Calculate rise and run, stringer length, materials, and costs for wood stairs. IRC building code compliance checks included.
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Enter your stair dimensions
Max Riser (R311.7.5.1): 7.75 inches
Min Riser: 4 inches
Min Tread (R311.7.5.2): 10 inches
Min Width: 36 inches
Riser Variation: ±3/8 inch
Ideal Riser: 7-7.5 inches
Ideal Tread: 10-11 inches
Comfort (2R+T): 24-25 inches
Best Angle: 30-37 degrees
Handrail if: ≥4 risers
The 7-11 Rule
Professional builders target 7" risers and 11" treads for the most comfortable stairs. The comfort formula 2R + T should land between 24 and 25 inches for the best feel underfoot.
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How do I calculate stairs?
Measure total rise (floor to floor), divide by 7.5" ideal riser height to get number of risers. Treads = risers minus 1. Total run = treads x tread depth.
What is the 7-11 rule?
A guideline for 7" risers and 11" treads. IRC code allows up to 7.75" risers with minimum 10" treads. The comfort formula 2R+T should equal 24-25".
How many stringers do I need?
Minimum 3 stringers. Add one for every 16" of stair width. 36" stairs need 3 stringers, 48" stairs need 4.
What size lumber for stringers?
Use 2x12 boards. Length depends on your stringer calculation (rise² + run² = stringer²). Common sizes: 10ft or 12ft boards.
What is building code for stairs?
IRC requires: 4-7.75" riser height, ≥10" tread depth, ≥36" width, ±3/8" riser variation, handrails for ≥4 risers.
Calculate Rise and Run
Divides your total rise by the ideal 7.5" riser height to find the number of risers, then calculates actual riser height for even spacing. Total run = (risers - 1) x tread depth.
Check Building Code
Validates your dimensions against IRC residential code: riser height (4-7.75"), tread depth (≥10"), comfort formula (2R+T = 24-25"), and stair angle (30-37° ideal).
Generate Material List
Calculates stringer length (Pythagorean theorem), selects the next available lumber length, determines stringer count based on width, and totals all materials with cost estimates.
Key Formulas
Risers = round(Total Rise / 7.5)
Stringer = √(Rise² + Run²)
Angle = atan(Rise / Run)
Stringers = max(3, ceil(Width / 16) + 1)