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Stair Stringer Calculator

Calculate stair stringer dimensions, number of steps, and total run for wood stairs.

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Number of steps steps
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Building stairs is one of the most math-intensive carpentry tasks. Getting the rise and run wrong by even a small amount creates a tripping hazard and violates building code. This calculator takes your total rise measurement and computes the number of steps, the actual rise per step, and helps you plan the total horizontal run so you know how much floor space the staircase will consume. The total rise is the vertical distance from finished floor to finished floor. Measure this carefully and measure it more than once. Being off by half an inch gets divided across all the steps and creates uneven risers that feel awkward underfoot. The building code maximum variance between the tallest and shortest riser in a flight is 3/8 inch. The International Residential Code sets a maximum riser height of 7.75 inches and a minimum tread depth of 10 inches. Most carpenters target a 7 to 7.5 inch rise and a 10 to 11 inch run as the sweet spot for comfortable stairs. A common rule of thumb is that rise plus run should equal 17 to 18 inches. Stringers are the diagonal boards that support the treads and risers. For standard residential stairs, 2x12 pressure-treated lumber is the go-to material. After cutting the notches for treads and risers, you need at least 3.5 inches of solid wood remaining at the thinnest point of the stringer. This is called the effective throat depth and determines the structural integrity of the stringer. For stair width, code requires a minimum clear width of 36 inches. Use three stringers for stairs up to 36 inches wide, one on each side and one in the center. Wider stairs need a center stringer for every 16 to 24 inches of additional width to prevent the treads from bouncing underfoot. A 48-inch wide stair should have four stringers. Always lay out your stringer on the ground with a framing square and stair gauges before making any cuts. Mark every tread and riser, double check your total rise against the marks, and only then start cutting. Measure twice and cut once is nowhere more important than on stair stringers.