Stair Calculator

About this calculator

This calculator works out the number of steps, the actual riser height, total horizontal run, and stringer length for any staircase from a single floor-to-floor rise measurement. Use it when framing a new deck stair, planning a basement staircase, or checking that an existing flight meets residential building code.

How to read your results

The headline figure is the number of risers — the count of vertical steps you will cut and install. Below it the summary card shows actual riser height (total rise divided by the riser count), total run (how much horizontal floor space the stair consumes), and stringer length (the diagonal board length). A compliance badge immediately tells you whether the layout satisfies IRC R311.7: riser height must not exceed 7.75 in and tread depth must be at least 10 in. The step diagram below the card lets you visually confirm the proportions before cutting any lumber.

Worked example

A deck is 108 in above grade (9 ft floor-to-floor). Target riser height 7.5 in, tread depth 10 in.

The calculator rounds to 14 risers. Actual riser height is 108 / 14 = 7.71 in — within the IRC 7.75 in limit. With 13 treads at 10 in each the total run is 130 in. The stringer length is sqrt(108^2 + 130^2) = 169 in (roughly 14 ft 1 in). The layout is code-compliant.

Frequently asked questions

Why does the number of treads equal risers minus one?

Each riser lifts you to a new level; the last riser deposits you onto the landing, which is not a tread. So a stair with 14 risers has exactly 13 treads between the bottom floor and the top landing.

What are the IRC R311.7 limits for residential stairs?

The International Residential Code (IRC) R311.7 sets a maximum riser height of 7.75 in and a minimum tread depth of 10 in. The calculator flags any combination that exceeds these limits so you can adjust your target riser height before building.

How do I use the stringer length?

The stringer is the inclined board that supports the treads and risers. Cut two (or three for wider stairs) to the computed length, mark the rise-and-run pattern with a framing square, and the resulting notched boards form the structural skeleton of your stair.

What is nosing and does it affect the run?

Nosing is the overhang of a tread beyond the face of the riser below it — typically 0.75 in to 1.25 in. The structural tread depth (the 10 in minimum in IRC) is measured from nosing to nosing, so nosing does not change the stringer length calculation but it does affect the finished tread board width you need to purchase.

Can I adjust the target riser height to get a better layout?

Yes. The calculator divides total rise by the nearest whole number of risers to the target you enter. Trying a slightly different target — say 7 in instead of 7.5 in — can change the riser count by one and produce a more even actual height. Experiment until you find a riser height that is both uniform and within code limits.

How it's calculated

The riser count is found by rounding the total rise divided by the target riser height to the nearest integer (Math.round). The actual riser height is then total rise divided by that integer count, ensuring all risers are equal. The tread count is risers minus one. Total run is tread count multiplied by tread depth. Stringer length is the hypotenuse of the right triangle formed by total rise and total run, computed as Math.sqrt(rise^2 + run^2). Code compliance compares actual riser height against the IRC R311.7 maximum of 7.75 in and tread depth against the minimum of 10 in. Formulas follow the standard stair-layout conventions documented at buildingcodetrainer.com and inchcalculator.com.