Lay out deck stair stringers: board size (2x10 vs 2x12), throat depth, stringer count and spacing, with the cut geometry for an outdoor flight.
The stringer is the notched board that carries the treads. This calculator focuses on the stringer side of a deck stair: board recommendation, throat depth, stringer count, and cut length.
A cut stringer must keep at least 3.5 inches of sound wood behind each notch; most need a 2x12.
At 16-inch spacing, divide width by 16 and add one.
Commonly 16 inches on-center for standard decking.