About Egress Window Calculator (IRC R310)
The egress window calculator applies IRC 2021 R310.2 to emergency escape and rescue openings — the operable window every basement, habitable attic, and sleeping room must have. R310.2.1 requires a net clear opening of at least 5.7 square feet (820.8 square inches), with a net clear height of at least 24 inches and a net clear width of at least 20 inches; the exception drops the area to 5.0 square feet (720 square inches) for grade floor or below grade openings. R310.2.2 caps the sill at 44 inches above the floor.
In size-it mode, enter the net clear dimension you already know and the tool returns the other dimension needed to satisfy the area rule, floored at the 24-inch or 20-inch minimum, and tells you which constraint governs. In check-it mode, enter the as-built net clear width, height, and sill height for a rule-by-rule verdict with the margin on each. The window well card checks the R310.2.3 minimums — 9 square feet of horizontal area with the projection and width each at least 36 inches — and flags the permanently affixed ladder or steps R310.2.3.1 requires once the well is deeper than 44 inches.
How It Works
- Pick size-it or check-it mode. Both work on NET CLEAR dimensions — the actual unobstructed opening measured with the window in its normal fully open position, not the glass size or the rough opening.
- Size-it: choose which dimension you know (width or height), enter it, and tick grade floor / below grade if the exception applies. The tool divides the required area (820.8 or 720 square inches) by your dimension, floors the result at the 24-inch height or 20-inch width minimum, and reports which rule governs. The displayed value is rounded up at the second decimal so building to it always meets the area.
- Check-it: enter the as-built net clear width, height, and sill height. Each rule — area, width, height (R310.2.1), and sill height (R310.2.2) — gets its own pass/fail badge with the actual value, the code limit, and the margin. A dimension exactly at a limit passes; anything beyond it fails.
- Window well: for openings with a sill below grade, enter the well area, its horizontal projection from the foundation wall, its width along the wall, and its depth. The well must offer at least 9 square feet with both the projection and the width at least 36 inches, and it must let the window open fully (R310.2.3).
- If the well is deeper than 44 inches, the tool flags the permanently affixed ladder or steps required by R310.2.3.1 (inside width at least 12 inches, projecting at least 3 inches from the wall, rungs at most 18 inches on center; it may encroach up to 6 inches into the required well dimensions).
Worked Example
A basement bedroom casement window offers a net clear opening of 30 inches wide by 28 inches high with the sill 40 inches above the floor, and it is not a grade floor opening. The net clear area is 30 x 28 = 840 square inches = 5.83 square feet, which beats the 820.8 square inch (5.7 sq ft) requirement by 19.2 square inches. The width clears the 20-inch minimum by 10 inches, the height clears the 24-inch minimum by 4 inches, and the sill sits 4 inches under the 44-inch maximum — all four R310.2 rules pass.
Formulas
- Net clear opening area (R310.2.1)
A_net = W_clear x H_clear >= 820.8 in2 (5.7 ft2); grade floor / below grade: >= 720 in2 (5.0 ft2)- Required other dimension (size-it mode)
D_required = max(D_min, A_required / D_known)- Sill height (R310.2.2)
H_sill <= 44 in above the floor- Window well (R310.2.3 / R310.2.3.1)
A_well >= 9 ft2 AND projection >= 36 in AND width >= 36 in; ladder required when depth > 44 in
Standards & References
- IRC 2021 R310.1 -- emergency escape and rescue opening required (basements, habitable attics, every sleeping room)
- IRC 2021 R310.2.1 -- minimum opening area 5.7 ft2; net clear height 24 in, width 20 in; grade floor / below grade exception 5.0 ft2
- IRC 2021 R310.2.2 -- maximum sill height 44 in above the floor
- IRC 2021 R310.2.3 -- window wells: 9 ft2 horizontal area, 36 in projection and width
- IRC 2021 R310.2.3.1 -- permanently affixed ladder or steps for wells deeper than 44 in
Frequently Asked Questions
What counts as the net clear opening — is it the glass size?
No. The net clear opening is the actual unobstructed rectangle a person can pass through with the window in its normal fully open position, measured between the sash, frame, stops, and hardware. It is always smaller than the glass or the unit call-out size — a 3-0 x 4-0 double-hung typically nets only about half its sash area because just one sash opens. Manufacturers publish net clear opening dimensions for each unit; use those, not the rough opening or glass dimensions.
When does the 5.0 square foot grade floor exception apply?
IRC R310.2.1 Exception allows a net clear opening of 5.0 square feet instead of 5.7 for grade floor openings and below grade openings — windows whose sill is within 44 inches of, or below, the adjacent finished grade, such as typical basement windows opening into a window well. The 24-inch height and 20-inch width minimums and the 44-inch maximum sill height still apply unchanged.
Can a window meet 5.7 square feet at the minimum 24 x 20 dimensions?
No — 24 x 20 inches is only 480 square inches (3.33 sq ft), well short of the 820.8 square inches required. At the minimum 20-inch width the height must reach 41.04 inches, and at the minimum 24-inch height the width must reach 34.2 inches. Every compliant opening trades between those extremes, which is exactly what size-it mode computes.
What are the window well requirements for a basement egress window?
When the sill is below grade, R310.2.3 requires a window well with a horizontal area of at least 9 square feet and with both the horizontal projection from the wall and the width along the wall at least 36 inches, and the well must allow the window to open fully. If the well is deeper than 44 inches, R310.2.3.1 adds a permanently affixed ladder or steps (at least 12 inches wide, projecting at least 3 inches from the wall, rungs no more than 18 inches apart), which may encroach up to 6 inches into the required well dimensions.
Do replacement windows have to meet the full egress size?
Not always. IRC 2021 R310.5 (replacement windows for emergency escape and rescue openings) gives allowances for replacements in existing openings: a replacement window is generally acceptable where it is the manufacturer's largest standard size that fits the existing frame or rough opening and is of the same or a more openable style, rather than being forced to meet the full new-construction numbers. Adoption and amendments vary by state, so confirm the replacement provisions with your local building department.
Which rooms need an emergency escape and rescue opening?
IRC R310.1 requires at least one operable emergency escape and rescue opening in basements, habitable attics, and every sleeping room, and each basement sleeping room needs its own. The opening must operate from inside without keys, tools, or special knowledge and open directly to a public way or to a yard or court that leads to one. Storm shelters and basements of 200 square feet or less used only for mechanical equipment are excepted.
Does a window exactly at a limit — say a 44-inch sill — pass?
Yes. The code language is "not more than 44 inches" and "not less than" for the minimums, so a value exactly at the limit complies: a 44.00-inch sill, a 20.00-inch width, or exactly 5.7 square feet all pass, while 44.01 inches or 19.99 inches fail. The calculator applies the same at-the-limit rule with a tolerance for floating-point rounding, but leave real-world margin — field measurements and inspector tapes are not that precise.