Laying out a stringer with a framing square
The calculator does the arithmetic, but the cut still happens on a real board. This is the method most carpenters use, and it's how the plate is drawn.
- Pick a straight board and find the crown. Sight down the edge. Lay the board crown up so the stringer settles flat under load instead of sagging further.
- Set the stair gauges. Clamp one gauge on the tongue (the narrow leg) at your riser height and one on the blade at your tread depth, both on the outside edge.
- Step it off. Hold the gauges tight to the top edge and trace along the tongue and blade. Slide the square up so the blade mark meets the last tongue mark, and repeat once for every riser.
- Drop the stringer. Measure the first riser and cut it shorter by the thickness of one tread. The calculator lists this as the first riser cut.
- Mark the bottom level cut and top plumb cut. The level cut sits on the landing pad; the plumb cut bears against the deck rim or hangs in a hanger.
- Cut, then finish by hand. Run a circular saw up to the lines but not past them, and finish each inside corner with a handsaw or jigsaw. Overcuts weaken the throat.
Cut one stringer, test it in place, then use it as the pattern for the rest.
The throat is what holds the stair up
Every notch takes wood out of the stringer. What's left between the inside corner of a notch and the bottom edge is the throat, and it carries the whole load. The deeper the notch, the thinner the throat.
notch depth = riser × tread ÷ √(riser² + tread²)
throat = board width − notch depth
For a 7″ riser and 11 1/4″ tread, the notch is 5.94″ deep. A 2×12 is 11 1/4″ wide, which leaves a 5 5/16″ throat. That's why 2×12 is the default for cut stringers and why the American Wood Council's deck guide (DCA 6) shows 5″ of throat as the minimum. If the calculator flags your throat, go to wider stock or support the stringer at mid-span.
Stringer length and what to buy
The stringer length here is measured along the board, from the toe of the bottom level cut to the far end of the top plumb cut. It's slightly more than risers × diagonal because of the cuts at each end. The calculator then rounds up to the next stock length: 6, 8, 10, 12, 14, 16, 18 or 20 feet.
Long stringers need support. The DCA 6 guide limits how far a cut stringer can span between supports, and a long run often needs a post and beam under the middle or a landing partway down. Check the guide or your inspector before you plan a single 16-footer.
Mistakes worth avoiding
- Forgetting the drop. The bottom step ends up an inch too tall and the top step an inch short. Inspectors measure both.
- Measuring rise at the house. If the ground slopes away, the real rise at the landing is bigger. Measure where the stairs land.
- Untreated stringers on concrete. Use lumber rated for ground contact if the stringer touches the pad, or hold it off the concrete.
- Cutting into the stringer from the bottom edge. Don't. Cut from the top edge only and keep the throat intact.
Need to compare riser counts or check headroom first? Use the full stair calculator, then come back with the numbers.