How the Egress Window Size Checker works

The residential code calls an egress window an emergency escape and rescue opening. Section R310 of the 2021 IRC requires one in every sleeping room, in a basement and in a habitable attic (with a few exceptions this tool does not evaluate), and sets four size rules for it. This tool checks each rule separately, because passing three of them tells you nothing about the fourth.

1. Area. The net clear opening must be at least 5.7 sq ft. For a grade-floor opening the minimum is 5 sq ft. The code defines that as an opening whose bottom is no more than 44 in above or below the finished ground next to it.
2. Height. The net clear opening must be at least 24 in high.
3. Width. The net clear opening must be at least 20 in wide.
4. Height off the floor. The bottom of the clear opening must be no more than 44 in above the floor.

Where the bottom of the clear opening is below the ground outside, the code also requires a well in front of it, which the 2021 IRC calls an area well. Its horizontal area must be at least 9 sq ft, and its width and its projection from the wall must each be at least 36 in. A well with a vertical depth of more than 44 in needs a permanently fixed ladder or steps, which may reach up to 6 in into those dimensions.

The trap is in the first three rules. An opening exactly 20 in wide and 24 in high meets both dimension rules and has only 3.33 sq ft, far short of either area. One dimension has to be well over its minimum. The table gives the least clear height for a range of clear widths. Both figures in a cell are rounded up, the fraction to the next 1/16 in and the decimal to the next 0.01 in, so the checker on the tool page passes every number in it.

What to measure. The code says the net clear opening dimensions are those that result from normal operation of the opening, and that the opening has to work from inside without keys, tools or special knowledge. It does not define the term any further. This tool takes it as the free hole left when the window is opened the way it is meant to open: the space between whatever is still in the way, such as a sash that does not move, the frame, the stops and the hardware. Measure it on the window itself or take it from the manufacturer's clear opening figure. It is not the same as the frame size or the size on the label.

Where the wording can be read two ways, this tool takes the reading that fails more, and the form enforces it:

• The 5 sq ft minimum, above the ground only. The code's definition of a grade-floor opening reaches 44 in below the adjacent finished ground as well as above it. Beside a window well, the ground next to the opening could mean the floor of the well or the yard around it, and the code does not say which. So this tool gives the 5 sq ft minimum only to an opening you say is at or above the ground, with no well, and only once you enter a height of 44 in or less from the ground up to its bottom. Every opening below the ground is held to 5.7 sq ft. That can fail an opening of 5 to 5.7 sq ft behind a shallow well that a building department might accept; the result says so when it happens. The 2018 edition gave the smaller minimum to any below-grade opening; the 2021 and 2024 editions do not.
• Below the ground, the well is part of the answer. The form asks whether the bottom of the opening is below the ground outside. Until that is answered and the height off the floor is entered, the result reads "Not checked yet", never a pass. If the answer is yes, every well field is required and a failing well fails the window.
• Well depth, the deepest way. The code says "vertical depth" and gives no point to measure from. This tool asks for the depth from the highest point of the well's top edge (the rim of the well wall, or the ground beside it if that is higher) to the lowest point of the well floor. A well 43 in below the ground with a 3 in rim is 46 in deep here and needs a ladder.
• No rounding in your favour. Both handouts listed under Sources print 20 in by 41 in as a 5.7 sq ft opening. It is 5.694 sq ft, 0.8 sq in short, and this tool fails it and says why; your inspector may accept the handout figure, so ask. Areas are shown rounded down, required dimensions rounded up, and your own measurements as you typed them.
• Exactly at a limit passes, anything past it does not. The sill rule is a maximum that 44 in itself satisfies, and the ladder rule applies only to a well deeper than 44 in, so a 44 in sill passes and a 44 in deep well needs no ladder. A sill of 44 1/16 in fails.
• Numbers that might be in the wrong unit are refused. Every field is in inches. A bare number under 12 for the height off the floor, the height above the ground or the well depth could be feet, and a bare number over 72 for the clear opening (over 96 for the well) could be centimetres; in each case reading it as inches would be the kinder mistake. The field asks you to type the unit, as in 4' or 4", and every value is repeated under the verdict as it was read. These bounds are this tool's, not the code's.

Curved wells. The code does not say how width and projection apply to a well that is not a rectangle. This tool supports two shapes. A rectangular well has an area of width times projection. A rounded well is taken as half an oval with its flat side on the house wall, with an area of π ÷ 4 (0.785) × width × projection, and the 36 in minimums are checked at its widest and deepest points. That last choice is this tool's reading, not the stricter one, so a rounded well never gets a plain pass on those two lines: they are tagged Assumed, and the verdict reads "Meets the size rules if your building department accepts a rounded well measured at its widest points". The shape matters: a rounded well 40 in by 36 in has 7.85 sq ft, not the 10.00 sq ft of a rectangle that size. For any other shape, get the horizontal area from the maker or measure it; this tool cannot work it out.

Not checked here. Size is only part of the section. The tool does not check that the window opens without keys or tools; bars, grilles, covers or screens over the opening or the well; window opening control devices; the clear path required under a deck or porch; that the opening leads to a yard or public way; that the well lets the window open fully; that the ladder or steps reach no more than 6 in into the required well dimensions and are not blocked by the open window; their rung and tread sizes; or well drainage. A separate rule (R312.2.1) requires window fall protection where the bottom of an opening is less than 24 in above the floor and more than 72 in above the ground outside; the result points to it for a low sill but does not check it.

What the tested examples cover. The examples marked Published come from two building department handouts: a warning that the two minimum dimensions do not make the area, one drawn opening, six pairs from their width and height charts, and one well size. Neither handout prints a worked example for the sill limit, the ladder rule, a below-ground opening or a rounded well; those are tested against examples worked by hand from the code text.

Scope. Windows in one- and two-family dwellings and townhouses built under the International Residential Code, checked against the 2021 IRC. The 2018 and 2024 texts were read as well. They give the same figures; the 2018 edition numbers them differently and words the grade-floor exception more broadly, and the 2024 edition moves the whole section to R319. The edition your jurisdiction has adopted, with its local amendments, is what governs, so confirm with your building department before you cut a foundation wall. This tool does not cover buildings under the International Building Code, doors used as the escape opening, or the separate provisions for replacement windows and for existing basements, which can allow a smaller opening.

This tool cannot be published unless it reproduces every example below. 9 are copied from a published source. 20 were worked by hand from the cited rule.