Stair Calculator
Calculate rise, run, steps, and stringer length for any staircase.
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How to Calculate Stair Dimensions
The stair calculator takes your total rise and desired tread depth and computes everything else automatically. Getting stair dimensions right is not just a comfort issue — IRC building code sets strict limits on riser height and tread depth, and violations can cause a project to fail inspection.
The Rise-Run Relationship
The fundamental principle of comfortable stair design is: riser height + tread depth should equal approximately 17–18 inches. A stair with 7-inch risers should have 10–11-inch treads. A stair with 6.5-inch risers can have 11–11.5-inch treads. This ratio produces a natural walking rhythm.
Total rise ÷ ideal riser height = number of steps (round to nearest whole number). Actual riser = total rise ÷ number of steps. For a 108-inch (9-foot) total rise: 108 ÷ 7 = 15.4, round to 15 steps. Actual riser = 108 ÷ 15 = 7.2 inches.
IRC Code Requirements
The International Residential Code requires: riser height 4 inches minimum, 7¾ inches maximum. All risers must be within ⅜ inch of each other. Tread depth 10 inches minimum (from nosing to nosing). Stair width 36 inches minimum clear. Handrail required when there are 4 or more risers, at height 34–38 inches. Headroom clearance 80 inches minimum measured vertically above any stair nosing.
Calculating Stringer Length
The stringer is the diagonal board running the full height and length of the stair. Its length = √(total rise² + total run²). For 15 steps with 7.2-inch risers and 11-inch treads: total rise = 108 inches, total run = 15 × 11 = 165 inches. Stringer = √(108² + 165²) = √(11664 + 27225) = √38889 ≈ 197 inches (16.4 feet). You need a board at least this long plus extra for notching at top and bottom.
Number of Stringers
For a 36-inch wide stair, use 2 stringers (one on each side). For widths over 36 inches, add a center stringer. For a 48-inch wide stair, use 3 stringers. Each stringer is typically a 2×12 board. Cut notches into the stringer for each tread and riser, leaving at least 3.5 inches of structural depth at every notch.
Deck Stairs vs. Interior Stairs
Exterior deck stairs typically have open risers (no vertical boards between treads), which allows water and debris to pass through. Interior stairs usually have closed risers for a finished look. Exterior stairs must use pressure-treated or naturally rot-resistant wood (cedar, redwood). Interior stairs can use any wood species or engineered lumber. The rise-run rules and code limits are the same for both applications.
Frequently Asked Questions
IRC code allows 4"–7¾" risers. The most comfortable riser height is 7–7.5 inches. All risers on a flight must be within ⅜" of each other to prevent trip hazards.
The IRC minimum is 10 inches (nosing to nosing). A 11-inch tread feels comfortable for most users. Exterior stairs often use 11–12 inch treads for a more open, relaxed feel.
Measure vertically from the top of the finished floor at the bottom of the stairs to the top of the finished floor at the top. Account for finished flooring thickness if not yet installed.
The stringer is the diagonal structural board that supports the treads. Its length equals the hypotenuse of the triangle formed by total rise and total run. Most stairs need 2–3 stringers depending on width.
Yes. Any stair inside or attached to a home must comply with local building codes based on the IRC. Get a permit and inspection for any new staircase construction.