How to lay out a stair stringer
Turn a finished rise and a tread depth into equal steps, a throat, and the board to buy.
Start from the finished rise
Measure total rise before you mark anything. The rise guide covers finished floor to finished floor, both sides, and the taller reading. Divide that rise by the maximum riser and round up. Then divide the rise by the whole number of risers. Every riser on the board is that same height.
With the top tread below the upper floor, you mark one fewer tread than risers. The run is the tread count times the tread depth. Clamp stair gauges to a framing square at the riser height and the tread depth, and step the square along the board. Each step of the square is one riser and one tread.
Throat is the wood you leave
Throat is the wood left after the triangular steps are cut, measured square to the slope. A 2x12 is 11 1/4 inches wide, and a 2x10 is 9 1/4 inches wide. Use that actual width, not the name size. For an outdoor stair, AWC DCA 6 Figure 28 calls for at least 5 inches of throat. A thinner throat fails that check outdoors. The stringer calculator draws the throat as a dashed line so you can see it before you cut.
A 40.5 inch rise with a 10.25 inch tread is 6 risers at 6.75 inches, a 73.64 inch stringer, and a 5.61 inch throat. The stock length to buy is 8 feet.
The board to buy
The sloped length is the number of risers times the diagonal of one riser and one tread. Round up to a stock length from 8 feet through 20 feet. Three stringers is the minimum. A wider stair adds stringers so the tread material stays within its spacing. Mark every stringer from the same rise and run. The stringer calculator prints the layout, the throat, and the stock length. Plan the opening first on the stair calculator when headroom is part of the job.
This guide explains the measurement the calculators use. It is not a permit review.
Use it on a calculator
FAQ
How many risers do I mark?
Divide the total rise by the maximum riser, usually 7 3/4 inches, and round up to a whole number. Each riser is then the total rise divided by that count, so the steps match. You can use more risers. You cannot use fewer if that would push one riser over the maximum. The count is the number of vertical faces, not the number of treads.
Why is there one fewer tread?
With the top of the stringer below the upper floor, the floor itself is the last tread. The tread count is one less than the riser count, and the run is that count times the tread depth. A flush top puts a tread level with the upper floor, so the tread count equals the riser count and the run is one tread longer. Throat and stringer length stay the same either way.
What board do I buy?
The sloped length is the number of risers times the diagonal of one riser and one tread. Round that length up to the next stock length from 8 feet through 20 feet. A 2x12 is 11 1/4 inches wide, and the throat is what remains after the step cuts. For an outdoor stair, AWC DCA 6 Figure 28 calls for at least 5 inches of throat. Three stringers is the minimum.
By Lumber Calc Editorial Team · Verified against 2021/2024 IRC & AWC DCA 6 · Last updated Oct 24, 2026