Stair Calculator

The stair calculator takes the total vertical rise between two floors and breaks it into code-compliant risers and treads, then outputs the number of risers, the exact riser height, total horizontal run, stringer length, and the resulting stair angle. Multiple flights can be added to plan multi-story or split-landing staircases in a single session.

• Total rise. The vertical distance from the finished floor of one level to the finished floor of the next; small measurement errors compound across many risers, so this figure matters more than any other input.
• Desired riser height. A typical comfortable riser height runs 7 to 7.5 inches (178 to 190 mm). The calculator divides total rise by this value, rounds to the nearest whole number of risers, then recalculates the actual riser height so the rise distributes evenly across that whole number of risers.
• Tread depth. The horizontal distance from the front edge (nose) of one tread to the front edge of the next, excluding the nosing overhang. IRC 2021 sets a 10 inch (254 mm) minimum.
• Stair width. Does not affect the rise-and-run geometry; used only to estimate the total stair footprint area.
• Flights. Each flight is calculated independently with its own total rise, riser height, and tread depth, which is what a split-landing staircase or a multi-story run needs.
• Code limits. Nosing overhang, minimum headroom, maximum riser height, and minimum tread depth are all adjustable, and a flight that exceeds any of them is flagged.
• Accepted formats. Decimals (7.5), fractions (3/4), and mixed numbers (5 1/16 or 1-3/8) are all parsed automatically.

Risers equal the total rise divided by the desired riser height, rounded to the nearest whole number. Actual riser height equals total rise divided by that riser count. Total run equals (risers minus 1) times tread depth, since the top riser lands on the upper floor rather than adding another tread. Stringer length equals the square root of total rise squared plus total run squared. Stair angle equals the arctangent of total rise divided by total run, in degrees.

A deck 42 inches above grade, with a 7 inch desired riser height, needs 42 divided by 7, or 6 risers, at exactly 7 inches each. With a 10.5 inch tread depth, total run is (6 minus 1) times 10.5, or 52.5 inches. Stringer length is the square root of 42 squared plus 52.5 squared, about 67.2 inches, and the stair angle is the arctangent of 42 divided by 52.5, about 38.7 degrees.

Stair geometry affects safety, comfort, and code compliance in every building project that connects two levels. The following scenarios illustrate where precise riser and tread calculations prevent costly rework.

• Residential New Construction: A two-story home with 108 inches of total rise from the first-floor slab to the second-floor subfloor, at a desired riser height of 7.5 inches, works out to 14 risers at 7.714 inches each, a 136.5 inch total run, and a 38.4-degree angle, all within IRC limits.
• Deck and Porch Stairs: An exterior deck 42 inches above grade, at a 7 inch desired riser, needs 6 risers at 7 inches each, helping determine how far the staircase extends into the yard.
• Basement Renovation: Older basements often have non-standard floor-to-floor heights; adjusting the desired riser height until the actual riser stays below the 7.75 inch code maximum avoids a failed inspection.
• Split-Landing Staircases: A staircase with an intermediate landing splits into two separate flights; independent rise values for each, for example 54 inches lower and 54 inches upper, verify that both flights share consistent riser heights for a uniform feel.
• Stringer Cutting: Cutting stair stringers from dimensional lumber needs the exact riser height and tread depth to mark the sawtooth pattern; the stringer length output confirms whether a standard 12-foot or 14-foot board provides enough material.
• Metric Projects: Metric mode converts all values to millimeters, for projects in regions that use the metric system.