Rough Opening Calculator
Rough openings and framing for windows and doors, with the egress check — where meeting both minimums still fails the code.
Emergency escape
Two minimums don't make a maximum
An escape window has three requirements, and people read them as one. At least 20″ of clear width, at least 24″ of clear height, and at least 5.7 sq ft of clear area. A window at exactly both dimensional minimums is 20 × 24 — which is 3.33 sq ft, short of the area requirement by a factor of 1.7. The dimensional minimums are floors on each side, not a combination that satisfies anything. At the minimum width you need 41″ of height; at the minimum height you need 34″ of width.
And the area is the net clear opening, not the window. It's what's left when the sash is fully open, so the style decides it more than the size does. A casement swings clear and gives up little; a double-hung only ever opens half its height, however tall the unit. At 36″ wide, a casement clears 5.7 sq ft at a 31″ unit — a double-hung needs 58″. That's the difference between a modest window and one that dominates the wall, and it's settled before anything is framed.
The callout is feet and inches, not inches. A "2/8" door is two feet eight — 32″, not 28. A "3050" window is 3'0″ × 5'0″. Reading a callout as inches under-frames every opening on the drawing by a consistent and invisible amount, and it's the commonest ordering error there is.
- A door's rough opening is measured from the subfloor. The finished floor comes out of the gap underneath, so decide the floor covering before framing — a door framed tight and then given ¾″ hardwood won't swing.
- Door allowances aren't square. 2″ on the width and 2½″ on the height, because the head jamb needs the extra.
- An awning window rarely satisfies egress at any practical size, because the sash pivots outward and blocks its own opening. Treat one as daylight and ventilation.
- A slider loses half its width rather than half its height — so a wide short slider can pass where a tall narrow one can't. The same arithmetic problem, rotated 90°.
- Use the manufacturer's published net clear opening. They publish it for exactly this reason, and it varies by several inches between products of the same nominal size.
How to use
- Enter the size callout — 3050, 2/8 x 6/8 and 36 x 60 all work.
- Pick the opening type, since doors and windows get different allowances.
- Tick the egress box for a bedroom window and check all three requirements.
- Order from the manufacturer’s published rough opening, not from this one.
Frequently asked questions
What size is a 2/8 door?
Thirty-two inches. Door and window callouts are written in feet and inches, so 2/8 means two feet eight — not twenty-eight. A 3/0 door is thirty-six inches, and a 3050 window is three feet by five feet. Reading a callout as inches under-frames every opening on a drawing by a consistent and invisible amount, and it is probably the commonest ordering error in the trade.
What is the rough opening for a window?
The unit size plus about an inch each way, giving half an inch of shim space on each side. Manufacturers differ and some ask for three quarters of an inch per side, so the published figure for the unit you are actually buying governs. It is a small allowance and it exists to let you plumb and square the window in an opening that is neither.
What is the rough opening for a door?
Two inches on the width and two and a half on the height for a standard pre-hung. The allowances are deliberately not square: the extra half inch on the height is what leaves room for the head jamb and its shims. A thirty-two inch door in an eighty inch height wants a thirty-four by eighty-two-and-a-half opening.
Is a door rough opening measured from the subfloor?
Yes, and this catches people. The two and a half inch height allowance assumes you are measuring from the subfloor, and the finished floor comes out of the gap underneath the door. Frame tight to the subfloor and then lay three-quarter inch hardwood and the door will not swing. Deciding the floor covering before framing the openings avoids a repair that is genuinely unpleasant.
How big does an egress window need to be?
It has three requirements and all of them apply: at least twenty inches of clear width, at least twenty-four inches of clear height, and at least 5.7 square feet of clear area. Grade-floor openings get 5.0 square feet. The sill may be no more than forty-four inches above the floor, which is a separate requirement again.
Does a 20 by 24 inch window meet egress?
No, and this is the trap. Twenty by twenty-four is 3.33 square feet, against a requirement of 5.7 — short by a factor of 1.7. The dimensional minimums are floors on each side rather than a combination that satisfies anything, and a window meeting both of them still fails badly on area. At the minimum width you need forty-one inches of height; at the minimum height you need thirty-four inches of width.
What is net clear opening?
What is actually left when the sash is fully open, which is not the window size and is not the rough opening either. It is the hole a person could climb through, so frames, sashes and hardware all come off it. This is what the code measures, and it is why two windows of the same nominal size can differ by several square feet.
Why do double-hung windows fail egress?
Because only the lower sash opens, so you never get more than half the height however tall the unit is. At thirty-six inches wide, a casement clears the area requirement at about a thirty-one inch unit while a double-hung needs around fifty-eight. That is the difference between a modest window and one that dominates the wall, and it is decided before anything is framed.
Which window style is best for egress?
A casement, comfortably. The sash swings clear of the frame so almost the whole opening counts, which is why casements turn up in basements so often. A horizontal slider loses half its width rather than half its height, so a wide short one can work where a tall narrow one cannot. An awning pivots outward and blocks its own opening, and rarely satisfies egress at any practical size.
How accurate are the net clear opening estimates?
They are estimates from typical frame and sash losses, and they will be out by several inches against a specific product. Manufacturers publish the actual net clear opening for every unit precisely because this is what it gets checked against. Use theirs to order and use these to work out which units are obviously not worth pricing in the first place.
How high can an egress window sill be?
Forty-four inches above the finished floor. Above that a person cannot reliably climb out, particularly a child or somebody disoriented by smoke, which is the whole point of the requirement. A permanently installed step below the window does not automatically fix it — it has to meet its own requirements to count.
Do these numbers work everywhere?
They are the common requirements in widely adopted editions, but egress rules in particular differ between editions and get amended locally, and every manufacturer publishes its own rough openings which govern over any general allowance. Whether an opening needs a header at all depends on whether the wall is bearing. Check the unit instructions and your adopted code before cutting anything.
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