Stair Calculator
Calculate the number of steps, actual rise per step, and total run for any staircase from total rise height.
Stairs are one of those things that look simple until you start calculating. The number of steps is always a whole number, which means the actual rise per step almost never equals the target, it's the total rise divided by the number of steps, and that division almost never comes out even. This calculator does the rounding correctly and shows all the derived dimensions.
The rise+run rule: the sum of one riser height plus one tread depth should equal 17–18 inches for comfortable stairs. A 7-inch rise and 11-inch tread sums to 18, comfortable. A 7.5-inch rise and 10.5-inch tread sums to 18 as well.
How stair design works
The number of risers is calculated by dividing total rise by target rise and rounding to the nearest whole number. The actual rise per step is then total rise divided by the number of risers. Total run is the number of treads (one fewer than risers for a closed stair landing at top) multiplied by tread depth. Stringer length is the hypotenuse of the rise-run triangle, calculated with the Pythagorean theorem.
Building code requirements (IBC)
The International Building Code (IBC) specifies: maximum riser height of 7¾ inches, minimum tread depth of 10 inches, maximum variation between risers of ⅜ inch (all risers must be consistent within this tolerance), and minimum headroom of 6 feet 8 inches. Residential codes (IRC) allow up to 8¾ inch risers and 9-inch treads in some jurisdictions, check your local code. The calculator flags when values exceed common code limits.
The rise+run comfort formula
Several formulas exist for comfortable stair proportions. The most common: Rise × 2 + Run = 24–25 inches (the "two risers and one tread" formula). Alternatively, Rise + Run = 17–18 inches. A 7-inch rise with 11-inch tread: 7 + 11 = 18 (good). A 7.5-inch rise with 10-inch tread: 7.5 + 10 = 17.5 (acceptable). These formulas are guidelines, code compliance is the legal standard.
Stringer length and lumber
The stringer is the diagonal structural member that supports the treads and risers. Its length determines what lumber length you need. Add 6–12 inches to the calculated stringer length for the structural portion that sits on the lower landing. Standard construction lumber comes in 2-foot increments, round up to the next available length.
Frequently asked questions
Why can't I just use my target rise exactly?
Because the total rise divided by your target rise almost never produces a whole number of steps. You must round to a whole number of steps, then recalculate actual rise as total rise ÷ steps. Every riser in a staircase must be the same height (within code tolerance), using the recalculated actual rise for all steps ensures consistency.
What's the difference between risers and treads?
Risers are the vertical faces of each step, the height. Treads are the horizontal surfaces you step on, the depth. A staircase with 14 risers has 13 treads (the top landing is not a tread). Total run uses the tread count, not the riser count.
How do I account for tread nosing?
The nosing (the portion of the tread that overhangs the riser below) is included in the tread depth measurement but not in the run calculation, the run is measured from riser face to riser face, not including the nosing overhang. IBC requires a minimum ¾-inch nosing projection.