Free stair calculator computes steps, riser height, tread depth, stringer length and stair angle from your rise and target riser, using code-friendly design rules.
Building a set of stairs - indoors or outdoors, wood or concrete - starts with getting the rise and run right. The two numbers that matter are the total rise (floor-to-floor height) and your target riser height, and everything else follows from them. This calculator applies the classic Blondel rule (2 x riser + tread = about 63 cm), which is the basis of comfortable stair design used in building codes worldwide.
Enter the total rise and a target riser height. The calculator rounds up to a whole number of steps, recomputes the exact riser height, and derives a comfortable tread depth and the total horizontal run your staircase will occupy. It also reports the stringer length and stair angle, so you can order lumber and lay out cuts without a separate calculation.
Most building codes, including the US International Residential Code (IRC R311.7), limit riser height to 7.75 inches (about 19.7 cm) and require a minimum tread depth of 10 inches (25.4 cm). The calculator keeps your riser close to the 18 cm target so your stairs stay comfortable and code-friendly.
Codes typically require a landing whenever a stair flight rises more than about 12 feet (3.7 m) or changes direction. A landing must be at least as deep as the stair width. If your total rise is large, split the run into two flights with a platform between them.
No. Spiral stairs follow a different geometry (treads radiate around a center column, with minimum tread width measured at a set distance from the narrow end). Use a dedicated spiral stair calculator and local code for those projects.
The stringer is the diagonal that connects the total rise and the total run: Stringer = sqrt(Rise² + Run²). The stair angle is the arctangent of rise divided by run. For a 280 cm rise and a 420 cm run, the stringer is about 5.05 m and the angle is about 33.7°. Buy stringers a little longer than the calculated length and cut them to fit.