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Stair Stringer Calculator: Rise, Run & Layout

Calculate riser height, tread depth, stringer length and throat depth from your total rise, with a live blueprint of the stringer layout and IRC R311.7 code checks.

AU (NCC) / UK (Part K) / Eurocode Stair Layout

Stair Rise & Run Inputs (Metric mm)

Finished lower floor to finished upper floor (2.7 m)
mm
300mm5,000mm
Standard step rise. Australian & UK residential code specifies max 190 mm
mm
140mm220mm
Horizontal step depth. AU Code specifies min 240 mm (UK min 220 mm)
mm
220mm380mm
Finished tread thickness (used to drop bottom stringer cut)
mm
15mm50mm
3 stringers for 900mm width
pcs
2pcs6pcs
Stock timber board cost
$
$0$150
Live Stringer CAD Blueprint
Angle: 35.8°
Lower Finished FloorUpper Floor
Total Rise: 2700 mm (2.7 m)Total Run: 3500 mm (3.5 m)
Stringer Cut SpecificationsBuilding Code Compliant
Number of Risers15@ 180 mm
Number of Treads14@ 250 mm depth
Bottom Riser Cut155 mmMinus 25 mm tread
Board Stock Needed4.8 m TimberDiagonal: 4420 mm (4.42 m)
Complies with AU/UK & Eurocode (≤ 190 mm). Riser is 180 mm.
Complies with AU/UK Code (≥ 240 mm). Tread run is 250 mm.
Solid stringer meat (139.7 mm throat remaining ≥ 90 mm standard code).
Optimal gait ergonomics (2R + T = 610 mm, ideal range 600–640 mm).
3 × 45x290 Lumber:$96
Carpentry Rules & Stair Geometry

How to Calculate Stair Rise and Run: Building Code Specifications

Quick Answer & IRC Code Benchmark

To calculate stairs: divide Total Rise by 7.5 and round to find the number of risers. Divide total rise by risers to get exact riser height (max 7-3/4" by IRC code). Number of treads = risers − 1 with minimum 10" run. Crucial: cut tread thickness off the bottom of the stringer so the first step matches all others.

Stairs are governed by strict building codes because even small variances in riser height (greater than 3/8 in) create a severe tripping hazard. Our tool calculates exact architectural fractions (1/16 in) and checks every dimension against the International Residential Code (IRC R311.7).

1. IRC Residential Code Limits (IRC R311.7)

  • • Maximum Riser Height: 7-3/4 in (7.75" / 197 mm)
  • • Minimum Tread Run: 10.0 in (254 mm)
  • • Minimum Stairway Width: 36.0 in (914 mm)
  • • Minimum Headroom Clearance: 6 ft 8 in (80")
  • • Maximum Riser Variance: 3/8 in between greatest and smallest step
  • • Minimum Stringer Throat: 3.5 in effective meat remaining

2. The Bottom Riser Drop Rule

The most common amateur stair cutting error is failing to drop the bottom cut:

Bottom Stringer Cut = Riser Height − Tread Thickness

If your calculated riser is 7-1/2" and your tread board is 1" thick, the bottom cut on the lumber must be 6-1/2". When the 1" tread is installed, the bottom step rises exactly to 7-1/2".

Choosing Lumber for Stair Stringers: 2x10 vs 2x12

Always use 2x12 dimensional lumber (actual width 11.25 in) for exterior deck stairs and flights exceeding 3 risers. When cutting 7.5" risers and 10" treads into a 2x10 (9.25"), the remaining triangular throat depth is often under 3.0", violating the 3.5" code minimum and making stringers vulnerable to cracking under heavy loads.

Frequently Asked Questions

What is the maximum stair riser height allowed by building code?

Under the International Residential Code (IRC R311.7.5.1), the maximum riser height for residential stairs is 7-3/4 inches (7.75 inches / 197 mm). The difference between the tallest and shortest riser in a flight cannot exceed 3/8 inch (0.375 inches / 9.5 mm).

What is the minimum stair tread depth allowed by code?

Under IRC R311.7.5.2, the minimum tread run depth is 10 inches (254 mm) measured horizontally from the nose of the step to the vertical riser or front edge of the next tread.

Why do you cut the tread thickness off the bottom of a stair stringer?

When you install finished tread boards (typically 1.0 to 1.125 inches thick), they add height to every step. If you do not cut the tread thickness off the bottom of the stringer, your bottom step will be 1 inch too high and your top step will be 1 inch too short. Cutting the tread thickness off the bottom foot of the stringer ensures every single riser is identical in finished height.

What is Blondel's Rule for comfortable stair walking?

Blondel's ergonomic rule states that twice the riser height plus the tread run should equal approximately 24 to 25 inches (2 * Rise + Run = 24" to 25"). This ergonomic formula matches the natural human walking stride.