Stair Calculator

How to calculate and cut stair stringers

By · Published 2026-06-18 · Updated 2026-07-22 · 8 min read

A stringer is just a 2×12 with a series of identical right triangles cut out of it. The math is easy - the calculator hands you riser height and tread depth. The craft is in transferring those two numbers onto lumber accurately, nine or fourteen times in a row, without accumulating error. This guide covers both halves: first the numbers, then the layout and cuts.

How to calculate stringer length

The stringer is the hypotenuse of the right triangle formed by the stair's total rise (floor-to-floor height) and total run (horizontal distance the stair covers). So stringer length comes straight from the Pythagorean theorem:

Stringer length = √(total rise² + total run²)

Say your total rise is 108″ and your total run is 130″. Then the stringer length is √(108² + 130²) = √(11,664 + 16,900) = √28,564 ≈ 169″, or just over 14 feet. To get there you first need riser and tread counts, which set the total run:

  • Total rise is measured finished-floor to finished-floor. Divide it by a target riser (~7.5″) and round to get your number of risers and steps.
  • Total run is the run per tread × the number of treads — not the depth of a single step. A 14-riser stair has 13 treads; at 10″ each that is a 130″ total run.
  • Stringer length ≠ the board you buy. The formula gives the diagonal of the sawtooth; buy your 2×12 longer (a 16-footer here) to leave room for the plumb and level end cuts.
The calculator computes stringer length automatically for every configuration, alongside riser height, tread depth and the code checks — so you can confirm the number before you buy stock, then use this guide to lay it out.

What you need

  • 2×12 stock, longer than your calculated stringer length (a 108″ rise / 130″ run needs ≈169″. Buy a 16-footer)
  • Framing square with stair gauges (the little brass clamps. About $5 and non-negotiable)
  • Circular saw and a handsaw or jigsaw for finishing corners
  • Sharp pencil, tape, and your calculator output
Pick straight, dry stock with no knots along either edge. A crowned or twisted stringer telegraphs into every step. Southern yellow pine and Douglas fir are the standard choices.

Step 1: Set the gauges

Clamp one stair gauge on the square's tongue (narrow arm) at your riser height, and one on the blade (wide arm) at your tread depth. Say the calculator gave you 7.71″ rise and 10″ run: gauge the tongue at 7.71″ (that's 7 11/16″ on the scale. Use the fraction table on the homepage) and the blade at 10″. From now on the square rides the same edge of the board for every step, and the gauges make each placement identical. That repeatability is the entire trick.

Step 2: Mark the sawtooth

Lay the 2×12 across sawhorses, crown edge away from you. Starting near one end, seat both gauges against the board's edge and trace the outside of the square: the tongue line is a riser, the blade line is a tread. Slide the square along, align the tread gauge with the end of the previous riser line, and repeat until you've marked every step. Count when you're done. The number of riser lines must equal your calculated riser count. Recount. Erasing a pencil line is cheaper than a 16-foot board.

Step 3: Mark the ends

The two end cuts are where first-timers go wrong:

  • Bottom (level) cut: extend the last tread line full width. The stringer stands on this.
  • Top (plumb) cut: extend the first riser line full width. This face bears against the ledger or rim joist.

Step 4: Drop the stringer

If you cut now, the first step would end up one tread-thickness too tall and the top step too short, because tread stock raises every step except the floor. The fix: trim one tread-thickness off the bottom level cut. Installing 1″ treads? Take 1″ off the bottom. This is the "drop," and forgetting it is the single most common stringer mistake. (Running the calculator's advanced mode with tread thickness included shows you the corrected first riser directly.)

Step 5: Cut

Cut the long lines with a circular saw, stopping exactly at each inside corner: an overcut weakens the remaining wood at the stringer's narrowest point, which is where it carries the load. Finish each corner with a handsaw or jigsaw. Cut on the waste side of every line; a saw kerf per step of accumulated error adds up to a code violation by the top.

Step 6: Test and duplicate

Stand the first stringer in place. Check the top riser and bottom riser with a level and confirm equal step heights. The IRC allows at most ⅜″ variation across the whole flight. When it fits, use this stringer as the template to trace the others (a 36″-wide stair needs three). Never re-lay-out from scratch; you'd double your error sources.

Rule of thumb for stringer count: one every 16″ of stair width with 1″ treads; every 12″ if treads are thinner or the stair carries heavy traffic. Building an L- or U-shaped stair? Each flight is laid out as its own straight stair — the U-shaped stair calculator gives you each flight's stringer length separately.

Common mistakes checklist

  • Forgot the drop → first step too tall, fails the ⅜″ uniformity rule
  • Overcut inside corners → cracked stringer under load
  • Measured total rise from subfloor, installed finish floor later → bottom riser off by the flooring thickness
  • Used a warped board → steps rock even though the math was right

Run your numbers in the calculator first. The printable cut sheet lists riser height, tread depth and stringer length in both imperial and metric, checked against code before you pick up a saw.