What is the minimum size of an egress window?

Under the International Residential Code, a net clear opening of 5.7 sq ft (5.0 sq ft at grade floor or below grade), at least 24 in high and 20 in wide, with the sill no more than 44 in above the floor.

Is a 24 by 20 inch window enough for egress?

No: 24 × 20 in is 3.3 sq ft. Those are the minimum height and width, but the area must reach 5.7 sq ft too, so a 20 in wide opening needs about 41 in of clear height.

What size is an escape window in the UK?

Approved Document B asks for an unobstructed openable area of at least 0.33 m², at least 450 mm high and 450 mm wide, with the bottom of it no more than 1100 mm above the floor.

Does a double-hung window count as egress?

Only if its clear opening, with one sash moved, reaches the minimums. A double-hung window opens about half its height, so it has to be large; casements are the usual choice.

Windows · Escape

Egress window size calculator

In the US an egress window needs a net clear opening of at least 5.7 sq ft (5.0 sq ft at grade floor or below), at least 24 in high and 20 in wide, with the sill no more than 44 in above the floor. In England the escape window needs 0.33 m² unobstructed, at least 450 mm high and wide, with the bottom no more than 1100 mm up. 24 × 20 in is only 3.3 sq ft, so meeting both minimums is not enough: a 24 in wide opening needs to be about 34 in high. Measure the opening clear of the sash, not the rough opening.

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Clear opening

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US, IRC R310

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UK, Approved Doc. B

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Height needed at this width

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The rules

	US, IRC R310.2	England, Approved Document B vol. 1
Clear opening area	5.7 sq ft (0.53 m²); 5.0 sq ft at grade floor or below	0.33 m² unobstructed
Minimum height	24 in (610 mm)	450 mm
Minimum width	20 in (508 mm)	450 mm
Highest sill	44 in (1118 mm) above the floor	1100 mm above the floor

The example above, a casement opening 24 in wide and 41 in high with a 40 in sill: 6.83 sq ft, so it passes in the US above grade; it is 0.63 m², so it passes in the UK too.

Measuring it right

Net clear, not rough. Measure the opening the window leaves when it is fully open, between the frame and the open sash: the rough opening is several inches bigger on every side.
A double-hung or sliding window opens only half. A double-hung window 32 × 54 in gives about 29 × 23 in clear, 4.6 sq ft: under 5.7. Casements open their whole sash and are the usual egress window.
Below grade, the window well counts too. In the US a basement egress window needs a well at least 9 sq ft with a 36 in projection, and a ladder when it is deeper than 44 in.

What this page does not do

It does not replace your local code. US states and cities amend the IRC; Scotland, Wales and Northern Ireland have their own regulations. The inspector has the last word.
It does not check the glass. A window near the floor may need safety glazing.

Questions people ask

What is the minimum size of an egress window?: Under the International Residential Code, a net clear opening of 5.7 sq ft (5.0 sq ft at grade floor or below grade), at least 24 in high and 20 in wide, with the sill no more than 44 in above the floor.
Is a 24 by 20 inch window enough for egress?: No: 24 × 20 in is 3.3 sq ft. Those are the minimum height and width, but the area must reach 5.7 sq ft too, so a 20 in wide opening needs about 41 in of clear height.
What size is an escape window in the UK?: Approved Document B asks for an unobstructed openable area of at least 0.33 m², at least 450 mm high and 450 mm wide, with the bottom of it no more than 1100 mm above the floor.
Does a double-hung window count as egress?: Only if its clear opening, with one sash moved, reaches the minimums. A double-hung window opens about half its height, so it has to be large; casements are the usual choice.