Stair Calculator — Risers, Tread Depth & Blondel Comfort Check

Laying out a safe staircase starts with one number: the total rise, meaning the exact floor-to-floor height the stair has to climb. Everything else is arithmetic built on top of that. This calculator first finds the smallest number of risers that keeps each individual riser at or under a maximum height — commonly 7.75 inches (196.85 mm) under typical US residential (IRC-style) limits, though exact limits vary by jurisdiction and are fully adjustable here. That's done by dividing total rise by the maximum riser height and rounding up, then dividing total rise by that riser count to get the real, exact riser height that will actually get built — which is usually a little shorter than the maximum, and shown to you precisely rather than rounded away.

Riser height alone doesn't make a stair comfortable, though — tread depth has to work together with it, and the relationship between the two has a name: the Blondel comfort formula, dating back to 17th-century French architect François Blondel's observation that a comfortable stair should preserve something close to a natural walking stride. The formula is simply 2 x riser height + tread depth, and it should land in roughly 24 to 25 inches (about 63 cm) for an average adult stride. This calculator solves that formula for tread depth given the riser height it just computed, so a taller riser automatically suggests a shallower tread and vice versa — the same trade-off a stair builder has to make by hand.

There's a genuine, well-known tension at the steep end of residential code: at the maximum legal riser height, the Blondel formula often calls for a tread shallower than the typical 10-inch (254 mm) code minimum tread depth. When that happens, this calculator uses the larger, code-compliant tread and says so explicitly rather than silently picking one number — a stair built to the code minimums at both ends is legal but can feel steeper than the comfort formula would prefer, and that's worth knowing before you commit to a run length. Total run is then treads (one fewer than the number of risers, since a stair starting and ending at floor level shares its top and bottom risers with the floors) multiplied by tread depth, checked against any available horizontal space you enter. This tool is a planning aid for sketching a layout before drawings — always verify the exact rise, run and riser/tread limits against your local building code, and have the final design reviewed by a qualified professional before any construction.