Pricing sources checked October 27, 2026 · How we calculate
Enter the clear opening when the window is fully open (not the frame size).
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The defaults (24 × 36 in clear opening, 42 in sill, below grade, 36 × 36 in well, 48 in deep) give 6.0 sq ft of clear opening, which passes. Because the well is deeper than 44 inches it also needs a permanent ladder or steps. If a check fails, the calculator shows the clear width you'd need at your clear height.
Egress code minimums (IRC R310)
| Requirement | Minimum / maximum |
|---|---|
| Net clear opening | 5.7 sq ft (5.0 sq ft for grade-floor or below-grade openings) |
| Net clear height | 24 in |
| Net clear width | 20 in |
| Sill height | No more than 44 in above the floor |
| Window well area | 9 sq ft, at least 36 in wide and 36 in projection |
| Well ladder | Required when the well is deeper than 44 in; rungs at least 12 in wide, projecting 3 in, no more than 18 in apart |
These are the International Residential Code Section R310 values, as reproduced in city building handouts (City of Boise (IRC R310 handout), 2022 handout of IRC Section R310; City of Saint Paul (IRC R310 handout), posted June 2023). A ladder may encroach up to 6 inches into the required well dimensions. Many cities amend the IRC, so confirm with your building department before you buy or cut.
Where egress openings are required
Under R310.1, basements, habitable attics and every sleeping room need at least one operable emergency escape and rescue opening. Where a basement has sleeping rooms, each sleeping room needs its own. The opening must lead directly to a public way, or to a yard or court that opens to one. Exceptions in the code include storm shelters, basements used only for mechanical equipment under 200 sq ft, and some sprinklered homes (City of Saint Paul (IRC R310 handout), posted June 2023; City of Boise (IRC R310 handout), 2022 handout of IRC Section R310).
How to measure the clear opening
- Open the window fully, the way you would in an emergency, using normal operation from the inside.
- Measure the clear width and height of the open space, between the sash, stops or frame, not the outside of the frame. The code measures the opening, so a window sold as 24 × 36 in has a smaller clear opening.
- Multiply and divide by 144 to get square feet: 24 × 36 = 864 sq in = 6.0 sq ft.
- Measure the sill height from the finished floor to the bottom of the clear opening.
- For a below-grade window, measure the well inside: width along the wall, projection out from the wall, and depth from grade to the well floor.
Which window styles work
Style matters because only the part that opens counts. A casement swings fully out of the way, so nearly its whole frame is clear opening. A double-hung or single-hung window opens only one sash, so roughly half of its height is clear, which is why many hung windows that look big enough fail the 24 in height or 5.7 sq ft test. Sliders have the same problem with width. Whatever the style, the opening must work from the inside without keys, tools or special knowledge, and any window opening control device has to comply with ASTM F2090 (City of Boise (IRC R310 handout), 2022 handout of IRC Section R310).
Replacing an existing egress window
R310.2.5 exempts replacement windows from the size and sill-height rules when the replacement is the manufacturer's largest standard size that fits the existing frame or rough opening, is the same operating style or one with an equal or greater opening, and isn't part of a change of occupancy (City of Boise (IRC R310 handout), 2022 handout of IRC Section R310). Creating a new opening, or converting a basement room to a bedroom, has to meet the full minimums.
Budgeting for a new egress window, well and permit? See the basement and egress window cost guide on Window Cost Estimate. To price ordinary above-grade replacement windows, use the window replacement cost calculator.
Sources
- City of Boise (IRC R310 handout): Egress Windows Code Handout 2022 (2022 handout of IRC Section R310; checked October 27, 2026).
- City of Saint Paul (IRC R310 handout): Egress Window requirements (posted June 2023; checked October 27, 2026).
Frequently asked questions
What size does an egress window need to be?
Under IRC R310, at least 5.7 sq ft of net clear opening (5.0 sq ft at grade floor or below grade), at least 24 in clear height and 20 in clear width, with the sill no more than 44 in above the floor. Local codes can differ.
Does a basement egress window need a ladder?
Only if the window well is deeper than 44 inches. Then IRC R310 requires a permanently fixed ladder or steps usable with the window fully open.
Do I measure the window frame or the opening?
The opening. Code measures the net clear opening when the window is opened normally from the inside, which is always smaller than the frame.
Does a replacement window have to meet the full egress size?
Not always. R310.2.5 exempts a replacement that is the manufacturer's largest standard size fitting the existing opening, in the same or a larger-opening style, when there's no change of occupancy.