Window Well Size Calculator
Calculate window well dimensions required by code for egress basement windows. Enter values for instant results with step-by-step formulas.
Reviewed by Abdullah, Technical Content Specialist
Formula
Well Width = Window Width + 18 in | Well Depth = Sill Depth + Window Height + 4 in
Where Window Width is the rough opening width, 18 inches provides 9 inches clearance on each side, Sill Depth is the distance from grade to the window sill, and 4 inches allows space below the window for gravel drainage. The well projection must be at least 36 inches per IRC code.
Worked Examples
Example 1: Standard Egress Window Well
Problem:Size a window well for a 36-inch wide by 24-inch tall basement window with the sill 36 inches below grade.
Solution:Well width = 36 + 18 = 54 inches\nWell projection = max(36, 54/2) = 36 inches\nWell depth = 36 + 24 + 4 = 64 inches (5.3 ft)\nClear area = (36 x 24) / 144 = 6.0 sq ft (meets 5.7 min)\nDepth > 44 in: Ladder required
Result:54 x 36 in well | 64 in deep | Ladder required | Code-compliant
Example 2: Shallow Basement Window Well
Problem:Determine well requirements for a 32 x 20 inch window with the sill 18 inches below grade.
Solution:Well width = 32 + 18 = 50 inches\nWell projection = 36 inches (minimum)\nWell depth = 18 + 20 + 4 = 42 inches (3.5 ft)\nClear area = (32 x 20) / 144 = 4.4 sq ft\nDoes NOT meet 5.7 sq ft minimum - larger window needed
Result:50 x 36 in well | 42 in deep | No ladder needed | Window too small for egress
Frequently Asked Questions
What are the building code requirements for egress window wells?
The International Residential Code (IRC) sets specific requirements for egress window wells in basements used as habitable space or sleeping rooms. The window must have a minimum net clear opening area of 5.7 square feet, with a minimum opening height of 24 inches and minimum opening width of 20 inches. The window sill cannot be more than 44 inches above the finished floor. The window well itself must have a horizontal area no less than 9 square feet, with a minimum projection of 36 inches from the foundation wall. If the well depth exceeds 44 inches, a permanently attached ladder or steps must be provided. Local codes may have stricter requirements, so always verify with your local building department.
How do I determine the correct window well size for my basement window?
Start by measuring your basement window opening width and height, then calculate the net clear opening area to verify it meets egress requirements. The window well width should be at least 6 inches wider than the window on each side, giving a minimum of 12 inches total additional width. The projection from the foundation wall should be at least 36 inches to allow a person to climb out in an emergency. The well depth is determined by the window sill height below grade plus the full window height plus a few inches below the window for drainage gravel. A standard approach is to select a well that is one to two standard sizes larger than the minimum requirement to provide comfortable egress access.
Do I need a window well cover and what type should I use?
Window well covers are strongly recommended for all egress window wells and may be required by local building codes. They prevent water, snow, leaves, debris, and animals from accumulating in the well, which can cause basement flooding and block emergency egress. Covers must be easily openable from inside the well without tools, keys, or special knowledge, as required by egress codes. Clear polycarbonate covers allow natural light into the basement while providing weather protection. Metal grate covers offer ventilation but do not keep out water and debris. The cover should support at least 400 pounds to safely hold a person who might step on it. Custom-sized covers are available for non-standard well sizes.
What drainage system do I need at the bottom of a window well?
Proper drainage is essential to prevent water from pooling in the window well and leaking into the basement. At minimum, place 4 to 6 inches of clean gravel or crushed stone at the bottom of the well to allow water to percolate into the surrounding soil. For deeper wells or areas with high water tables, install a drain pipe connected to the foundation drainage system or a dedicated sump pit. A 4-inch perforated drain pipe set in gravel at the bottom of the well works effectively when connected to the existing footing drain system. In areas with clay soil that does not drain well, a dedicated French drain leading away from the foundation may be necessary to prevent water accumulation.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy