Stair Calculator

Building code-compliant stairs starts with measuring the total rise — the vertical distance from finished floor to finished floor. Divide that by the ideal riser height (7–7.75 inches per IRC code) to find the number of steps.

Stair Dimensions Explained

Rise is the vertical height of each step. Run is the horizontal depth of each tread. The IRC requires a maximum rise of 7.75 inches and minimum run of 10 inches. The number of treads is always one fewer than risers because the top floor serves as the final tread.

Stringer Length Formula

Stringer length = √(total rise² + total run²). This gives you the diagonal length of the structural boards that support the treads. Most residential stairs use 2×12 stringers cut to this length.

Building Code Requirements (IRC)

The International Residential Code sets: maximum riser height of 7¾", minimum tread depth of 10", maximum variation between risers of 3/8", minimum stair width of 36", and minimum headroom of 6’8".

Frequently Asked Questions

What is the standard stair rise and run?

The standard residential stair has a 7–7.5 inch rise and 10–11 inch run. This produces a comfortable angle between 30° and 37° and complies with IRC building codes.

How many steps for an 8-foot ceiling?

With an 8-foot (96-inch) ceiling plus roughly 10–12 inches of floor framing, the total rise is 106–108 inches. At 7.5 inches per step, you need 14–15 risers (13–14 treads).

How do you calculate stringer length?

Use the Pythagorean theorem: stringer = √(total rise² + total run²). For 108" rise and 140" run, the stringer is about 177 inches (14.75 feet). Purchase 2×12 lumber at least 16 feet long.