Stair Stringer Calculator

IRC R311.7 (residential): maximum riser height 7.75", minimum tread depth 10", maximum variance 3/8" between any two risers in the same flight (and 3/8" between any two treads), minimum 36" width, minimum headroom 6'8" above the tread nosing line. Nose projection 3/4" to 1.25". Handrails required if more than 4 risers. Open risers must not allow a 4" sphere to pass through. Commercial code (IBC) is slightly different - 7" max riser, 11" min tread (consistent with ADA).

Start with total rise (floor-to-floor distance in inches; example 108" for typical 9-ft floor). Divide by ideal riser height 7": 108/7 = 15.4 → round to 15 risers, giving 7.2" actual riser height (within IRC limits). Number of treads = risers - 1 = 14. Total run = 14 × 11" = 154". Stringer length = √(108² + 154²) = √(11,664 + 23,716) = √35,380 = ~188" = 15'8". Use 2×12 stringers minimum.

Three stringers minimum for any stair 36" or wider - center stringer prevents tread bounce. Stairs over 48" wide may need 4 stringers. Stringer spacing typically 16" OC for solid feel. For exterior decks, use pressure-treated 2×12 stringers minimum; interior can use SPF or fir 2×12 if spans aren't exceeding ~13 ft. Always notch stringers AFTER laying out - calculator output should be checked against actual lumber dimensions and trimmed.

Closed stringer (also "housed stringer"): treads and risers are routed into the stringer side, finished appearance, more carpentry skill. Common indoors. Open stringer (also "cut stringer" or "sawtooth"): triangular notches cut from the top edge of the stringer to receive treads and risers, exposed end-grain shows from the side. Faster to build, used for exterior stairs and basement stairs. Strength is identical when properly sized and supported.