Stair Riser and Tread Calculator

Stair layout is one of those calculations where being off by a fraction of an inch matters. Uneven risers make people trip, because they expect every step to feel the same.

The total rise is the vertical distance from the lower floor to the upper floor (or deck surface).
The total run is the horizontal distance the stairs can occupy.
These two numbers determine everything else.

The target riser height for comfortable stairs is 7 to 7.5 inches.
The IRC residential building code sets hard limits:

• Maximum riser height: 7.75 inches
• Minimum riser height: 4 inches
• Minimum tread depth: 10 inches (measured from nosing to nosing)

A useful comfort check: riser + tread should be between 17 and 18 inches.
At a 7-inch riser and a 10.5-inch tread you get 17.5, which is about as good as stairs get.

The calculation:

n_risers = round(total_rise ÷ 7.0) riser = total_rise ÷ n_risers tread = total_run ÷ (n_risers − 1)

That minus one is the part people get wrong, and it is worth a paragraph. A flight with 7 risers has only 6 treads, because the seventh step lands you on the floor above rather than on a board you cut. Total run is measured across the treads you actually build, so dividing it by the riser count gives you a tread depth that is too shallow and a stair that will not fit the space you measured. On an exterior deck stair it works the same way: the top tread is the deck surface.

The riser has to be identical on every step. The IRC allows 3/8 inch between the tallest and the shortest in a flight, which sounds generous until you remember that the bottom riser changes height by exactly the thickness of the tread when you forget to subtract it. That is the single most common stair-building mistake.

If the run is fixed by a tight space you may need to add a step to keep the tread legal.
This calculator shows every legal step count near the target so you can pick the fit.

Worked example. A 49-inch rise into a space with 63 inches of run: 49 ÷ 7 = 7 risers, each 7.000 inches exactly. Six treads across 63 inches gives 10.5 inches each, legal and comfortable, and 7.0 + 10.5 = 17.5 lands in the middle of the comfort range. The stringer runs √(49² + 63²) = 79.8 inches before you add anything for the attachment.

Stringer length: use the Pythagorean theorem, so stringer = √(total_rise² + total_run²).
Add 6 to 8 inches for the top attachment and cut allowances.