Stair Stringer Calculator

By Emiliano · Last updated 2026-11-13

Stair Stringer Results

Number of Risers:0
Number of Treads:0
Actual Rise per Step:0
Total Run:0
Stair Angle:0
Stringer Length (per board):0
Number of Stringers:0
Board Type:0
Recommended Board Length:0
Total Stringer Lumber:0
How this calculator works

The calculator determines stringer length from two geometric inputs: total rise (floor-to-floor height) and total run (number of treads × tread depth). Stringer length = √(rise² + run²) plus a standard 16-inch landing allowance (8 inches per end for the top and bottom notches). The number of risers is total rise ÷ target rise, rounded up to the nearest whole number; actual rise per step is then total rise ÷ number of risers so the top step and bottom step are equal. Board type is 2x10 for risers up to 7 inches, or 2x12 for risers up to 7.75 inches (the IRC maximum).

Source: IRC R311.7 (Stairway rise, run, and stringer requirements) and AWC NDS for structural lumber sizing.

How to Calculate Stair Stringers

Stair stringers are the angled support boards that carry every tread and riser in a staircase. Getting the stringer cut length right is the difference between a stair that fits its opening and one that needs to be rebuilt. The math comes from the Pythagorean theorem: stringer length = the square root of (total rise² + total run²). Total rise is the floor-to-floor height; total run is the horizontal distance from the bottom riser's face to the top riser's face.

To find the number of risers, divide the total rise by your target rise per step (typically 7 to 7.75 inches). Round up to the next whole number, then divide again to get the actual rise. For a 30-inch rise with a 7-inch target, you get 5 risers at 6 inches each — under the 7.75-inch code maximum. The number of treads is one fewer than the number of risers, because the top step is the landing itself.

Once you have total rise and total run, multiply the diagonal by 1.05 to 1.15 for landing notches and kerf, then round up to the next standard board length (6, 8, 10, 12, 14, 16 feet). For a typical deck stair with 30 inches of rise and 44 inches of run, the diagonal is about 53 inches; with landing allowance you need a 6-foot board.

If you are pouring concrete steps instead of building wood stairs, our concrete steps calculator handles volume and bag counts for the same rise/run dimensions. For pricing the lumber you'll use to build the stringers and treads, our board foot calculator converts dimensions into board feet and total cost.

What Affects Stair Stringer Design

Several factors change the stringer length and board choice beyond the basic geometry. First, the rise per step must be at or below 7.75 inches per the International Residential Code (IRC R311.7.5). A 2x10 stringer can handle risers up to 7 inches with enough wood remaining below the notch. A 2x12 is required for risers between 7 and 7.75 inches. Anything taller needs a mid-stair landing.

Stringer spacing also matters. Standard 16-inch on-center spacing keeps the treads from sagging. For 36-inch-wide stairs, that means 3 stringers. For 48-inch wide, 4 stringers. Narrow 24-inch stairs can use 2. For wider stairs above 72 inches, add one stringer per additional 14 inches of width. Three stringers is the standard minimum for any stair over 30 inches wide.

The stringer's actual cut length also depends on how it lands at the top and bottom. A stringer that sits on a concrete slab needs a small notch or plumb cut where it meets the slab. A stringer that attaches to a rim joist needs a longer horizontal tail that tucks behind the deck rim. Both add to the raw diagonal length; standard practice is to add 8 to 10 inches per end.

If your stairs are part of a deck build, our deck calculator estimates deck boards, joists, and post footings in the same pass. If you are building an exterior concrete staircase instead of wood, our concrete stairs calculator handles the full volume, bag counts, and rebar length for the same rise and run.

Frequently Asked Questions

How do I calculate stair stringer length?

Stringer length is calculated using the Pythagorean theorem: square root of (total rise squared + total run squared). Add 8 to 10 inches per end for landing notches, then round up to the next standard lumber length (6, 8, 10, 12, 14, or 16 feet). A typical deck stair with 30 inches of rise and 44 inches of run needs a 6-foot 2x10 or 2x12 stringer.

What size lumber should I use for stair stringers?

Use 2x10 for risers up to 7 inches. Use 2x12 for risers between 7 and 7.75 inches (the IRC maximum). After cutting the tread and rise notches, the stringer should retain at least 3.5 inches of solid wood at the thinnest point.

How many stringers do I need for a staircase?

Standard stringers are spaced 16 inches on center. A 36-inch wide stair needs 3 stringers; 48-inch wide needs 4; 60-inch wide needs 5. Narrow stairs under 30 inches can use 2. Add one stringer per additional 14 inches of width beyond 72 inches.

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