you need 34.2.
How is the clear opening measured?
Measure it with the window open by its normal operation, between the parts that stay in the way. The unit size and rough opening do not count. A double-hung window gives you only the lifted sash, and a casement's sash and hardware can reduce the width.
Does a replacement window have to meet egress?
Sometimes not in full. The code has a separate rule for replacing a window in an existing emergency opening, and states have amended it. This calculator does not check that rule, so ask your building department before ordering.
When do I need a professional?
You need a permit and usually a contractor for cutting a new or larger opening in a foundation or wall, since it can affect the structure and how water is kept out. Have the inspector confirm the finished opening, and ask a contractor about any well deeper than 44 inches.
This egress window check is a guide to common code minimums, using 2021 IRC section R310, and newer editions renumber it as R319. Your town or county may have amended it, and the local building code and your inspector decide. A pass here does not make a room legal as a bedroom, and escaping a fire may depend on this opening, so have it checked.
Sources and review
- 2021 residential code, chapter 3, as adopted in Colorado, UpCodes, 2021.
- 2021 residential code, chapter 2, definitions, UpCodes, 2021.
- Emergency escape and rescue openings, Jefferson County, Colorado, 2026.
- B10 emergency escape and rescue openings, Boulder County, Colorado, no date shown, read November 2026.
- Emergency escape and rescue openings handout, Jersey County, Illinois, 2022.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Net clear area
Area = Clear width × Clear height ÷ 144
- Clear width, Clear height
- Measured with the window open, in inches
- 144
- Square inches in one square foot
Worked through: 28 × 30 ÷ 144 = 5.83 sq ft
Height needed
Height = Required area × 144 ÷ Clear width, but at least 24 in
- Required area
- 5.7 sq ft, or 5 sq ft for a grade-floor opening
Worked through: 5.7 × 144 ÷ 24 = 34.2 in
The constants, and where they come from
MIN_CLEAR_AREA_SQ_FT
Published figureMinimum net clear opening, in square feet
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: International Residential Code 2021, chapter 3, as adopted in Colorado (R311.7 stairways), International Code Council, read on UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
Also from: Emergency escape and rescue openings, Jefferson County, Colorado, published 2026-07, read 2026-11.
2021 IRC R310.2.1, read through a state adoption on UpCodes since ICC's own pages refused the request. Jefferson County prints the same figure from the 2024 edition's R319.
GRADE_FLOOR_AREA_SQ_FT
Published figureMinimum net clear opening for a grade-floor opening, in square feet
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: International Residential Code 2021, chapter 3, as adopted in Colorado (R311.7 stairways), International Code Council, read on UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
The exception to R310.2.1. Handouts written to the 2018 edition say grade floor or below grade, and that wording should not be copied for 2021.
GRADE_FLOOR_WITHIN_INCHES
Published figureHow far a grade-floor opening's sill may be above or below the ground, in inches
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: 2021 residential code, chapter 2, definitions, UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
The definition in R202. A deep basement window with its sill more than 44 inches below grade does not qualify.
MIN_CLEAR_HEIGHT_INCHES
Published figureMinimum net clear opening height, in inches
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: International Residential Code 2021, chapter 3, as adopted in Colorado (R311.7 stairways), International Code Council, read on UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
2021 IRC R310.2.2. The opening must result from normal operation of the window.
MIN_CLEAR_WIDTH_INCHES
Published figureMinimum net clear opening width, in inches
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: International Residential Code 2021, chapter 3, as adopted in Colorado (R311.7 stairways), International Code Council, read on UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
2021 IRC R310.2.2. Meeting both minimums alone gives only 3.33 square feet, so area still has to be checked.
MAX_SILL_INCHES
Published figureHighest the bottom of the opening may be above the floor, in inches
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: International Residential Code 2021, chapter 3, as adopted in Colorado (R311.7 stairways), International Code Council, read on UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
2021 IRC R310.2.3, stated as not greater than 44 inches above the floor.
MIN_WELL_AREA_SQ_FT
Published figureMinimum horizontal area of a window well, in square feet
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: International Residential Code 2021, chapter 3, as adopted in Colorado (R311.7 stairways), International Code Council, read on UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
Also from: B10 emergency escape and rescue openings, Boulder County, Colorado, no date on the page, read 2026-11.
2021 IRC R310.4.1. The well must also let the window open fully, which this cannot check.
MIN_WELL_SIDE_INCHES
Published figureMinimum well projection and width, in inches
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: International Residential Code 2021, chapter 3, as adopted in Colorado (R311.7 stairways), International Code Council, read on UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
2021 IRC R310.4.1 requires both a horizontal projection and a width of not less than 36 inches.
WELL_LADDER_ABOVE_INCHES
Published figureWell depth above which a ladder or steps are required, in inches
Applies to: Places using the 2021 IRC. Local amendments differ, and the 2024 IRC moves this to R319
Source: International Residential Code 2021, chapter 3, as adopted in Colorado (R311.7 stairways), International Code Council, read on UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
2021 IRC R310.4.2 requires an approved, permanently affixed ladder or steps, which may encroach no more than 6 inches.
SQ_IN_PER_SQ_FT
DefinitionalSquare inches in one square foot
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
Twelve inches in a foot, and an area is a length times a length, so 12 × 12 = 144.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the egress window size calculator
- 1
Open the window as far as it normally goes
- 2
Measure the clear width and height, between the parts that stay in the way
- 3
Measure the sill from the finished floor and from the ground outside
- 4
Add the window well sizes if there is one
- 5
See which minimums pass, and what height would pass at your width
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- You measured the net clear opening with the window open by normal operation, not the unit size or rough opening
- Section numbers are from the 2021 IRC. We read them through a state adoption, as ICC's own pages refused our request
- It does not check the exceptions, such as sprinklered houses, or the separate rules for replacement windows
- It cannot check that a well lets the window open fully, or that bars and covers release from inside without a key or tools
Pro tips
- Choose a casement if you can, since it gives far more clear opening for its size
- Measure after any new floor finish has been planned, since it moves the sill
- Ask for the clear opening figure on the maker's data sheet before ordering