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

Under IRC Section R311.7.5.1, the maximum unit riser height is 7-3/4 inches (197 mm), and the minimum unit tread run is 10 inches (254 mm) with a minimum 3/4-inch nosing projection (or 11 inches without nosing).

Why do you have to cut the bottom of a stair stringer by the tread thickness?

If you don't subtract the tread thickness from the bottom riser cut, adding the tread on top will make the bottom step too tall by the exact thickness of that tread, while making the second step normal. Trimming the bottom ensures all finished risers are identical.

What is Blondel's stair comfort formula?

Blondel's formula states that two times the unit rise plus one unit run (2R + T) should equal between 24 and 25 inches (61 to 63.5 cm). This mimics the natural human walking stride pace on an incline.

How thick must the solid wood throat of a cut stringer be?

The 'throat' or solid meat of an open cut 2x12 stringer should never be less than 3.5 inches (typically 5 inches recommended). If too much wood is cut away, the stringer loses shear strength and can crack under foot traffic.

How many stringers are needed for a deck staircase?

For 5/4 deck boards or composite treads, stringers must be spaced no more than 12 inches on center (meaning a 36-inch wide stair requires 4 stringers). Standard 2x lumber treads can span up to 16 inches on center (3 stringers for a 36-inch stair).

DIY & Trades100% Client-Side Local Execution•Updated 2026

Stair Stringer Rise, Run & Tread Layout Calculator (IRC Code)

Name: Stair Stringer Rise, Run & Tread Calculator (IRC Code)
Author: AnshuTechy

Calculate exact stair riser heights, tread runs, stringer throat depth, total stringer board length, and angle of inclination compliant with International Residential Code (IRC).Documentation & FAQs ↓

Stair Stringer Rise, Run & Tread Calculator (IRC Code) — Interactive Console

Runs locally in your browser • Instant output

Exact Unit Rise7.71″14 Risers total

Unit Tread Run11.0″13 Steps / Treads

Total Stairway Run11′ 11.0″143″ horizontal footprint

Stairway Incline Angle35.0°Stringer: ~14.9 ft 2x12

Total Floor-to-Floor Rise: 108″Standard 8ft ceiling + subfloor ≈ 108″

Target Tread Depth: 11″IRC Code minimum is 10.0″ (11″ ideal)

Max Riser Code Limit: 7.75″

IRC Code Compliant (Unit Rise ≤ 7.75″ & Run ≥ 10″)Blondel Ratio (2R + T): 26.4″ (Ideal: 24″–25″)

2D Elevation Cutout & Stringer Profile

Step-by-Step Instructions

4 Steps to Completion

1Phase 1

Measure the total vertical rise from finished lower floor/ground surface to the finished upper deck or landing.

2Phase 2

Set your target unit riser height (typically 7" to 7.5") or desired tread run (standard 10" to 11").

3Phase 3

Enter your tread thickness (e.g., 1" composite or 1.5" 2x lumber) to apply automatic bottom riser deduction.

4Phase 4

Inspect stringer layout measurements, total horizontal run, cutting angles, and 2x12 board length requirements.

Practical Use Cases & Real-World Scenarios

Outdoor Deck & Porch Stair Construction

Determine exact step cuts and stringer lumber orders from deck joist ledger height down to ground concrete patio.

Basement & Attic Remodeling

Plan code-compliant replacement staircases with restricted horizontal run and strict headroom clearances.

Custom Furniture & Loft Step Ladders

Calculate uniform riser increments for loft access steps where standard pre-cut 3-step stringers will not fit.

Stair Stringer Rise, Run & Tread Layout Calculator (IRC Code)

2D Elevation Cutout & Stringer Profile

Step-by-Step Instructions

Practical Use Cases & Real-World Scenarios

IRC Code Compliance Guard

Lumber Cut Length & Throat Depth

Bottom Tread Thickness Compensation

Incline Pitch & Headroom Angle

Frequently Asked Questions

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