Calculate Stair Stringer Layout
Calculate rise, run, number of steps, and stringer length for any set of stairs. Code-compliant dimensions.
Stair Layout Basics
Number of risers = Total Rise ÷ Ideal Riser Height (round to nearest whole number). Then: Actual riser = Total Rise ÷ Number of risers. For a 9-foot (108") floor-to-floor rise at 7.5" ideal: 108 ÷ 7.5 = 14.4, round to 14 risers. Actual rise: 108 ÷ 14 = 7.71" per step. The total horizontal run = (risers - 1) × tread depth: 13 × 10" = 130" = 10.8 feet.
Building Code Requirements (IRC)
Residential stair codes per the International Residential Code: Riser height: Maximum 7.75" (ideal 7-7.5"). Tread depth: Minimum 10" (ideal 10-11"). Width: Minimum 36" clear. Headroom: Minimum 6'8". Rise + Run sum: Should total 17-18" for comfortable stairs. Uniformity is critical — all risers must be within 3/8" of each other. Uneven stairs are the leading cause of stair falls.
The "18 rule" is the fastest way to check stair comfort: riser height + tread depth should equal approximately 17-18 inches. A 7.5" rise + 10" tread = 17.5" — perfect. A 6" rise + 12" tread = 18" — also comfortable. Outside this range, stairs feel either too steep (sum under 17") or too shallow (sum over 18").
