What size board do I need for stair stringers?

A 2x12. A standard notch on a 2x12 leaves about 5.5 inches of throat, just over the 5 inch minimum that keeps the stringer from splitting. A 2x10 drops to roughly 3.5 inches at code-max risers and will crack. AWC DCA 6 also caps a cut 2x12 at 6 feet of run.

How many stringers do I need?

This calculator returns two up to 36 inches of width, three up to 72, and four beyond, following about 16 inch spacing. With 2x6 wood treads on a 36 inch stair, many builders add a third in the middle so the treads do not flex. Composite treads want stringers closer, often 12 inches on center.

How do you cut stair stringers?

Set a framing square to the riser height on the tongue and the tread depth on the body, then step it down the board scribing each notch. Cut the notches, then trim the bottom by one tread thickness so the first step matches the rest. Keep the throat behind each notch at 5 inches or more.

How long is a stair stringer?

It is the diagonal: the square root of total rise squared plus total run squared. A 42 inch rise with a 50 inch run gives a stringer about 65 inches long. Buy the next stock length up so you have material to square the end cuts.

MATERIALFOREMAN · SHEET S-002 · STAIR WORKS

CALC-002 · DRAWING NO. S-002 · REV A

Stair stringer calculator.

Drafted to scale · cited sources · honest numbers

Enter the total rise and pick a board. The calculator lays out the stringer: length, count, the riser and tread marks for your framing square, and the throat depth that keeps a notched 2x12 from splitting. Built for deck and porch stringers cut on site.

◈ DRAFTING PANEL · STAIR TAKEOFF · N.T.S. SHEET S-002 · REV A

Redline · scope notice Cut layout and stringer geometry only. Stringer sizing is structural. AWC DCA 6 caps a cut 2x12 at 6 feet of run; longer spans, commercial stairs, and load-bearing connections need engineered design and a permit.

Note 01 · Field Procedure

How to lay out a stringer

Measure total rise from the finished lower surface, usually grade or a landing pad, to the top of the deck or porch surface.
The calculator splits the rise into equal risers and gives one riser height and one tread depth. Set a framing square to those two numbers: riser on the tongue, tread on the body.
Step the square down the face of the board, scribing each notch, until you have marked every riser.
Trim the bottom of the stringer by the thickness of one tread so the first step is not taller than the rest.
Before cutting, check the throat: the wood left behind the deepest notch. Keep it at 5 inches or more or the stringer splits.

Note 02 · Equation

The formula

stringer = √( rise² + run² ) · throat = board − ( R × T ) ÷ √( R² + T² )

risetotal vertical rise, lower surface to deck surface

runtotal run, (risers − 1) × tread depth

throatwood behind the deepest notch; keep it at 5 inches or more

R, Tone riser height and one tread depth

A cut 2x12 stringer is capped at 6 feet of run by AWC DCA 6. Past that, the stringer is engineered lumber, not a notched board.

Note 03 · Sanity Ranges

Stringer scenarios

T	Use case	Notes
36"	Low deck, 2 to 3 steps	Short run, plenty of throat on a 2x12 cut stringer.
42"	Typical deck	6 risers at 7 in, 5 treads. Throat about 5.5 in on a 2x12.
2x12	Default board	The only common board that holds 5 in of throat at a 7.75 in riser.
2x10	Shallow board	Throat drops under 5 in near code-max risers. Watch the warning.

Note 04 · References

Sources

Authorities cited on this sheet

AWC DCA 6: Prescriptive Residential Wood Deck Construction Guide · American Wood Council. The prescriptive reference for deck stair stringers: the 6 ft cut-stringer run limit and the notched-stringer detail behind the throat-depth check.
IRC R311.7 Stairways · 2021 International Residential Code, free public viewer at ICC. The 7.75 in maximum riser, 10 in minimum tread, and 3/8 in uniformity limit the calculator enforces.

Note 05 · Field Manual

What the sheet count does not tell you

A 42 inch deck stringer, laid out

A 42 inch rise is the everyday deck stair. The calculator splits it into 6 risers at 7 inches each, which leaves 5 treads. At a 10 inch tread depth, the total run is 50 inches and the stringer board runs the diagonal: the square root of 42 squared plus 50 squared, about 65.3 inches. Buy the next length up, a 6 foot 2x12, and you have room to square the cuts.

The framing-square settings are the two numbers that matter at the sawhorses: 7 inches on the tongue, 10 inches on the body. Step the square down the board six times, scribe, and cut. Trim the bottom of the stringer by one tread thickness so the first step lands at 7 inches like the rest. Skip that trim and the bottom step is an inch and a half tall, which fails the 3/8 inch uniformity rule and trips people in the dark.

Throat depth is what splits a stringer

The throat is the wood left behind the deepest notch, measured straight back from the inside corner of the cut. It is the weakest line on the board, and on a notched stringer it carries the whole load. The trade floor is 5 inches. Below that, the stringer cracks at the notch corner, usually months later under a full load of people.

This is the number that decides the board. A 2x12 is 11.25 inches deep, and a standard 7.5 by 10 notch leaves about 5.5 inches of throat. A 2x10 is only 9.25 inches deep, so the same notch leaves about 3.5 inches, well under the line. Switch the board selector to a 2x10 at a code-max riser and the calculator throws the throat warning. The fix is a 2x12, a shallower tread, or a closed (housed) stringer that is not notched at all.

How many stringers, and how far apart

Stringer count follows the stair width and the tread material. The calculator returns two stringers up to 36 inches, three up to 72, and four beyond, which tracks roughly 16 inch spacing. That spacing is a function of the tread, not the code minimum. Two 2x6 treads laid across a 36 inch stair with only edge stringers will flex underfoot, so many builders add a middle stringer on anything 36 inches and wider. Composite treads flex more than wood and want stringers closer still, often 12 inches on center, per the decking manufacturer's span chart. When in doubt, add a stringer; it is the cheapest insurance on the stair.

Note 06 · Common Questions

Common questions

What size board do I need for stair stringers?: A 2x12. A standard notch on a 2x12 leaves about 5.5 inches of throat, just over the 5 inch minimum that keeps the stringer from splitting. A 2x10 drops to roughly 3.5 inches at code-max risers and will crack. AWC DCA 6 also caps a cut 2x12 at 6 feet of run.
How many stringers do I need?: This calculator returns two up to 36 inches of width, three up to 72, and four beyond, following about 16 inch spacing. With 2x6 wood treads on a 36 inch stair, many builders add a third in the middle so the treads do not flex. Composite treads want stringers closer, often 12 inches on center.
How do you cut stair stringers?: Set a framing square to the riser height on the tongue and the tread depth on the body, then step it down the board scribing each notch. Cut the notches, then trim the bottom by one tread thickness so the first step matches the rest. Keep the throat behind each notch at 5 inches or more.
How long is a stair stringer?: It is the diagonal: the square root of total rise squared plus total run squared. A 42 inch rise with a 50 inch run gives a stringer about 65 inches long. Buy the next stock length up so you have material to square the end cuts.

PROJ MATERIALFOREMAN

SHT S-002 / 014

REV A · 2026-06-07

DRAWN MF