Stair Stringer Calculator

Measure the total rise from the landing spot to the top of the deck or floor. The calculator works out how many steps you need, the exact riser height, and how to lay out and cut the stringers.

–
risers of – each
Treads–
Total run–
Stringer length (along the slope)–
Board to buy–
Stair angle–
Bottom riser cut (drop)–
Stringers needed–

    How the stair math works

    Divide the total rise by the maximum riser height and round up. That gives the number of risers. Then divide the total rise by that number to get the exact riser height, so every step is the same.

    risers = ⌈rise ÷ 7.75⌉ · riser height = rise ÷ risers · treads = risers − 1

    There is one tread fewer than risers because the deck or floor itself is the top step. The total run is the number of treads times the tread depth. The stringer length is the diagonal of rise and run, and the board you buy needs to be about one step longer to leave room for the top and bottom cuts.

    Laying out the stringer

    Set a framing square on the board with the riser height on the tongue and the tread depth on the blade, and trace each step. Stair gauges clamped to the square keep every step identical.

    Before cutting, shorten the bottom riser by the thickness of one tread. This is called dropping the stringer. Without it, the first step ends up taller than the rest by the tread thickness and the top step shorter.

    Code rules this calculator checks

    RuleLimitSource
    Riser height7 3/4" maxIRC R311.7.5.1
    Tread depth10" minIRC R311.7.5.2
    Riser variation in one flight3/8" maxIRC R311.7.5
    Rise of one flight147" max, then a landingIRC R311.7.3
    Stair width36" minIRC R311.7.1
    Headroom6' 8" minIRC R311.7.2

    These are the International Residential Code limits most US jurisdictions adopt. Your local code may differ, and deck stairs often need a permit. The AWC deck guide (DCA 6) adds stringer rules: 2×12 stringers, at least three of them, no more than 18" on center, and at least 5" of solid wood left below the notches.

    Questions

    How many steps do I need for a 3-foot rise?

    36" ÷ 7.75" = 4.6, so 5 risers of 7 3/16" and 4 treads. With 10.5" treads the stairs run out 42".

    Do I need a landing?

    When the total rise is more than 147" (12' 3"), the stairs must be split into two flights with a landing between them. The calculator flags this.

    2×10 or 2×12 stringers?

    Use 2×12. A 2×10 often leaves less than 5" of wood below the notches, which makes the stringer weak. The throat check above shows the number.

    Do I need a handrail?

    Under the IRC, stairs with four or more risers need a handrail on at least one side.