How many stringers does a staircase need?

Three is the standard for most residential stairs up to 36 inches wide: two outer stringers plus one center stringer. Wider stairs need additional centers so treads are never left spanning long unsupported distances. Long or heavily loaded flights sometimes use two extra centers instead of one.

What size lumber should I use for stair stringers?

A 2x12 is the standard choice because its actual 11-1/4 inch depth leaves enough wood for standard riser and tread notches plus the 3-1/2 inch minimum throat. A 2x10 is too shallow for full notches, and engineered lumber such as LVL makes a stiffer, straighter stringer on long spans.

What is the drop cut on a stair stringer?

The drop cut shortens the bottom of the stringer by exactly the thickness of the tread material. Without it, the first riser would stand one tread thickness taller than all the others once the treads are installed. The calculator applies the drop automatically.

How do I calculate stringer length?

Treat the total rise and total run as the legs of a right triangle and take the hypotenuse: the square root of rise squared plus run squared. That gives the length along the slope; add a little extra stock for the top and bottom cuts. The calculator returns this length from your rise and tread depth.

What is throat depth and how much do I need?

Throat depth is the solid wood remaining below the deepest notch, measured perpendicular to the stringer's back edge. Standard practice requires at least 3-1/2 inches. If your notches leave less, switch to a deeper board, a shallower riser, or both.

Can I use LVL or engineered lumber for stringers?

Yes. LVL stays straighter and stiffer than sawn lumber on long flights, and it is a good choice for wide or heavily used stairs. It cuts like lumber with ordinary tools, and the layout rules for throat depth and notch dimensions are the same as for sawn stock.

Should the top tread be part of the stringer?

Usually not. On most stairs the upper floor or deck surface serves as the final tread, so the top of the stringer ends with a riser cut rather than a tread notch. The calculator treats the top landing as the last tread and lays out the final rise accordingly.

Stringer Calculator

What Is the Stringer Calculator?

A stringer is the notched diagonal board that carries every tread and riser on an open staircase, and it is the single part of the stair most builders fear cutting wrong, because once the saw comes out there is no adding wood back. This free stringer calculator turns your total rise and chosen tread depth into a complete cut layout: the number of risers, the exact riser height, the stringer length along the slope, the depth and spacing of every notch, the throat depth left after notching, and the drop cut at the bottom that keeps the first step from standing taller than the rest. The calculator assumes the classic cut stringer, sometimes called a sawtooth or open stringer, sawn from a 2x12 board. A nominal 2x12 actually measures 1-1/2 by 11-1/4 inches, and the layout is built on that true depth, since the notches for a 7-1/2 inch riser and a 10 inch tread consume most of the board. For stairs wider than 36 inches the calculator also tells you where a center stringer is needed so the treads do not flex underfoot. Whether you are framing a basement stair, a garage step, or a new main flight, the layout this tool produces is the same one a framing square traces onto the board.

How to Use This Calculator

Measure the total rise from the finished lower floor to the finished upper floor, including all flooring thickness at both ends. Every riser on the stair is derived from this one number, so measure it in two places and use the larger.
Enter your target riser height. The IRC caps risers at 7-3/4 inches, and most comfortable residential stairs land between 7 and 7-1/2 inches.
Enter the tread depth, the unit run per step. The IRC minimum is 10 inches measured nosing to nosing, and deeper treads between 10-1/2 and 11 inches climb more comfortably.
Set the stringer board, normally a 2x12. The calculator works from the actual dimensions, 1-1/2 by 11-1/4 inches, not the nominal size.
Review the stringer length, the long diagonal dimension along the bottom edge of the board, plus the total run and the number of risers.
Check the throat depth readout. Standard practice requires at least 3-1/2 inches of solid wood below the deepest notch; anything less means a deeper board or shallower notches.

Stair Stringer Calculator

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