Egress Window Sizing Calculator
Check whether a bedroom or basement window meets US egress code (IRC R310). Enter the window size and type to get the net clear opening — area, width, and height — checked against the minimum area, height, width, and sill-height limits, with a downloadable PDF report.
| Check | Your value | Requirement | Status |
|---|---|---|---|
| Enter your window to run the checks | |||
| Type | Net W × H | Area | Egress |
|---|---|---|---|
| Enter a frame size to compare types | |||
How to use the egress window calculator
Four quick steps turn your window’s frame size and type into the net clear opening — area, width, and height — then check it against every IRC R310 limit, with a pass/fail on each and a downloadable report.
Enter the frame size
Give the window’s overall (callout) width and height in inches. The tool deducts frame and sash for the net opening.
Pick the window type
Casement, single/double-hung, or slider — how it opens sets how much of the frame becomes clear opening.
Set the placement
Enter the sill height and whether the opening is above grade (5.7 sq ft) or grade-floor (5.0 sq ft).
Read results & save a PDF
See the net clear opening, a pass/fail on area, height, width, and sill, and a window-type comparison — then download a report.
How the net clear opening is calculated
The net clear opening is the frame reduced for frame and sash, adjusted for how the window opens, then checked against the four IRC R310 limits. Here is the exact math the calculator runs.
Worked example — 36″ × 48″ casement, 40″ sill, above grade (5.7 sq ft)
Net width: 36 × 1 − 3 = 33″ (≥ 20″ ✓) · Net height: 48 × 1 − 3 = 45″ (≥ 24″ ✓)
Net area: 33 × 45 / 144 ≈ 10.3 sq ft (≥ 5.7 ✓)
Sill: 40″ (≤ 44″ ✓)
✓ Meets IRC R310 on all four checks. The same frame as a hung window (½ height) would be far tighter.
Egress code reference charts
Handy lookups for the IRC R310 limits, how each window type reduces the opening, and why the minimum dimensions alone don’t hit the area. Confirm net clear on the spec sheet.
| Requirement | Limit | Note |
|---|---|---|
| Net clear area (above grade) | ≥ 5.7 sq ft | Standard minimum |
| Net clear area (grade-floor) | ≥ 5.0 sq ft | At-ground openings only |
| Net clear height | ≥ 24″ | Of the open dimension |
| Net clear width | ≥ 20″ | Of the open dimension |
| Sill height above floor | ≤ 44″ | So the opening is reachable |
All four checks must pass on the net clear opening — the actual free space when fully open, not the frame or glass.
| Type | Opens | Effect on net clear |
|---|---|---|
| Casement | Full sash swings out | Nearly the whole frame — most efficient |
| Single-hung | Bottom sash only | ≈ half the height |
| Double-hung | One sash at a time | ≈ half the height |
| Slider | One panel slides | ≈ half the width |
A casement meets egress with the smallest window; hung windows must be tall and sliders wide to compensate.
| Opening | Area | Meets 5.7 sq ft? |
|---|---|---|
| 20″ × 24″ (both minimums) | 3.33 sq ft | No |
| 20″ × 41″ | 5.69 sq ft | Just short |
| 24″ × 36″ | 6.00 sq ft | Yes |
| 30″ × 30″ | 6.25 sq ft | Yes |
Hitting 24″ × 20″ is only 3.3 sq ft — at least one dimension must be well above its minimum to reach the area.
Everything the calculator works out
A frame size and window type give you the estimated net clear opening plus a pass/fail on every R310 limit — so you know where a window stands before you order it.
Key figures behind the check
Built for a compliant escape opening
From a basement finish to a bedroom addition, the same calculator checks whether a window is a legal escape route — then the building department confirms.
Turning a basement into a bedroom and needs a window that passes egress so the room is legal.
- Enter frame & type
- Check all four limits pass
- Plan the window well too
Comparing windows and wants the smallest one that still meets egress for the opening they have.
- Compare casement vs hung
- See net clear by type
- Confirm with the spec sheet
Wants a quick pass/fail on an opening against the R310 limits before or during a job.
- Set above grade / grade-floor
- Read every check
- Attach the PDF to the set
7 things to know about egress windows
An egress window is a life-safety escape route, so the details decide whether a room is legal — and safe. These points keep you on track.
Egress window calculator FAQ
The sizing, code, and safety questions people ask most about egress windows.
Under the IRC (R310), an egress opening needs a net clear opening of at least 5.7 sq ft, a min clear height of 24″, a min clear width of 20″, and a sill no higher than 44″ above the floor.
The 5.7 applies above grade; grade-floor openings get a reduced 5.0 sq ft. You can’t hit the area with the minimums alone — 24 × 20 is only ~3.3 sq ft — so at least one dimension must be well above its minimum.
Every dimension is the net clear opening, not the frame or glass.
Net clear opening is the actual unobstructed space a person can climb through when the window is fully open — measured through the opening, not across the frame or glass.
It’s always smaller than the callout size because frame, sash, and hardware take up room. How much depends on type: a casement opens nearly the whole frame, a hung window ~half its height, a slider ~half its width.
The number that matters for code is the manufacturer’s published net clear opening, not the outside window size.
Casement windows are the most efficient because the sash swings fully open, so nearly the entire frame becomes clear opening — a smaller casement can meet 5.7 sq ft.
Single- and double-hung windows open ~half their height, so they must be tall to reach the area. Sliders open ~half their width, so they must be wide. Both work — they just have to be larger overall.
Where wall space is tight, a casement is usually the easiest way to hit egress.
Yes. Any sleeping room — including a basement bedroom — must have at least one emergency escape and rescue opening, and basements with habitable space generally need one too.
It’s a life-safety requirement: an escape route in a fire and a way for a firefighter in gear to enter. A basement bedroom without compliant egress isn’t legal, won’t pass inspection, and creates liability and resale problems.
Below-grade windows also need a window well large enough to open the window and climb out, with a ladder if deep.
The bottom of the clear opening can be no more than 44″ above the finished floor (IRC), so the opening is reachable in an emergency without climbing on furniture.
If the sill is higher than 44″, the window fails egress even if the opening is plenty large — you’d need to lower the window or add a code-recognized permanent step or platform.
Measure from the finished floor to the bottom of the net clear opening, not to the sill trim, since they can differ.
Yes, and it’s a common trap. The 24″ height and 20″ width are floors on each dimension, but a window at exactly 24 × 20 is only ~3.3 sq ft — well short of 5.7.
So you can’t use both minimums at once; at least one dimension must be significantly larger to reach the area. A narrow window can hit 20″ wide but must be much taller than 24″, and vice versa.
Area, height, width, and sill are separate checks that all have to pass — three of four still fails.
If the sill is below ground level — typical for a basement — yes. The well must be large enough for the window to open fully and a person to climb out, generally at least 9 sq ft with a minimum dimension of 36″.
If the well is deeper than 44″, it also needs a permanently attached ladder or steps. Covers must open from inside without special tools or knowledge.
This calculator checks the window opening, not the well — but a below-grade egress window isn’t compliant without a conforming well, so plan for both.
Measure the net clear opening with the window fully open: clear width at the narrowest point, clear height at the shortest point, then W × H / 144 for area in sq ft.
For a casement that’s roughly the full sash opening. For a hung window the clear height is only the sash travel (~half the frame). For a slider the clear width is only the open panel (~half the frame). Also measure the sill from finished floor to the bottom of that opening.
The surest figures come from the manufacturer’s spec sheet — use those to confirm before ordering or installing.
Estimate only — confirm the net clear opening. This calculator estimates the net clear opening from the frame size and window type using typical deductions; the true net clear opening is set by the manufacturer and listed on the spec sheet or NFRC label — always verify against that before ordering or installing. Checks are based on the IRC R310 model limits (5.7/5.0 sq ft area, 24″ height, 20″ width, 44″ sill); local jurisdictions may amend them, and this does not check window wells, ladders, covers, or other required provisions. An egress opening is a life-safety escape route: confirm the complete design with your local building department and permit set. For planning purposes only.