How to Measure an Egress Window Net Clear Opening

Why this matters

The window a firefighter needs to get through, and the one a child needs to get out of, is the hole that exists when the sash is fully open. Not the glass, not the frame, not the rough opening the window was ordered for. Those three numbers are all bigger than the real opening, sometimes more than twice as big, and they are the numbers people quote when they say a bedroom window "is plenty big." Any trade that adds a window air conditioner, a security bar, a built-in, a raised floor or a replacement sash needs to know what the opening actually is before and after. Measuring it takes five minutes and a tape. The rules are owned by the code card: see related, Egress Window Code Compliance Per IRC R310. This card is how you get numbers to put against them.

The gate you are measuring against

Under the 2021 IRC, as adopted by your jurisdiction, R310 requires an emergency escape and rescue opening in each sleeping room and in a basement with habitable space, and sets four figures for it. Check your adopted edition and any local amendment, because these are model-code numbers until a jurisdiction adopts them.

• Net clear opening area of at least 5.7 square feet, or 5.0 square feet for a grade-floor or below-grade opening.
• Net clear height of at least 24 inches.
• Net clear width of at least 20 inches.
• Sill height of not more than 44 inches, measured from the finished floor to the bottom of the clear opening.

All four apply at once. A window can clear 5.7 square feet and still fail on a 19-inch width. The section also requires the opening to work from the inside without a key, a tool or special knowledge; that is a separate check with its own card: see related, How to Check Bars, Grilles and Well Covers for Release From Inside.

What you are actually measuring

Net clear opening is the unobstructed space a body can pass through with the window open as far as its normal operation allows. On every window type it is bounded by the stops, the sash rails and the hardware, not by the frame or the glass.

DOUBLE-HUNG, from inside, lower sash raised
+--------------------------+
| upper sash, with raised  |
| lower sash behind it     |
+==========================+ <- bottom rail of raised
|                          |    sash = top of opening
|       CLEAR HEIGHT       |
|   rail to sill stop      |
|                          |
+--------------------------+ <- sill stop
|<- jamb liner to liner  ->|  = CLEAR WIDTH

CASEMENT, looking down on the sill, sash open
              outside
    open sash  \
                \
hinge  |=========|<---- CLEAR WIDTH ---->| latch
jamb    hinge track and                    stop
        operator arm
              inside (room)
Casement CLEAR HEIGHT = sill stop to head stop

On the double-hung, the raised lower sash sits behind the upper one, so its bottom rail is the top of the clear opening, and the side jamb liners or stops set the width. On the casement, the open sash itself, with its hinge track and operator arm, intrudes on one side, so the width runs from the inside edge of the open sash or its hardware to the latch-side stop, whichever is narrower. Height on a casement runs sill stop to head stop.

The steps

1. Open the window fully, the way an occupant would. Raise the sash or crank the casement to its full travel, using only the window's own operation. Do not remove a sash, tilt it out, or take off a limiter with a screwdriver to get a better number; the code measures the opening in normal operation, and a sash that only comes out with a tool is not part of an escape route. If the sash will not stay up because a balance is broken, record that the window does not currently provide a clear opening at all, and measure it held up only to record what it would provide once repaired.

2. Measure the clear height at the narrowest point. On a double-hung, from the top of the sill stop, where a body would slide over, to the underside of the raised sash's bottom rail. On a casement, from sill stop to head stop. Take it at both sides and the middle and keep the smallest, because sashes rack and rails sag. Record to the nearest quarter inch and round down, never up; a reading of 23.75 inches is under the 24-inch minimum, and rounding it to 24 would pass a window that fails.

3. Measure the clear width at the narrowest point. On a double-hung, jamb liner to jamb liner, or stop to stop, inside the sash track. On a casement, from the hinge-side obstruction to the latch-side stop. Take it at the top, middle and bottom and keep the smallest.

4. Multiply height by width and divide by 144. Inches times inches gives square inches; there are 144 square inches in a square foot. Work it twice: once by the multiplication, and once by checking that the answer is plausible (a 2-foot by 2.5-foot opening is 5 square feet, so a 24 by 30 inch opening must come out at 5.0).

5. Measure the sill height from the finished floor. Tape from the finished floor surface directly below the window, carpet or tile included, to the bottom of the clear opening, which on most windows is the top of the sill stop. Not to the bottom of the frame, not to the exterior sill, not to the top of the radiator cover in front of it. A built-in seat, a window bench or a heating enclosure under the window does not change the measurement unless the code official in your jurisdiction accepts it as a permanent step, and that is their call, not yours.

6. Compare all four numbers to the gate and record each one. Write down height, width, area, sill height and the window type, and which floor it is on, because the area threshold depends on it. Record "pass" or "fail" for each of the four, not one overall word.

One gate, two windows

A handyman is asked to put a window air conditioner in a second-floor bedroom and wants to know first which window is the escape opening. There are two.

Window A, a double-hung. The frame measures 34 by 58 inches, and someone has already told the customer that's "almost 14 square feet of window." 34 times 58 is 1,972 square inches, over 144 is 13.69 square feet. That is the frame, and it is not what anyone climbs through.

With the lower sash raised fully:

• Clear height, sill stop to underside of the raised rail, smallest of three readings: 26 inches.
• Clear width, jamb liner to jamb liner, smallest of three: 30 inches.
• Area: 26 times 30 is 780 square inches; 780 over 144 is 5.42 square feet. Check: 2.17 feet by 2.5 feet is 5.4, which agrees.
• Sill height from the finished carpet to the top of the sill stop: 22 inches.

Against the gate on a second floor: height 26 passes the 24 minimum, width 30 passes the 20, sill 22 passes the 44 maximum, and area 5.42 fails the 5.7 by 0.28 square feet. The frame number was 2.53 times the real opening, which is how a window that "looks huge" fails.

Window B, a casement. Cranked to its full travel:

• Clear height, sill stop to head stop, smallest of three: 41 inches.
• Clear width, inside edge of the open sash's hinge track to the latch-side stop, smallest of three: 21 inches. The frame is 27 inches wide; the sash and its track take the other 6.
• Area: 41 times 21 is 861 square inches; 861 over 144 is 5.98 square feet. Check: 3.42 feet by 1.75 feet is 5.98.
• Sill height: 40 inches from the finished floor.

Against the same gate: height 41 passes, width 21 passes by an inch, area 5.98 passes the 5.7, sill 40 passes the 44. Window B is the escape opening for this room. Window A is not, and in an older house that may be lawful as it stands, but it means Window B is the one nothing goes in.

So the air conditioner goes in Window A, and as it happens Window A is the double-hung, which is the type window units are built for. The handyman records both sets of numbers on the ticket, because the next person to touch either window needs them. The placement itself, and what to say when the only qualifying window is the one the customer wants to use, are another card: see related, How to Place a Window Air Conditioner Without Blocking the Only Egress.

What would change the result. If the casement's width had read 19.75 inches at its narrowest point, rounding down keeps it at 19.75, under the 20-inch minimum, and the room would have no qualifying opening at all, which is a finding the customer is told in writing. If the bedroom had been on a grade floor, where the 5.0 square foot figure applies, Window A's 5.42 would pass too, and the room would have two escape openings rather than one. If Window B had been fitted with a window opening control device that stops the sash at a few inches for fall protection, the device has to release for full opening without a key or tool, and you measure with it released; one that cannot be released has made the casement fail.

How you know you measured right

• You have three readings on each dimension and recorded the smallest.
• Your area is from height times width over 144, and a rough feet-times-feet check agrees with it.
• The sill height was taken from the finished floor to the bottom of the clear opening.
• You recorded all four results against the right area threshold for the floor the room is on.
• Nothing was removed or forced to get the opening.

References

• International Residential Code, 2021 edition, R310 (emergency escape and rescue openings), as adopted by your jurisdiction
• Window manufacturer data for clear opening dimensions, where the unit is new and documented
• See related: Egress Window Code Compliance Per IRC R310
• See related: The Egress Window and the Trades That Quietly Defeat It