How to Calculate & Cut Stair Stringers
Part of Decks, Fences & Roofing
Quick answer
To cut a stringer, mark each step on a 2x12 with a framing square set to your riser height on one arm and tread depth on the other. Step the square down the board for each riser/tread, then drop the bottom (first) riser by the thickness of one tread so all finished risers end up equal. Cut the notches, then the top and bottom plumb/level cuts.
A stringer is the sawtooth board that carries the treads. Lay one out accurately and the rest of the staircase falls into place — the whole job is transferring your calculated rise and run onto the board, repeated for every step.
Steps to lay out a stringer
- Get your numbers first: riser height, tread depth and the number of treads from the stair calculator.
- Clamp stair gauges (or tape) on a framing square at the riser height (tongue) and tread depth (body).
- Starting at one end of a 2x12, trace the square's right angle — that's one riser and one tread.
- Slide the square down so it picks up where the last tread ended, and repeat for every step.
- Mark the top plumb cut (against the header) and the bottom level cut (on the floor).
Don't forget the first-riser drop
After cutting the notches, trim the bottom of the stringer by the thickness of one tread. Without this 'dropping the stringer,' the bottom step ends up one tread thickness taller than the rest once the treads are installed — breaking the equal-riser rule.
- Cut the notches with a circular saw and finish the inside corners with a handsaw (don't overcut).
- Use the first stringer as a template to trace the others so every notch matches.
- Use a stringer at each side and one in the middle for stairs up to 36 in wide.
Run the stair calculator to get the exact riser, tread, total run and stringer length before you mark the board — it also flags if your rise/run falls outside code.
FAQs
What size board is a stair stringer?
Standard stringers are cut from 2x12 lumber, which leaves enough wood behind the notches (at least 3.5 in) for strength. Pressure-treated 2x12 is used for exterior and deck stairs.
Why do you drop the bottom of the stringer?
Because the treads add thickness on top of every step. If you don't trim the bottom by one tread thickness, the first step finishes taller than the others. Dropping the stringer keeps all the finished risers equal, as code requires.