Key takeaways
- The 2021 IRC figures are 5.7 sq ft of net clear opening, or 5 at grade floor, 24 in high, 20 in wide, and a sill no higher than 44 in.
- The minimum height and width together make only 3.33 sq ft, so meeting both isn't enough.
- Measure the clear opening with the window fully open, because this is about getting out in a fire.
This compares your measurements with figures in 2021 IRC Section R310. Measure the actual clear opening with the window fully open, rather than the frame or the size on the label. It doesn't confirm that a window meets your local code, so ask your building department.
The figures
| Item | 2021 IRC figure | Section |
|---|---|---|
| Net clear opening | At least 5.7 sq ft | R310.2.1 |
| Grade-floor opening | At least 5 sq ft | R310.2.1 |
| Net clear height | At least 24 in | R310.2.2 |
| Net clear width | At least 20 in | R310.2.2 |
| Sill height above floor | No more than 44 in | R310.2.3 |
The section applies to basements, habitable attics and every sleeping room, with some exceptions. The opening must work from inside without keys, tools or special knowledge.
The trap: minimum height × minimum width
A 24-inch high, 20-inch wide opening meets both of those numbers but gives only 3.33 square feet, well short of 5.7. These openings show how size adds up:
| Clear width | Clear height | Clear area | Compared with 5.7 sq ft |
|---|---|---|---|
| 20 in | 24 in | 3.33 sq ft | Below |
| 20 in | 41 in | 5.69 sq ft | Just below |
| 24 in | 34 in | 5.67 sq ft | Just below |
| 24 in | 35 in | 5.83 sq ft | At or above |
| 30 in | 28 in | 5.83 sq ft | At or above |
How to measure
- Open the window fully, as you normally would.
- Measure the clear width and height of the opening, inside the sash and any stops.
- Measure from the finished floor to the bottom of the clear opening.
- Enter those numbers, not the size on the window's label. Double-hung and sliding windows open only part of their frame, so their clear area is much smaller than their size suggests.
Situations with their own rules
- Replacement windows (R310.5): these are exempt from the size rules if they're the maker's largest standard size that fits the existing opening, of the same operating style or one with equal or greater opening area, and aren't part of a change of occupancy.
- Below grade (R310.4): area wells need at least 9 square feet, with at least 36 inches of width and projection. Wells deeper than 44 inches need a ladder or steps.
- Under decks and porches (R310.2.4): these need to be fully openable, with a path at least 36 inches high and wide to a yard or court.
- New basement bedrooms (R310.7): new sleeping rooms in existing basements need an opening.
We've read these in the 2021 edition. The 2024 edition's section numbers may differ, so check what your area uses.
Older homes
Replacing a window in a pre-1978 home falls outside the lead rule's "minor repair" exclusion, so ask any contractor about their RRP certification.
Frequently asked questions
What size is an egress window?
The 2021 IRC figures are at least 5.7 square feet of clear opening, at least 24 inches high and 20 inches wide, and a sill no more than 44 inches above the floor. A grade-floor opening can be 5 square feet.
Is a 24 × 36 inch opening enough?
It's 6 square feet, which is above the 5.7 figure. But your local code and how you measure decide the outcome, so confirm with your building department.
Does my existing bedroom window need to change?
The 2021 IRC's R310.5 and R310.7 cover replacements and alterations, and exceptions apply. Ask your building department about your project.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 3, Section R310
- 40 CFR 745.83