Net clear opening is the whole subject
Every number on this page hangs off one definition. Net clear opening is the actual unobstructed gap you could climb through with the window fully open. It is a smaller figure than the frame size printed on the order sheet, smaller than the rough opening framed into the wall, and different again from the glass area quoted in a brochure. Frame, sash, screen track and hardware all subtract from it.
Here is the gap that catches people out. A 24 in × 36 in unit measures 6.0 sq ft of frame, which reads as comfortably over the requirement. Once the sash and frame are accounted for, the same unit delivers roughly 4.5 – 4.9 sq ft of net clear opening and falls under the 5.7 sq ft minimum. It fits the hole, it looks right, and it fails inspection.
Manufacturers publish net clear opening on the spec sheet for every unit, usually as clear opening width, clear opening height and clear opening area. Order from those three numbers rather than from the nominal size, and ask your supplier to put the figures on the quote. That single habit removes most of the risk in this category.
The dimensions the code asks for
| Requirement | Minimum | How it is measured | What it is for |
|---|---|---|---|
| Net clear opening area | 5.7 sq ft | Sash fully open, width × height | A firefighter in gear has to pass through it in both directions |
| Net clear area, grade-floor opening | 5.0 sq ft | Same measurement | IRC allowance where the opening sits at ground level |
| Clear opening height | 24 in | Sill to head, window open | A floor that applies in addition to the area requirement |
| Clear opening width | 20 in | Jamb to sash, window open | A floor that applies in addition to the area requirement |
| Sill height above the floor | 44 in max | Finished floor to the sill | An adult has to climb out unaided, with no ladder inside |
| Window well area | 9 sq ft | Horizontal area of the well | Required where the sill sits below grade |
| Window well projection and width | 36 in | Out from the wall, and across | Keeps the 9 sq ft usable rather than a long slot |
| Ladder or steps in the well | Over 44 in deep | Well depth measured from grade | Permanently attached; may project 6 in into the required area |
| Window well drainage | Required | Gravel base plus a drain | Tied to the footing drain or to a dedicated line |
Source: Dimensions follow the widely adopted International Residential Code provisions for emergency escape and rescue openings in sleeping rooms and basements. Local jurisdictions adopt different editions and amend them, so confirm each figure with your building department before ordering. · Last reviewed July 30, 2026 · Reference data · How we compile these figures
Local jurisdictions adopt the IRC on their own schedule and amend it as they go. Your city may run a 2018 edition, a 2021 edition, or a state code with its own sill and well language, and a handful of jurisdictions set stricter numbers than the table above. Call the building department with your address before you order a window. The call is free, takes ten minutes, and settles which edition governs your permit.
The minimums do not add up to the area
Read the requirement as three separate tests rather than one. The area test is 5.7 sq ft. The height test is 24 in. The width test is 20 in. A window has to pass all three, and passing the two smaller ones does nothing to satisfy the first.
The arithmetic makes it plain. 24 in × 20 in is 480 square inches, which is 3.3 sq ft — a little over half of what the area test asks for. Meeting both dimensional floors leaves you more than 40% short. One dimension has to grow well past its minimum, and usually both do.
Work it the other way and the practical sizes appear quickly. Hold the height at the 24 in floor and the width has to reach roughly 34 in of clear opening. Hold the width at 20 in and the clear height has to reach about 41 in. Most egress retrofits land between those two poles, which is why the finished openings tend to look considerably larger than the minimums suggest.
Grade-floor openings get a concession: 5.0 sq ft instead of 5.7 sq ft, because the opening sits at ground level and nobody has to be lifted out of a well. The height, width and sill numbers stay exactly the same. Basement windows below grade use the full 5.7 sq ft.
Window sizes that qualify, and styles that struggle
| Nominal unit size | Net clear opening | Frame area for comparison | What happens at inspection |
|---|---|---|---|
| 24 in × 36 in casement | 4.5 – 4.9 sq ft | Frame area 6.0 sq ft | The classic near-miss. Frame, sash and hardware take the difference |
| 28 in × 48 in casement | 6.6 – 7.2 sq ft | Frame area 9.3 sq ft | Clears the requirement with margin. A common retrofit size |
| 32 in × 36 in casement | 6.0 – 6.5 sq ft | Frame area 8.0 sq ft | Wide and short. Check the 24 in height floor on the spec sheet |
| 48 in × 36 in slider | 4.5 – 5.0 sq ft | Frame area 12.0 sq ft | Half the width opens. Twice the frame, still short of 5.7 |
| 60 in × 40 in slider | 6.4 – 7.0 sq ft | Frame area 16.7 sq ft | Sliders qualify on width. This is roughly what it takes |
| 36 in × 24 in hopper | 2.5 – 3.2 sq ft | Frame area 6.0 sq ft | The sash swings into the opening and blocks part of it |
| 24 in × 20 in, the two minimums | 3.3 sq ft | Frame area 3.3 sq ft | Both floors satisfied, area requirement missed by more than 40% |
Source: Net clear opening ranges are typical for residential vinyl and clad-wood units at each nominal size; exact figures vary by manufacturer, sash profile and hardware. Verify against the specific unit's published clear opening area before ordering. · Last reviewed July 30, 2026 · Reference data · How we compile these figures
Style decides how much of the frame survives as opening. A casement swings the whole sash clear of the way, so it reaches 5.7 sq ft in the smallest rough opening of any style — which is why it is the default for basement egress and why it costs more per unit. A slider gives up half its width to the fixed pane, so it has to be roughly twice as wide to qualify, and that extra width is concrete you pay to cut. Hoppers and awning units swing into the opening and rarely reach the requirement in a basement wall at all.
Double-hung units face the same arithmetic vertically: only the lower sash opens, so the clear height is about half the frame height, and the unit has to be tall enough that half of it still clears 24 in. The cost consequences of each of those choices are broken out in egress window cost.
Window well dimensions where the sill sits below grade
A below-grade opening needs a well, and the well has its own minimums: 9 sq ft of horizontal area, with at least 36 in of projection out from the foundation and at least 36 in of width. Those two dimensions together are what keeps the 9 sq ft usable — a long narrow well can hit the area figure while leaving too little room to swing the window open and stand up.
Once the well is deeper than 44 in, it needs a permanently attached ladder or steps. Those may project up to 6 in into the required area, so a well sized exactly at the minimum still works with a ladder in it. The ladder has to be usable with the window fully open, which is one more reason to check the swing direction against the well layout before the order goes in.
Drainage is part of the dimensional package rather than an upgrade. The well needs a gravel base and a drain tied either into the footing drain or into a dedicated line. Built that way, the well sheds the water that lands in it. Built as a bare hole against the foundation, it collects roof runoff and delivers it to the new opening, which is how a dry basement acquires a leak in the same month it acquires a bedroom. Covers are worth having for the same reason, and code-compliant ones open from the inside without tools or a key.
Measure what you already have first
Before you price a new opening, measure the window that is already in the wall. Open it fully. Measure the clear width and the clear height in inches at the narrowest points, with the sash where it sits when open. Multiply them, divide by 144, and compare the result to 5.7 sq ft. Then measure from the finished floor to the sill and compare to 44 in.
Homes built since the mid-1990s frequently have basement windows that already comply or sit within an inch of it, and confirming that costs a tape measure and five minutes. When the existing opening passes, you have saved the $3,000 – $6,500 a complete new installation runs, and the money can go to the parts of the basement that actually need it. When it falls short by a small margin, a larger window in the existing opening at $1,200 – $2,800 often closes the gap without a new cut through the foundation.
The requirement itself attaches to sleeping rooms and to basements with habitable space. A basement used for storage, a workshop or a gym generally carries no egress obligation. It becomes mandatory the moment you frame a bedroom down there — and a finished bedroom without a compliant opening tends to reappear later as a failed inspection, an appraisal problem, or a room your listing cannot legally call a bedroom. Keep the permit, the inspection sign-off and the manufacturer's clear opening specification together in your house records. Those three documents are what an appraiser asks for.
Related guides
- Egress window cost — what each part of the job runs
- Basement repair cost
- Signs of foundation problems — check the wall before you cut it
- How to get estimates
- How we compile these figures