Stair Stringer Calculator

Worked example

With these inputs:

• Total rise — finished deck surface down to the landing: 42 in
• Preferred rise per step: 7" — most comfortable (recommended)
• Tread depth (run per step): 10-1/2" — two 5/4x6 boards with a 1/4" gap
• Stringer board: 2x12 — 11-1/4" actual (usual choice)
• Stair width: 42 in
• Tread thickness: 1" — 5/4 deck boards or composite
• Tread material: Wood — stringers up to 16" apart

Every step is 7.000" — cut them all from one marked stringer so they match. Codes allow no more than 3/8" variation between the tallest and shortest riser in a flight, and uneven steps are the single most common failure at inspection.

How it works

To lay out a stair stringer, divide the total rise by 7-3/4 inches and round up to get the number of risers, divide the rise by that number for the exact step height, multiply one fewer tread by the tread depth for the total run, and the stringer length is the square root of rise² + run². This calculator does all of it against the IRC R311.7 limits. It works out the stringer itself, not just the staircase: the cut length, how many stringers, the framing-square settings for marking them, and — the part most calculators skip — whether the board you plan to use still has enough material left after notching.

Frequently asked questions

What size board do I need for a stair stringer?

Almost always a 2x12. After you notch a stringer, what matters is the material left behind the notches — the throat — and it has to be at least 5 inches. At a typical 7 inch rise and 10-1/2 inch tread, a 2x12 leaves about 5.2 inches and passes, while a 2x10 leaves about 3.2 inches and fails. That is why deck stairs are cut from 2x12 even though a 2x10 looks big enough.

How many stringers do I need?

Three minimum for any staircase, then one more for roughly every 16 inches of width with wood treads. Composite and PVC treads flex more, so their manufacturers usually require 12 inch spacing — a 48 inch wide stair needs 4 stringers in wood and 5 in composite. Check the specific tread's instructions, because the spacing requirement is a condition of the warranty.

How do you calculate stringer length?

It is the hypotenuse of the total rise and the total run. Square the total rise, square the total run, add them and take the square root. For a 42 inch rise over a 52-1/2 inch run that is about 67 inches, or 5 feet 7 — so you would buy 8 foot boards and have room for the cuts at each end.

Why is the rise per step not a round number?

Because the total height rarely divides evenly. The right approach is to pick the number of risers, then divide the total height by it and cut every step to that exact figure, even if it is 6.857 inches. Rounding to a round number and letting the last step absorb the difference is what creates the uneven bottom step people trip on — and codes limit the variation between steps to 3/8 inch.

Do I measure to the ground or to the concrete pad?

To the finished surface the bottom step will land on. If you are pouring a pad, measure to where the top of that pad will be. Measuring to bare soil and then pouring concrete afterwards raises the landing and shortens the bottom step, which is a common way to end up outside code after everything is already cut.

Should the bottom step be shorter to allow for the tread thickness?

Yes — this is the classic stringer mistake. When stringers sit on the landing and treads are added on top, the bottom riser ends up one tread thickness taller than the rest unless you drop the bottom of the stringer by that amount. Cut the bottom of the stringer down by the thickness of your tread material, usually 1 to 1-1/2 inches.