Measuring Stair Stringers
Measure the height of your porch from the ground to the bottom of the porch decking material. Write down the measurement in inches, to the nearest 1/8 inch. This is your rise.
Measure from the porch along the ground to the point you want your stairs to end. Write the measurement down in inches to the nearest inch. This is your run.
Divide the first number as close as possible by 8 inches; this is the typical rise of most steps. This will tell you how many steps you need. You can divide the height evenly into all steps, or put the odd step at the bottom with all others being 8 inches tall. This is the height of your risers.
Divide the second number by the number of steps you need. If it does not divide out to at least 8 inches for each tread, lengthen the run to make each tread at least 8 inches. This is the depth of your tread.
Lay out stringers to the length of the diagonal between the rise and run. Use a tape measure to measure the distance between the top of the porch and the point where the stairs will end. This is the length of your stringer.
Mark steps on the stringer using a framing square. Clamp a straight-edged stick to the square with one end on top of the mark indicating the height of your riser. Clamp the other end on top of the mark on the opposite angle of the square that indicates the depth of your tread. This will form a triangle.
Place the straight board against the edge of your stringer; mark along the two outside edges of the resulting triangle to mark the corner your step will sit in. Repeat this for each step.
Measuring Risers and Treads
Use the riser height calculated for the stringer as the base height of your riser. Add the thickness of your step tread material. Risers are typically nailed between the stringers with the bottom edge of the riser resting behind the back edge of the tread below.
Measure the width you want your stairs to be. Subtract the thickness of both stringers. This is the width of your riser.
Use the depth of the tread from the stringer for the base depth of your riser. Add for a slight overhang along the front edge of the step. An inch is typical, but it can be slightly more or less, depending on the look you want.
Measure the width of your stairs including both stringers. This is the width of your stair treads.