Enter how far the window's sill sits below the ground and how wide the window is. You get the well's height and size under the building code, whether it needs a ladder and how many rungs, and the egress wells Bilco and RockWell sell that fit.
Your window well
A well 43½″ tall, 48″ wide and 36″ out, no ladderIRC R310.4: 9 sq ft, 36″ out and 36″ wide at least, as wide as the window · 39½″ deep: a ladder only past 44″ · Bilco: the floor 3½″ under the sill, the top 4″ above the ground
Quick answer: under the building code (IRC R310.4), an egress window well must have at least 9 sq ft of floor, 36″ out from the wall and 36″ wide, and be big enough for the window to open all the way. Deeper than 44″, it needs a fixed ladder or steps: rungs at least 12″ wide, 3″ off the wall and no more than 18″ apart. For the well’s height, Bilco adds 7½″ to the sill’s depth below the ground.
How to use the window well calculator
- Measure the sill’s depth: from the top of the window’s sill straight up to the ground outside. That is Bilco’s “Dimension A”.
- Measure the window’s width and say how it opens: a slider, a hung window or a hopper opens inside its frame, while a casement swings out into the well.
- You get the well’s height, how deep it goes below the ground, the size the code asks for, whether it needs a ladder and how many rungs, and the Bilco and RockWell wells that fit, drawn from the side to scale. Add your own well’s width and how far it comes out to check it.
Planning the room behind the window? The bedroom requirements checker runs a basement bedroom through the code, and the window size finder checks that the window itself is big enough to escape through.
The rules, and where they come from
When you need a well. IRC 2021 R310.4: “An emergency escape and rescue opening where the bottom of the clear opening is below the adjacent grade shall be provided with an area well.”
Its size. R310.4.1: “The horizontal area of the area well shall be not less than 9 square feet, with a horizontal projection and width of not less than 36 inches. The size of the area well shall allow the emergency escape and rescue opening to be fully opened.” A casement has to swing all the way open, so the well must reach past its sash.
The ladder. R310.4.2: “Area wells with a vertical depth greater than 44 inches shall be equipped with an approved, permanently affixed ladder or steps.” Rungs “shall have an inside width of not less than 12 inches, shall project not less than 3 inches from the wall and shall be spaced not more than 18 inches on center vertically for the full height of the area well”. Steps need a 12″ width, a 5″ tread and at most an 18″ riser. The ladder or steps may take up to 6″ of the 36″.
The height. Bilco’s sizing sheets: “Required Side Panel Height = Dimension A + 7-1/2″”. The well’s floor sits 3½″ below the sill, and its panels “must extend 4 inches above grade level”, with the ground sloped away from it.
Covers. R310.4.4: a cover or grate over the well must be “releasable or removable from the inside without the use of a key or tool or force greater than that required for the normal operation” of the window. Bilco says its dome cover holds up to 300 lb.
Well height and ladder by sill depth
| Sill below the ground | Well height | Depth | Ladder |
|---|---|---|---|
| 24″ | 31½″ | 27½″ | none |
| 30″ | 37½″ | 33½″ | none |
| 36″ | 43½″ | 39½″ | none |
| 40″ | 47½″ | 43½″ | none |
| 42″ | 49½″ | 45½″ | 2 rungs |
| 48″ | 55½″ | 51½″ | 2 rungs |
| 54″ | 61½″ | 57½″ | 3 rungs |
| 60″ | 67½″ | 63½″ | 3 rungs |
| 72″ | 79½″ | 75½″ | 4 rungs |
Depth is the well’s floor to the ground; the ladder’s count spaces rungs no more than 18″ apart from the floor to the top.
The wells you can buy
| Bilco ScapeWEL | Steps | Inside width | Out from the wall | Height | Window up to | Cover |
|---|---|---|---|---|---|---|
| 4048-42 | 2 | 42″ | 41″ | 48″ | 42″ | 4042C |
| 4048-54 | 2 | 54″ | 41″ | 48″ | 54″ | 4054C |
| 4048-66 | 2 | 66″ | 41″ | 48″ | 66″ | 4066C |
| 4862-42 | 3 | 42″ | 49″ | 62″ (81″ with extension) | 42″ | 4842C |
| 4862-54 | 3 | 54″ | 49″ | 62″ (81″ with extension) | 54″ | 4854C |
| 4862-66 | 3 | 66″ | 49″ | 62″ (81″ with extension) | 66″ | 4866C |
Bilco’s table gives the window width for a wall-mounted window; on a buck mount it is 4″ less. From the foundation wall to the face of the first step is 30″.
| Other egress wells | Width | Out from the wall | Heights |
|---|---|---|---|
| Bilco StakWEL (stacking modules) | 54″ | 40¼″ | 21″, 36⅜″, 51¾″, 67⅛″, 82½″, 97⅞″ |
| RockWell Rockwell Series (steps 16″ apart) | 66″ | 44″ | 36″, 48″, 60″, 72″, 84″, 96″ |
Bilco makes StakWEL “for use on 36″ and 48″ windows only”. RockWell’s Rockwell 42 Series (42″ wide, 23″ out) is sized for smaller windows: at 23″ it comes out less than the 36″ an egress well needs.
Worked examples
A well 43½″ tall, 48″ wide and 36″ out, no ladder: a 48″ slider with its sill 36″ below the ground. The floor is 39½″ down, under the 44″ that calls for a ladder. A Bilco ScapeWEL 4048-54 or a RockWell 48″ well fits.
A well 49½″ tall, 48″ wide and 36″ out, with a 2-rung ladder: six inches deeper puts the floor at 45½″, past 44″, so it needs a ladder. Two rungs at about 15″ apart keep every gap under 18″.
A well 57½″ tall, 60″ wide and 36″ out, with a 2-rung ladder: a 60″ window needs a well at least 60″ wide, which rules out StakWEL. A ScapeWEL 4862-66 or a RockWell 60″ well fits.
A well 43½″ tall, 36″ wide and 36″ out, no ladder: a 32″ casement swings out 32″, past the 30″ to a ScapeWEL’s first step, so it suits a StakWEL or a RockWell well instead.
Common mistakes
- Measuring from the floor. The well’s height comes from the sill’s depth below the ground outside, not from the basement floor.
- Setting the well flush with the ground. Bilco: the panels must stand 4″ above the ground, or rain runs in over the top.
- Forgetting the casement. The well must let the window open all the way, so a swing-out sash needs the well to reach past it.
- A cover you can’t push open. A locked or screwed-down cover fails R310.4.4: it must open from inside without a key or tool.
- Trusting the model code alone. Cities amend it: Fargo, North Dakota allows a 48″ sill below grade and a platform that cuts the depth to 42″.
More on basements at Dreamy Home Style
- Cozy ideas for unfinished basements
- Rustic basement ideas
- Basement playroom ideas
- Basement stairwell ideas
- Cheap basement ceiling ideas
Laying out the basement room? Draw it to scale in the free 2D floor planner.
Egress window well questions
What are the egress window well requirements?
Under IRC R310.4: at least 9 sq ft of floor, 36″ out from the wall and 36″ wide, big enough for the window to open fully, a fixed ladder or steps when it is over 44″ deep, and any cover openable from inside without a key or tool.
What size window well do I need?
At least 36″ out and 36″ wide, and as wide as the window. Its height is the sill’s depth below the ground plus 7½″, by Bilco’s rule: a sill 36″ down takes a 43½″ well, so a 48″ one.
Does an egress window well need a ladder?
Only when it is more than 44″ deep. Then it needs a permanently fixed ladder or steps that the open window does not block.
How far apart are the rungs on a window well ladder?
No more than 18″ on center, for the full height of the well. Each rung is at least 12″ wide inside and stands at least 3″ off the wall.
How deep can a window well be without a ladder?
44″. With Bilco’s 3½″ under the sill, that is a sill up to 40½″ below the ground.
What are standard window well dimensions?
Egress wells come about 40–49″ out and 42–66″ wide: Bilco’s ScapeWEL is 42, 54 or 66″ wide and 41 or 49″ out, its StakWEL 54″ × 40¼″, and RockWell’s Rockwell Series 66″ × 44″, 36–96″ tall.
Do egress window wells need a cover?
The code does not require one, but Bilco recommends a cover for fall protection. Whatever covers the well must open from inside without a key, a tool or more force than the window takes.