Stair Stringer Calculator

The stair stringer calculator divides total rise by your target riser height, rounds to a whole number of equal risers, and sets treads at one fewer. It returns the actual riser height, total run, stringer length and a check against the IRC riser and tread limits.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Number of Risers

Code Check

7.71 in. risers and 10.00 in. treads both meet the IRC's limits (max 7.75 in. riser, min 10 in. tread).

Breakdown
Number of Risers	14
Number of Treads	13
Actual Riser Height	7.71 in
Total Horizontal Run	130.0 in
Stringer Board Length	14.08 ft

How to Calculate Stair Stringer

Starting from the total rise (the exact floor-to-floor height the stairs need to climb), the calculator finds the number of risers. It divides rise by your target riser height and rounds to the nearest whole number. Stairs can't have a fractional riser, so every riser in a flight is required to be equal height. That is why the actual riser height (total rise ÷ number of risers) almost never lands on a perfectly round number like the target. Number of treads is always one fewer than risers, since the top riser is the landing/floor itself, not a separate tread. Total run is tread count × tread depth. The stringer board's length is the hypotenuse of the total-rise/total-run triangle (`√(rise² + run²)`). That is the diagonal cut line a stringer is laid out along before the individual step notches are cut into it.

Example: 108 in. (9 ft) floor-to-floor rise

A 108 in. (9 ft) floor-to-floor rise, targeting a 7.5 in. riser and a 10 in. tread run: 1. Number of risers: round(108 ÷ 7.5) = round(14.4) = 14 2. Actual riser height: 108 ÷ 14 = 7.71 in. (under the 7.75 in. IRC maximum) 3. Number of treads: 14 − 1 = 13 4. Total run: 13 × 10 = 130 in. 5. Stringer length: √(108² + 130²) = √(11,664 + 16,900) = √28,564 ≈ 169.01 in. = 14.08 ft 14 risers at 7.71 in. each, 13 treads at 10 in. each, and a stringer cut from a board at least 14.08 ft long. Add a bit extra for the birds-mouth notch overhang at each end.

Stair Stringer: Compare Scenarios

Scenario	Number of Risers	Number of Treads	Actual Riser Height	Total Horizontal Run	Stringer Board Length
Small (30 in. deck step-down)	4	3	7.50 in	30.0 in	3.54 ft
Medium (108 in. floor-to-floor stair)	14	13	7.71 in	130.0 in	14.08 ft
Large (144 in. walkout basement stair)	19	18	7.58 in	180.0 in	19.21 ft

What Is Stair Stringer?

A stair stringer calculator converts a stair's total rise (the vertical height it needs to climb) into the number of risers and treads. It also gives the actual riser height once that number is fixed, and the length of stringer board needed to frame it. Then it checks the result against the International Residential Code's riser-height maximum and tread-depth minimum. Those are the two dimensions building departments actually inspect on a set of stairs.

Stair Stringer: Pros and Cons

Pros

Forces every riser to come out equal height automatically, by rounding to a whole riser count first. An uneven riser is a common, code-flagged tripping hazard
Checks the result directly against the IRC's riser-height maximum and tread-depth minimum instead of leaving that check to a final inspection
Gives the stringer board length as a straightforward diagonal measurement, useful for buying stock before laying out the actual step notches

Cons

Doesn't lay out the birds-mouth (step) notches on the stringer for you. The diagonal length here is the framing member's length before those cuts are marked
Doesn't add nosing (the small overhang past the riser face some tread profiles have), which changes tread depth measurement slightly depending on the exact code edition and nosing profile used
A local building department can adopt amendments to the IRC riser/tread limits. Always confirm the exact figures your jurisdiction enforces before cutting stock

Stair Stringer: What a Pro Checks

Buy stringer stock a little longer than the calculated length. It doesn't include margin for the birds-mouth notch cut at the top and bottom, which uses up material this geometry alone doesn't account for.
Check your local building department's specific riser and tread limits before cutting. Jurisdictions can adopt amendments to the baseline IRC R311.7.5 figures used here.
Don't assume this sizes the stringer board itself. It solves for riser count, tread count, and length as a geometry problem, not the lumber size or span your stairs need structurally.

Frequently Asked Questions

What is the maximum riser height for stairs?

7.75 in. (7 3/4 in.), per the 2021 International Residential Code, Section R311.7.5.1. The IRC is a common residential building code, adopted (sometimes with local amendments) across most US jurisdictions.

What is the minimum tread depth for stairs?

10 in., per IRC Section R311.7.5.2. It's measured horizontally from the leading edge of one tread to the leading edge of the next, not along the diagonal of the stair.

How do you calculate the number of stair risers?

Divide the total rise (floor-to-floor height) by your target riser height and round to the nearest whole number. Then divide the total rise by that whole number again to get the actual, equal riser height every step will use.

Why does my riser height come out to an odd decimal like 7.71 in.?

Because a stair's total rise almost never divides evenly by a round riser height. The calculator rounds to a whole number of risers first, since a fractional riser isn't buildable. It then divides the total rise by that whole number. That produces the precise, equal height every riser in the flight needs to be.

How long does a stair stringer need to be?

At least the diagonal distance between total rise and total run, `√(rise² + run²)`. Buy stock a bit longer than that minimum to leave margin for the birds-mouth notch cut at the top and bottom of the stringer.

Does this calculator size the stringer board itself (structural)?

No. It calculates riser count, tread count, and stringer length as a geometry problem. It doesn't determine the stringer's required lumber size, spacing, or maximum span. Those are structural sizing decisions that depend on your local code and are outside this calculator's scope.

Sources

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