Stair Calculator (Riser & Tread Dimensions)
Calculate number of risers, tread depth, stair stringer length, and incline angle conforming to building code.
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How to Use the Stair Calculator
- 1Measure and enter the total vertical rise (floor-to-floor height in inches).
- 2Enter your desired target riser height (typically 7 inches for residential building code).
- 3Enter target tread depth (minimum 10 or 11 inches).
- 4Review the optimal step count, actual riser height, total stair run, and stringer board length.
Formula & How It Works
Blondel's Rule & IRC Standards
Residential code specifies a maximum riser of 7.75 inches and minimum tread of 10 inches. Blondel's comfort formula dictates that 2 × Rise + Run should equal 24 to 25 inches for ergonomic climbing.
Worked Examples
Standard Deck Staircase
Input: Total Rise: 56 inches, Target Riser: 7 in, Target Tread: 11 in
Explanation: 56 / 7 = exactly 8 risers. Number of treads = 8 - 1 = 7. 7 × 11 in = 77 in total run.
Basement Stairs
Input: Total Rise: 105 inches
Explanation: 105 / 14 = 7.5 inches per riser (within code maximum of 7.75 in).
Frequently Asked Questions
Why is there one fewer tread than risers?▼
The top riser meets the upper finished floor level, so the upper landing acts as the final step.
What is the maximum allowed riser height by code?▼
Under the International Residential Code (IRC), the maximum riser height is 7.75 inches (197 mm).
What is the ideal angle of incline for stairs?▼
A comfortable residential stair angle is typically between 30 and 37 degrees.
How do I calculate stringer length?▼
Stringer length is determined using the Pythagorean theorem: Stringer = √[(Total Rise)² + (Total Run)²].
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