Stair Calculator

How to Calculate Stairs — Rise, Run & Stringer Layout

Building code-compliant stairs starts with accurate rise and run calculations. The total rise (floor-to-floor height) determines how many risers you need, and each riser must fall within code limits. Getting this wrong means tearing out framing and starting over — or worse, a dangerous staircase.

IRC Stair Code Requirements (R311.7)

• Maximum riser height: 7-3/4" (196 mm)
• Minimum tread depth: 10" (254 mm)
• Minimum width: 36" clear above handrails
• Maximum variation: 3/8" between largest and smallest riser/tread
• Headroom: Minimum 6'-8" measured vertically from stair nosing
• Nosing: 3/4" to 1-1/4" projection, uniform throughout

Rise & Run Relationship

The classic comfort formula is: Riser + Tread = 17" to 18". A 7.5" riser with a 10" tread (17.5" total) is the most common residential configuration. Steeper stairs save horizontal space but are harder to climb. Shallower stairs are more comfortable but require more floor space.

Stringer Cutting Technique

• Step 1: Calculate the stringer length: √(total rise² + total run²)
• Step 2: Use a framing square with stair gauges set to your riser and tread dimensions
• Step 3: Mark and cut notches from a 2×12 (minimum 3.5" of wood must remain below the cut)
• Step 4: Drop the stringer by subtracting one tread thickness from the bottom riser
• Step 5: Use the first stringer as a template for remaining stringers

Stringer Board Sizing

Most residential stairs use 2×12 stringers. After cutting the notches, you must maintain at least 3.5" of solid wood below the notch throat. For spans over 6 feet without intermediate support, verify that your stringer board meets structural requirements. Engineered LVL stringers are available for longer spans or heavy-duty applications.