How to use the egress window calculator
Every bedroom, and every basement with habitable space, needs a way out other than the door. The International Residential Code calls this an emergency escape and rescue opening, set out in section R310, and the window is usually that opening. The rules are short but easy to get wrong, because the size that counts is the clear opening, not the window's nominal or rough opening size. This calculator checks all of the dimensional rules at once.
- Pick the window location. Openings above the grade floor need 5.7 ft²; grade-floor and below-grade openings need 5.0 ft².
- Enter the clear opening width and height with the sash fully open, from the manufacturer's data or measured on site, and the sill height from the finished floor to the bottom of the clear opening.
- Add the window well if the sill is below the ground outside: its projection from the wall, its width along the wall and its depth.
- Read the results: clear area, a pass or fail for each rule, the minimum height that would work at your width, and whether the well needs a ladder.
Egress window rules and formulas
Net clear area = clear width × clear height
Area ≥ 5.7 ft² (820.8 in², 0.53 m²), or 5.0 ft² (720 in², 0.46 m²) at grade floor or below grade
Clear height ≥ 24 in (610 mm), clear width ≥ 20 in (508 mm)
Sill height ≤ 44 in (1,118 mm)
Well area ≥ 9 ft², projection and width ≥ 36 in, ladder if depth > 44 in
Meeting the minimum width and minimum height isn't enough on its own: both have to be met and the area too. That is why the tool also tells you the height needed at your chosen width, which is the quickest way to pick a window size from a catalogue.
| Window type | Share of the frame that opens | Notes for egress |
|---|---|---|
| Casement | Almost all | Easiest to comply; check the hinge side clearance |
| Double-hung / single-hung | About half the height | Needs a tall unit, often 60 in or more |
| Slider | About half the width | Needs a wide unit to reach 20 in clear |
| Awning / hopper | Limited by the opening angle | Often fails; use egress hardware if offered |
Worked examples
A second-floor bedroom casement
A casement with a clear opening of 24 × 36 inches and a 40 inch sill. The area is 864 in², or 6.0 ft², above the 5.7 ft² needed. Width, height and sill all pass, so this window works for egress.
A double-hung that looks big enough
A 32 × 54 inch double-hung opens to about 28.5 × 23.5 inches clear. The area is 670 in², or 4.65 ft², and the clear height is under 24 inches, so it fails twice. At a 28.5 inch clear width the opening needs to be at least 28.8 inches tall, so a taller unit or a casement is needed.
A basement bedroom in metric
A below-grade casement of 600 × 900 mm clear has 0.54 m², above the 0.46 m² basement minimum, with a 1,050 mm sill. The well is 950 mm out from the wall, 1,200 mm wide and 1,250 mm deep. Its area is 1.14 m², above the 0.84 m² minimum, but at 1,250 mm deep it is over 1,118 mm, so a fixed ladder is required.
Who this calculator helps
- Architects and residential designers choosing window sizes for bedrooms and basements at the design stage.
- Basement and loft conversion contractors checking that a new habitable room can pass inspection.
- Window suppliers and installers confirming that a replacement window keeps the egress opening.
- Home inspectors and real estate agents flagging bedrooms that may not count as legal sleeping rooms.
- Interior designers making sure furniture and built-ins leave the window usable.
- Homeowners planning a basement bedroom or a replacement window.
Practical tips
- Ask the manufacturer for the clear opening dimensions; most publish an egress size chart for each unit.
- Keep the window operable from inside without keys, tools or special knowledge. Security bars need a quick-release.
- Check the room's floor area for the listing or code with the square footage calculator, and its daylight with the daylight factor calculator.
- On commercial projects, check exits and egress widths with the occupancy load calculator.
- Leave the window clear of beds and desks; the furniture clearance guide helps plan the layout.
Planning a basement bedroom? Show the client the finished room before work starts with Room Redesign.
