How do you calculate stair rise and run?

Measure the total rise (floor to floor height in inches). Divide by 7 and round to the nearest whole number to get the number of risers. Divide total rise by number of risers to find exact riser height. Set run (tread depth) to satisfy the comfort formula: 2×rise + run = 24–25 inches. With a 7-inch rise, a 10–11 inch run is appropriate and code-compliant.

What's the code for residential stairs?

Under the IRC (International Residential Code), the maximum riser height is 7-3/4 inches, minimum tread depth is 10 inches, minimum stair width is 36 inches, and handrails are required when there are 4 or more risers. Handrails must be 34–38 inches above the stair nosing. Local jurisdictions may have stricter requirements, so always check with your local building department.

How many steps can a staircase have without a landing?

Under the IRC, a stair flight can have a maximum vertical rise of 12 feet before a landing is required. At a typical 7-inch riser height, that's about 20 risers (20 × 7 = 140 inches = 11.67 feet) without needing a landing. Most exterior deck stairs and interior stairways in typical homes fall well within this limit.

What size lumber for stair stringers?

2×12 lumber is the standard for stair stringers. After making the rise and run notch cuts, you need a minimum of 3.5 inches of solid wood remaining in the stringer's effective depth (the narrowest point after cuts). A 2×12 (actual 11.25 inches) provides enough material to meet this requirement with typical riser/tread dimensions. Never use 2×10 for notched stringers — it won't leave enough material.

How do I calculate stringer length?

Use the Pythagorean theorem: stringer length = √(total rise² + total run²). Where total rise is the vertical height of the entire staircase in inches, and total run is the total horizontal distance (number of treads × tread depth) in inches. Add 12 inches to the calculated length to account for the top and bottom seat cuts. This gives you the minimum board length needed.

Stair Building Guide: Rise, Run, Stringer Length & Codes

Understanding Rise and Run

Every stair calculation starts with two measurements: rise (the vertical height of each step) and run (the horizontal depth of each step tread). The total rise is the vertical distance from finished floor to finished floor (or ground to landing). The total run is how much horizontal space the staircase occupies.

A comfortable stair follows this relationship: 2 × rise + run = 24–25 inches. This ergonomic formula produces a natural walking stride. A 7-inch rise with an 11-inch run = 7×2 + 11 = 25 — comfortable and code-compliant.

IRC Building Code Requirements

The International Residential Code (IRC) sets minimum standards for residential stairs. Most US jurisdictions adopt these codes:

Maximum riser height: 7-3/4 inches (7.75 in)
Minimum tread depth (run): 10 inches
Minimum tread width: 36 inches
Maximum riser height variation: 3/8 inch between tallest and shortest riser in the flight
Handrail required when: 4 or more risers
Handrail height: 34–38 inches above the stair nosing
Headroom clearance: Minimum 6 feet 8 inches above any stair nosing

Calculating Number of Steps

To find the number of steps (risers) needed:

Measure the total rise (vertical distance from floor to floor) in inches.
Divide by 7 (target riser height) to get the approximate number of steps. Round to the nearest whole number.
Divide the total rise by the number of steps to find the exact riser height.

Example: Total rise = 104 inches (8 ft 8 in). 104 ÷ 7 = 14.9 → round to 15 risers. 104 ÷ 15 = 6.93 inches per riser (well under the 7.75 max). Total run = 14 treads × 10 inches = 140 inches = 11.67 feet of horizontal space. Note: there is always one fewer tread than riser — the top landing is the floor itself.

Stringer Length Calculation

A stair stringer is the diagonal structural member that supports the treads. To calculate stringer length, use the Pythagorean theorem:

Stringer length = √(total rise² + total run²)

Using the example above: √(104² + 140²) = √(10,816 + 19,600) = √30,416 = 174.5 inches (14.5 feet). Add 6–12 inches for the stringer seat cut (where it rests on the landing) and header cut at top. Use 2×12 lumber for stringers — you need at least 3.5 inches of "effective depth" remaining in the stringer after cuts.

Deck Stairs: Key Differences

Deck stairs have a few requirements beyond interior stairs:

Footing at the base: The bottom of the stringer must rest on a concrete footing or concrete pad, not on soil. Frost depth requirements apply in cold climates.
Post support: For tall deck stairs (over 6 feet), intermediate support posts may be required.
Composite or treated lumber for treads: Exterior exposure requires pressure-treated wood or composite decking. 5/4×6 decking boards work well as stair treads.
Open vs. closed risers: Deck stairs often have open risers (no vertical board between treads). IRC allows this as long as a 4-inch sphere cannot pass through any opening.

Stringer Material

Use 2×12 lumber for stair stringers — this gives enough material depth after making the rise and run cuts. For exterior stairs, use pressure-treated 2×12. For three-cut open stringers (the decorative notched kind), you typically need 3 stringers for a 36-inch wide stair. For closed (solid) stringers, 2 stringers plus solid risers are standard for interior stairs.