Toolrack

Stair Stringer Calculator

Finished floor to finished floor
Comfortable range: 7–7.75 in
Excluding nosing overhang
Risers—
Riser height—
Treads—
Total run—
Stringer length—
Angle—

Enter your total floor-to-floor height and this calculator returns riser count, exact riser height, total run, stringer length and stair angle — then checks each result against IRC limits.

How stair dimensions are calculated

A stair is solved from the top down, not the bottom up. The total rise is fixed by the building, so the only free choice is how many equal steps to divide it into.

  1. Divide and round. Total rise divided by your preferred riser height gives a fractional number of risers. Round it to the nearest whole number — that is your riser count.
  2. Divide back. Total rise divided by that whole riser count gives the actual riser height. It will rarely be a round number, and that is correct.
  3. Multiply out the run. Tread count is normally one fewer than riser count, and total run is tread count times tread depth.
  4. Solve the diagonal. Stringer length is the hypotenuse of the rise and run triangle.

Comfort rules worth knowing

Three formulas have survived from 18th-century French carpentry and still predict whether a stair feels right underfoot:

RuleFormulaTarget
Blondel2 × riser + tread24–25 in (610–635 mm)
Sumriser + tread17–18 in (432–457 mm)
Productriser × tread71–74 in² (458–477 cm²)

A stair that satisfies Blondel's rule but violates code is still illegal. Code first, comfort second.

Frequently asked questions

What is the maximum riser height allowed by code?

The International Residential Code (IRC R311.7.5) caps riser height at 7 3/4 inches (196 mm) for residential stairs and requires a minimum tread depth of 10 inches (254 mm). Commercial stairs under the IBC are stricter: 7 inch maximum rise and 11 inch minimum tread. Local amendments override both, so confirm with your building department before cutting.

How do I measure total rise correctly?

Total rise is the vertical distance from the finished lower floor to the finished upper floor — not the subfloor. If the upper floor will get 3/4 inch hardwood and the lower gets tile, include both thicknesses. Measuring to the subfloor is the most common reason a finished stair ends up with one riser out of tolerance.

Why must every riser be the same height?

Code allows no more than 3/8 inch (9.5 mm) variation between the tallest and shortest riser in a flight. People climb stairs by muscle memory after the first two steps, so an odd riser is a trip hazard. This is why riser height is calculated by dividing total rise by a whole number of risers rather than picking a round number.

How long does the stringer board need to be?

The calculator returns the diagonal length of the stringer, but buy stock at least 12 inches longer. You need material beyond the first and last cut for the top hanger connection and the bottom kick plate, and a 2x12 that is cut exactly to the diagonal leaves no room for error.

Is there one tread fewer than risers?

Usually yes. When the top step is the upper floor itself, the stair has one fewer tread than risers. If you add a separate top tread level with the floor, tread count equals riser count. This calculator assumes the upper floor acts as the top step, which is the standard residential arrangement.

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