Stair Stringer Spacing Calculator
Eliminate bouncy steps. Determine exact center-to-center stringer spacing and required stringer count for wood and composite decking treads.
Input Dimensions
Stringer Results
Enter dimensions and click Calculate Stringer to see results.
Interactive Cut Stringer Blueprint
Standard Staircase Worked Examples
Click any preset to automatically populate the calculator with proven dimensional specifications.
How Stringer Spacing Prevents Deflection
Learn why on-center spacing dictates maximum tread live load deflection and user security.
Measure Total Rise
Measure the exact vertical distance between finished floor surfaces using a plumb bob or laser level. Always account for finished flooring thickness.
Calculate Number of Steps
Divide total rise by your target riser height (typically 7.5"). Round to the nearest whole integer to find the exact number of equal risers.
Determine Riser & Run
Divide total rise by the number of risers for exact height. Select tread depth (minimum 10") and verify with Blondel’s comfort formula (2R + T = 24–25").
Layout & Deduct Bottom Foot
Step off cuts with a framing square and stair gauges. Deduct the thickness of one tread from the bottom cut so the finished first step matches the rest.
Tread Material vs Maximum Stringer Span
Allowable on-center spacing by tread material and code criteria.
Building Code Comparison (IRC vs IBC vs OBC vs UK)
| Code Authority | Max Riser | Min Tread | Min Width | Min Throat |
|---|---|---|---|---|
| IRC Residential (US) | 7.75" (196 mm) | 10.0" (254 mm) | 36" (914 mm) | 3.5" (89 mm) |
| IBC Commercial (US) | 7.00" (178 mm) | 11.0" (279 mm) | 44" (1118 mm) | 3.5" (89 mm) |
| OBC (Ontario, Canada) | 8.25" (210 mm) | 8.25" (210 mm) | 33.5" (860 mm) | 3.5" (89 mm) |
| UK Approved Doc K | 8.66" (220 mm) | 8.66" (220 mm) | 31.5" (800 mm) | 3.5" (89 mm) |
Lumber Selection vs Notch & Throat Depth
| Nominal Size | Actual Width | Max Cut Notch | Remaining Throat | Recommendation |
|---|---|---|---|---|
| 2×8 | 7.25" (184 mm) | 4.50" | 2.75" (Deficient) | Not Code Compliant |
| 2×10 | 9.25" (235 mm) | 5.75" | 3.50" (Borderline) | Shallow stairs only |
| 2×12 | 11.25" (286 mm) | 6.25" | 5.00" (Excellent) | Standard / Best Choice |
| LVL / Engineered | 11.875" (302 mm) | 6.50" | 5.375" (Super-Duty) | Heavy Commercial / Long Spans |
Stringer Spacing FAQs
Answers to common questions regarding stringer spans and tread materials.
What is standard stair stringer spacing?
Standard stair stringers are spaced 16 inches on center (O.C.) for dimensional lumber treads (2×10 or 2×12 wood). For 5/4 decking or composite materials, spacing is reduced to 12 inches O.C. to prevent tread flex.
How many stringers are needed for a 36-inch wide stair?
A 36-inch stair requires at least 3 stringers (one on each outside edge and one centered), giving an on-center spacing of 18 inches or 16 inches with rim allowances.
Why do composite treads need tighter stringer spacing?
Composite and PVC deck boards are more flexible than natural dimensional lumber, especially in hot direct sunlight. Spacing stringers at 12 inches on center ensures compliance with manufacturer deflection warranties.