How to Calculate Stairs

Stair math starts with one measurement — the total rise — and everything else falls out of it. Here's the sequence the pros use.

Step 1 — Measure total rise

Rise is finished floor to finished floor. Include the thickness of both floor coverings — tile upstairs over hardwood downstairs can add over an inch and change every riser in the flight.

Step 2 — Count risers

Divide rise by a target riser height (7½" is comfortable) and round to the nearest whole number. A 106" deck-to-house rise gives round(106 ÷ 7.5) = 14 risers, so each actual riser is 106 ÷ 14 = 7.57" — snapped to 7 9/16" on a tape.

Step 3 — Count treads and total run

Treads = risers − 1 when the top tread is the landing itself. At 10" treads: 13 × 10 = 130" of run. Check that run fits the opening before cutting anything.

Step 4 — Check comfort and code

• Risers ≤ 7¾", ≥ 4"
• Treads ≥ 10" (11" with nosing overhang rules in some editions)
• Headroom ≥ 80" measured vertically from nosing tips
• Comfort rule: 2 × riser + tread ≈ 24–25"
• All risers in a flight must match within ⅜"

Step 5 — Cut the stringers

Stringer length = √(rise² + run²). For our example √(106² + 130²) ≈ 167.7" — buy 18-ft lumber. Set your framing square to the actual riser on one leg and tread on the other, and step down the stringer; drop the first tread thickness to seat on the ground correctly.

Run your numbers through the stair calculator — it flags code violations and prints the full layout math.