A stair-stringer calculator works out the riser height, tread depth, and number of steps from total floor-to-floor height and total run. The Toolenza calculator returns the stringer length (the diagonal board that supports the steps) using the Pythagorean theorem.

Building-code requirements (US IRC)

• Riser height — 7.75 in maximum, 4 in minimum. Most comfortable: 7 to 7.5 in.
• Tread depth — 10 in minimum (nosing included).
• Variation — between any two steps in a flight: 3/8 in maximum. Between the largest and smallest in the same flight: 3/8 in maximum. The cumulative variation rule is what trips up DIY builds.
• Headroom — 6 ft 8 in minimum above the line of nosings.
• Width — 36 in minimum (above any handrails).
• Handrail — required on at least one side for flights of 4+ risers; 34–38 in above the tread nosing.

The "riser + tread" formula

A comfortable, code-compliant flight follows one of these rules of thumb (any one is enough):

• 2 × riser + tread = 24–25 in — most common.
• Riser + tread = 17–18 in.
• Riser × tread = 72–75 sq in.

For a typical 7.5-in riser, that gives 9.5–10 in treads — code-minimum tread, comfortable rise.

Working out the count

1. Measure total rise (floor to floor) precisely. Don't round.
2. Divide by your target riser height (say 7.5 in). Round to a whole number — that's your number of risers.
3. Divide total rise by number of risers — that's the exact riser height to cut.
4. Number of treads = risers − 1 (the top floor counts as the last "step").
5. Total run = treads × tread depth.
6. Stringer length = √(rise² + run²) plus a couple inches for the bottom kick and top connection.

Pitfall

The first step from a deck or porch counts. Many homeowners get caught when adding a small step at the bottom of the stairs creates an inconsistent rise — failing inspection. Plan the whole staircase including transitions.