What is the minimum tread depth (run)?

IRC requires a minimum run of 10 inches (254mm) measured horizontally. This doesn't include the nosing overhang (typically 0.75-1.25 inches). Deeper treads are more comfortable.

What is the 17-18 rule for stairs?

For comfortable stairs, rise + run should equal approximately 17-18 inches. For example: 7.5 inch rise + 10.5 inch run = 18 inches. This ratio provides a natural walking stride on stairs.

How do I calculate stringer length?

Stringer length is calculated using the Pythagorean theorem: √(Total Rise² + Total Run²). Add several inches for the bottom cut and top attachment. Use 2x12 lumber for stringers.

How many stringers do I need?

Most residential stairs use 3 stringers: one on each side and one in the center. For stairs wider than 36 inches, a center stringer prevents tread bounce. Very wide stairs may need 4 stringers.

What's the minimum headroom for stairs?

IRC requires minimum 6 feet 8 inches (80 inches or 203cm) of headroom measured vertically from the stair nosing to any obstruction above.

How steep can stairs be?

Comfortable residential stairs are typically 30-37 degrees. Steeper than 42 degrees is considered a ladder by many codes. Shallower than 25 degrees may feel like a ramp.

What if my stairs don't meet code?

If auto-calculated stairs exceed code limits, try: increasing the number of steps, extending the available run length, or reconsidering the total rise. Sometimes floor-to-floor height requires a landing with a turn.

Construction

Stair Calculator

Calculate stair rise, run, tread dimensions, stringer length, and verify building code compliance for safe, comfortable stairs

Unit System

Calculation Method

Quick Preset

Total Rise (Height)

Optional Inputs

Available Run Length

Desired Run per Step

Headroom Clearance

Stair Width

IRC Building Code Reference

Max Rise: 7.75 in

Min Tread: 10 in

Min Headroom: 80 in

Min Width: 36 in

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Calculate Perfect Stair Dimensions Every Time

Building stairs that are safe, comfortable, and code-compliant requires precise calculations. Our stair calculator determines the optimal rise, run, and tread dimensions based on your total height, validates against IRC building codes, and estimates materials needed for your project.

How Stair Calculations Work

Safe stairs require careful balance between rise (step height) and run (tread depth). The IRC building code mandates maximum rise of 7.75 inches and minimum run of 10 inches. Comfortable stairs follow the '17-18 rule' where rise + run equals 17-18 inches. Our calculator optimizes dimensions within these constraints.

Key Formulas

Rise = Total Height ÷ Steps | Stringer = √(Rise² + Run²)

Why Use Our Stair Calculator?

IRC Code Validation

Automatically validates against IRC building codes for permit approval with clear compliance status.

Comfort Optimization

Applies the 17-18 rule to ensure comfortable stride and safe step dimensions.

Multiple Calculation Modes

Choose auto-calculate for optimal results, or specify fixed steps or fixed rise per step.

Stringer Length Calculation

Calculate precise stringer length using Pythagorean theorem for accurate lumber cuts.

Material Estimates

Get board feet estimates for stringers, treads, and risers to plan your material purchases.

Quick Presets

Pre-configured settings for common applications like decks, basements, and standard floors.

How to Calculate Stair Dimensions

Common Use Cases

Deck & Porch Stairs

Calculate outdoor stairs from deck to ground with proper rise and weather-resistant materials.

Basement Stairways

Design basement stairs with headroom clearance verification and code compliance.

Interior Floor-to-Floor

Plan main stairways between floors with optimal comfort and safety dimensions.

Contractor Estimates

Get accurate material quantities for stringer lumber, treads, and risers before bidding.

Building Permit Applications

Verify dimensions meet IRC code requirements before submitting plans for approval.

DIY Renovations

Calculate replacement stair dimensions or add stairs to existing spaces safely.

Frequently Asked Questions

The IRC (International Residential Code) specifies a maximum rise of 7.75 inches (196mm) per step. Some local codes may be stricter. Our calculator flags any rise exceeding this limit.

How Stair Calculations Work

Key Formulas

Rise = Total Height ÷ Steps | Stringer = √(Rise² + Run²)