Stair Calculator

Enter your deck height and get the exact riser count, tread count, total run, stringer length, and lumber list — with automatic IRC code-compliance checking. Inputs are saved in the URL.

Stair Dimensions

The total vertical height from ground to deck surface. Measure in inches.

How tall each step should be. IRC max is 7-3/4". Default 7.5" is comfortable and code-safe.

How deep each step is (the part you stand on). IRC minimum is 10". Default 10.5" is comfortable.

Width of the staircase. Residential minimum is 36". Wider stairs need more stringers.

↓ How the math works

Enter your total rise to calculate stair dimensions.

Lumber List

Enter total rise to generate a lumber list.

Important: These calculations use typical IRC model-code values. Local codes always take precedence. See our deck code reference. Consult a licensed contractor for structural confirmation.

Related Calculators

Deck Calculator · Railing & Baluster Calculator · Fence Calculator

Methodology

How the stair calculator works.

Riser count

Risers = max(2, round(totalRise ÷ targetRiser)). If the resulting actual riser height exceeds 7.75" (the IRC maximum), the tool flags it in red and recommends adding one riser.

Treads and total run

Treads = risers − 1 (a staircase with N risers has N−1 treads, because the landing counts as the top "step"). Total run = treads × tread depth.

Stringer length

Stringer length = √(totalRise² + totalRun²) — the hypotenuse of the rise-run triangle. The calculator recommends the next standard 2×12 length (8, 10, 12, 14, or 16 ft) that is long enough.

Stringer count

Stringers = max(3, ⌊ width ÷ 16" ⌋ + 1). At 36" wide you get 3 stringers; a 48" wide stair gets 4 (16" spans exactly); wider stairs get more.

Code note

These calculations use typical IRC model-code values. Your local jurisdiction may have different maximums or minimums. Always confirm with your building department before purchasing materials. Our deck code requirements guide has more detail.

FAQ

Frequently asked questions.

What is the IRC maximum riser height?

The International Residential Code (IRC) sets the maximum riser height at 7-3/4" (7.75"). This calculator automatically flags any calculation that exceeds this limit and recommends adding a riser to bring it into compliance. Your local code always takes precedence — verify before you build.

What is the minimum tread depth?

IRC requires a minimum tread depth of 10 inches. The nosing (overhang beyond the riser) can add up to 1-1/4" beyond the tread. This calculator uses the horizontal tread depth; the common starting point for comfortable residential stairs is 10.5".

What is a stringer?

A stringer is the diagonal board that runs along each side (and center) of a staircase, supporting the treads and risers. Stringers are typically cut from 2×12 pressure-treated lumber. This calculator tells you how many stringers you need and what length board to buy.

How many stringers do I need?

Residential stairs typically need at least 3 stringers for a 36" wide stair: one on each side and one in the center. This calculator uses the formula max(3, ⌊width ÷ 16"⌋ + 1) to keep stringer spans at or under 16 inches. A 48" stair gets 4 stringers; wider stairs get more.

What is the difference between total rise and riser height?

Total rise is the full vertical distance from the lower floor to the upper landing — measured in inches. Riser height is how tall each individual step is. The calculator divides total rise by the target riser height and rounds to get the number of steps, then computes the actual riser height.

Can I bookmark my stair calculation?

Yes — all inputs are saved in the URL. Bookmark the page or share the link to save or send your exact calculation.