Stair Calculator
Calculate stair dimensions: number of risers, riser height, tread depth, total run, and stringer length. Includes IRC code compliance check.
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Number of Risers
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Actual Riser Height —
Tread Depth (2R+T=25) —
Total Run —
Stringer Length —
Extended More scenarios, charts & detailed breakdown ▾
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Number of Risers
—
Actual Riser Height —
Tread Depth —
Total Run —
Stringer Length —
Stair Angle —
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Stair Dimensions
Number of Risers —
Actual Riser Height —
Tread Depth —
Total Run —
Stringer Length —
Code Compliance (IRC)
IRC Riser Height Check —
IRC Tread Depth Check —
Headroom Check —
Material Estimates
Handrail Length —
Tread Material (board ft) —
Stringer Material (board ft) —
Estimated Material Cost —
How to Use This Calculator
- Enter the total rise — the floor-to-floor height in inches.
- Set the ideal riser height (default 7.5 inches — the IRC sweet spot).
- The calculator shows number of risers, actual riser height, tread depth (2R+T rule), total run, and stringer length.
- Use the Professional tab for IRC code compliance, headroom check, and material estimates.
Formula
Number of Risers = Round(Total Rise / Ideal Riser Height)
Actual Riser = Total Rise / Number of Risers
Tread Depth = 25 − (2 × Actual Riser) [2R+T=25 rule]
Total Run = Tread Depth × (Number of Risers − 1)
Stringer Length = √(Total Run² + Total Rise²)
Example
Example: Floor-to-floor height = 108 inches, ideal riser = 7.5"
- Risers = Round(108 / 7.5) = 14 risers
- Actual Riser = 108 / 14 = 7.71"
- Tread = 25 − (2 × 7.71) = 9.58"
- Total Run = 9.58 × 13 = 124.5"
- Stringer = √(124.5² + 108²) = 164.9"
Frequently Asked Questions
- The most comfortable riser height is 7 to 7.75 inches. IRC building code requires risers to be no more than 7.75 inches high and no less than 4 inches.
- The ergonomic formula 2 × Riser + Tread = 24–25 inches ensures comfortable stairs. For a 7-inch riser, the tread should be 10–11 inches.
- Divide the total rise (floor-to-floor height) by your desired riser height, then round to the nearest whole number. Recalculate the actual riser height by dividing total rise by the number of risers.
- A stringer is the diagonal structural board that supports the treads and risers. Its length is calculated using the Pythagorean theorem: √(total run² + total rise²).
- IRC requires a minimum of 80 inches (6'8") of headroom measured vertically from any stair tread nosing. Most residential stairs need at least 84 inches for comfortable use.