Stair Calculator
Calculate stair rise, run, number of steps, and stringer length for any staircase.
The Formula
Run = Tread depth (typically 10–11 in)
Total Run = Run × (Steps − 1)
Ideal: Rise + Run = 17–18 inches
Target rise: 7.5 in → Steps = 108/7.5 = 14.4 → 15 steps
Actual rise = 108/15 = 7.2 in
Total run = 10 × 14 = 140 inches
Stair Building Fundamentals
Every staircase has two key dimensions: the rise (the vertical height of each step) and the run (the horizontal depth of each tread). Building codes set limits on both to ensure stairs are safe and comfortable. The International Residential Code (IRC) for residential buildings allows a maximum riser height of 7-3/4 inches and requires a minimum tread depth of 10 inches. The classical comfort formula used by builders for centuries is: two times the rise plus the run should equal 24–25 inches. A rise of 7 inches and run of 11 inches satisfies this formula (7 x 2 + 11 = 25) and makes for a very comfortable stair.
How to Calculate Your Staircase
Start with the total rise — the vertical distance from the finished floor at the bottom to the finished floor at the top. Divide that distance by your desired riser height (typically 7–7.5 inches) and round to the nearest whole number. That is your number of risers. Divide the total rise by the number of risers to get the exact riser height. Multiply the number of treads (always one fewer than risers) by your chosen tread depth to find the total horizontal run of the staircase. This tells you how much floor space the stair will occupy.
Deck Stairs vs. Interior Stairs
Deck stairs and exterior stairs can use slightly different standards than interior stairs — check your local building code, as some jurisdictions allow up to 8-inch risers for exterior stairs. Deck stairs must use pressure-treated or naturally rot-resistant wood (cedar, redwood). Stringer spacing is critical for strength — stringers should not span more than 16 inches without blocking or support. For stairs over 30 inches high, a handrail is required by most codes.
Frequently Asked Questions
How do I calculate the number of stairs?
Measure the total vertical rise from floor to floor. Divide by your desired riser height (try 7 or 7.5 inches as a starting point) and round to the nearest whole number — this is your number of risers. Divide the total rise by the number of risers to find the exact riser height. The number of treads is always one fewer than the number of risers.
What is the maximum riser height per building code?
The International Residential Code (IRC) allows a maximum riser height of 7-3/4 inches and minimum tread depth of 10 inches for residential stairs. The International Building Code (IBC) for commercial buildings is stricter: maximum 7-inch riser and minimum 11-inch tread. Local codes may vary. Consistency is as important as the absolute dimension — all risers in a flight of stairs must be within 3/8 inch of each other to prevent tripping.
How long should my stair stringers be?
Stringer length is the hypotenuse of the right triangle formed by the total rise and total run. Use the Pythagorean theorem: stringer length = square root of (total rise squared + total run squared). For example, a stair with a 9-foot total rise and 12-foot total run needs stringers of approximately 15 feet. Always buy stringers 6–12 inches longer than your calculation to allow for the angle cuts at top and bottom.
How many stringers do I need?
Most stair builds use three stringers: one on each side and one in the center. For stairs under 36 inches wide, two outer stringers may be sufficient if treads are thick and solid. For stairs over 36 inches wide, a center stringer is strongly recommended. For deck stairs, outdoor stringers should be pressure-treated 2x12 lumber and spaced no more than 16 inches apart under composite or thin wood treads.
What is the standard stair width?
Residential stairs must be at least 36 inches wide per IRC requirements. Commercial stairs require at least 44 inches of clear width. Most comfortable residential stairs are 36–42 inches wide. Grand staircases in foyers are often 48–60 inches wide. Spiral staircases can be narrower but have specific code requirements for the minimum width at the walking line.