A stair stringer — the long notched board that carries every tread — is laid out from just two numbers: the total rise and the total run. Measure those two correctly and the rest is marking and sawing. But the commonest failure happens before a single line is drawn on the lumber: measuring the rise to the wrong floor surface. A stringer laid out from a wrong rise produces a staircase where every step is off by the same amount, and there is no fixing it once the notches are cut.
This guide works through the layout in four steps: measuring the true total rise, dividing it into equal risers, marking the notches with a framing square, and dropping the bottom of the stringer so the first step lands at the right height. Follow them in order and the stringer will come out right the first time.
Step 1: Measure the total rise — finished floor to finished floor
The total rise is the vertical distance from the finished floor at the bottom of the stairs to the finished floor at the top. The word finished carries the whole measurement. If the upstairs landing will get 3/4 in hardwood and the downstairs is bare subfloor today, you must add that 3/4 in to the tape reading — the stairs will be walked on after the flooring goes in, not before.
Measuring to subfloor at either end is the classic mistake, and it is easy to make because during framing the subfloor is all there is to measure to. List the flooring build-up planned at the top and at the bottom — hardwood, tile and backer board, carpet and pad — and adjust the measurement by those thicknesses. For outdoor stairs the same rule holds: measure from the finished grade, the ground as it will sit when the work is done rather than the dug-up soil of mid-project, up to the top face of the deck boards.
Step 2: Divide the rise into equal risers
Divide the total rise by your target riser height, round to the nearest whole number of risers, then divide the total rise by that count to get the actual, equal riser height. Every riser in the flight must match — under US rules all risers must sit within 3/8 in of each other, and the reliable way to pass is to make them identical. The Stair Calculator does this division for you, and from the same three inputs it also works out the total run, the stringer length and the stair angle.
Never absorb the leftover into the top or bottom step. Feet learn the rhythm of equal risers within a few steps, and a single odd one out is what trips people. If the division gives an awkward riser height, nudge the target a little and recalculate — small changes to the target shift the actual riser by fractions of an inch, and the Stair Calculator updates the whole layout as you type.
Step 3: Lay out the notches with a framing square and stair gauges
Stringers are cut from 2×12 lumber, which is deep enough to carry the notches without weakening the board. Clamp a pair of stair gauges (small brass stops) to the framing square at your two marks: the actual riser height on one leg and the tread depth — the going — on the other. The gauges let you step the square along the board and mark the identical notch, over and over, without remeasuring.
Start at one end of the 2×12 and work along it, marking each rise-and-going pair in turn. Keep the square oriented the same way for every step — flip it and the notches will mirror. Mark the full flight on one stringer first, then use that stringer as a template for the others so every stringer matches exactly.
Step 4: Drop the first step by the tread thickness
Here is the step almost everyone misses the first time. The layout you just marked assumes every tread sits in a notch cut into the stringer. But the bottom tread sits on the floor, not in a notch — so if you cut the stringer exactly as marked, the first riser will be one full tread thickness taller than all the others. To fix it, shorten the bottom of the stringer by the tread thickness before you cut. If your treads are 1 in thick, cut 1 in off the bottom of the stringer; the first riser then lands at exactly the same height as the rest.
Worked example: deck stairs
Say the deck surface sits 42 in above the finished grade, so the total rise is 42 in. With a target riser of 7.5 in, divide 42 by 7.5 to get 5.6, which rounds to 6 risers at exactly 7.00 in each. Six risers mean five treads, and at a 10 in going the total run is 5 x 10 = 50 in. The stringer is the hypotenuse: sqrt(50^2 + 42^2) = 65.3 in. An 8 ft 2×12 covers that with room to spare for the top and bottom cuts. Run the same numbers through the Stair Calculator any time the rise changes, because a different rise means a different riser count. And use the Deck Calculator when you plan the framing, so the stair opening, the joists and the stringers all agree before you buy lumber.
Choosing stringer stock
Use straight, pressure-treated 2×12 lumber for outdoor stringers (indoor stairs can use standard framing-grade 2×12). Sight along each board and reject any with a twist or a deep crown — a warped stringer makes warped steps. And never notch deeper than the layout requires: the uncut wood left below the notches is what carries the load, so cutting the notches oversize weakens the very board the stairs depend on.
Frequently asked questions
What size lumber for stair stringers?
A 2×12 is the standard stringer material. It is deep enough that the notched-out steps still leave plenty of solid wood to carry the load. Avoid narrower boards — they leave too little material below the notches.
Why is my first step taller than the rest?
The stringer was not dropped by the tread thickness at the bottom. The layout assumes every tread sits in a notch, but the first tread sits on the floor, so without the drop the first riser comes out one tread thickness too tall. Shorten the bottom of the stringer by the tread thickness before cutting.
How accurate must riser heights be?
Under US rules every riser in a flight must be within 3/8 in of every other riser. In practice, aim for exactly equal — the Stair Calculator divides the total rise into identical risers so the flight passes the rule automatically.
Can I buy pre-cut stringers?
Yes, for common deck heights — but check their riser height against your own total rise first. A pre-cut stringer only fits if its rise matches yours; if your finished grade or deck height differs, its steps will not land evenly and you are better off laying out your own.
Estimates only — measure twice, cut once; stairs are regulated, so check local code.