Stair Stringer Calculator
Code & layout checks
Estimates only. Verify against your locally adopted building code and have stairs inspected as required.
A typical 36-inch-wide staircase needs 3 stringers — one at each side and one in the middle — spaced about 18 inches on center, and a cut stringer should be a 2x12 so it keeps at least 5 inches of uncut throat after notching. Enter your total rise, stair width, and board size below for the exact stringer count, remaining throat depth, board length to buy, and the rise/run marks to set on your framing square.
How many stringers by stair width
| Stair width | Stringers (16" o.c.) | Resulting o.c. spacing | Typical board |
|---|---|---|---|
| 36" (code minimum) | 3 | 18" o.c. | 2x12 cut |
| 42" | 4 | 14" o.c. | 2x12 cut |
| 48" | 4 | 16" o.c. | 2x12 cut |
| 60" | 5 | 15" o.c. | 2x12 cut |
| 72" | 6 | 14.4" o.c. | 2x12 cut |
How to lay out a stair stringer
A stringer is the sawtooth-cut board that carries the treads and risers. Its layout comes straight from the stair geometry: the rise(riser height) and run (tread depth) of one step are the two numbers you set on a framing square, and you step that triangle down the board once per riser. The board itself runs the diagonal, so its length is the hypotenuse: √(total run² + total rise²). Because lumber sells in even-foot lengths, buy the next stocked length above that diagonal plus a few inches for the plumb and level end cuts.
Why a cut stringer is a 2x12
Notching a stringer removes a triangular bite out of each step, and what's left below the notch — the throat — is the material that actually carries the load. The perpendicular depth of the notch is (rise × run) / √(rise² + run²), so a common 7.2-inch rise with a 10-inch run cuts about 5.8 inches out of the board. A 2x12 measures 11.25 inches deep, leaving roughly 5.4 inches of throat — just over the 5-inch minimum. A 2x10 (9.25 inches) would be left with about 3.4 inches, which is why a notched stringer is a 2x12 in practice. The same rise-over-run triangle drives the roof pitch calculator — there it's the slope factor and rafter length instead of the stringer. The calculator flags the throat live so you can see it fail before you cut.
How many stringers you need
Put a stringer under each edge of the stair, then add interior stringers so no gap is wider than your spacing limit (about 16 to 18 inches on center for standard 2x tread material — closer for thinner or composite treads). A 36-inch-wide stair takes 3 stringers, a 48-inch stair takes 4. The calculator divides your width by the spacing, rounds up the number of gaps, and adds one for the count, then reports the actual on-center spacing that results.
Common mistakes
- Notching a 2x10. It rarely keeps the 5-inch throat once it's cut. Use a 2x12 for cut stringers, or switch to a solid (uncut) stringer with cleats or mounting brackets.
- Cutting the bottom riser full height. The bottom step is cut shorter by one tread thickness so every finished step ends up equal — the stair calculator gives that first-step cut.
- Forgetting the end allowance. The diagonal is the working length; buy a board longer than that so you have material for the plumb cut at the top and the level cut at the bottom.
Choosing 2x12 vs 2x10 — and how far a stringer can span
Two different limits decide the stringer board, and it's worth keeping them separate. The first is the throat, which sets the board depth: a notched 2x12 keeps the 5-inch minimum throat while a notched 2x10 usually doesn't on a typical 7-inch riser, which is why cut stringers are 2x12s. That's the check the calculator runs live.
The second is the unsupported span — how far the stringer runs between its top and bottom bearing points. This is about the stair's horizontal run, not the board depth: a deeper board doesn't buy you a longer span here. Prescriptive deck guidance (AWC's DCA6) recommends a cut stringer span no more than about 6 feet horizontally between supports; run longer than that and you add a mid-span support beam or intermediate footing rather than reaching for a bigger board. A tall interior staircase usually breaks into flights with a landing before it ever hits that limit, so the span rule bites most often on long exterior deck stairs.
Deck stairs and stairs with a landing
Deck stairs
Deck stringers use the same layout math as interior stairs, with a few outdoor differences. Use pressure-treated 2x12 stock, keep spacing tighter — NADRA recommends 12 inches on center and code caps it near 16 to 18 — because exterior treads and 5/4 decking flex more than a solid interior tread. The bottom of the stringers should bear on a concrete pad or footing (about a 12-inch bearing is typical) rather than sitting on soil, so the run doesn't settle and throw the riser heights out.
Stairs with a landing or turn
A landing splits one tall staircase into two shorter flights, each with its own stringers. Calculate each flight on its own: measure the total rise from the floor to the landing for the lower flight, and from the landing to the upper floor for the top flight, then run each rise through the calculator separately. Treat the landing as its own level surface — its height is a riser in whichever flight lands on it, not an extra step to squeeze into a single stringer. Splitting a tall rise this way is also how you stay under the unsupported-span limit without a mid-span support.
Frequently asked questions
How many stringers do I need for my stairs?
A standard 36-inch-wide stair typically uses three stringers: one at each outer edge and one in the middle. As the stair gets wider you add interior stringers so the gap between them stays roughly 16 to 18 inches, which keeps the treads from flexing underfoot. A 48-inch stair usually takes four. Thinner tread material and composite decking need tighter spacing than solid 2x lumber, so check the tread manufacturer's span rating. Enter your stair width above for the exact count and resulting on-center spacing.
What size lumber is used for stair stringers?
Cut (notched) stringers are almost always made from 2x12 lumber. Once you notch a board for the treads and risers, code requires at least 5 inches of uncut material left below the notch, and a 2x12 (which actually measures 11.25 inches deep) is the smallest common board that reliably keeps that 5-inch throat. A 2x10 usually fails the throat check on a typical 7-inch riser. The calculator computes your remaining throat for the board you pick and flags it if it drops below 5 inches.
What is the throat of a stair stringer?
The throat is the solid, uncut depth of material left below the notches once the stringer is cut for the steps. Notching removes a triangular bite from the board, and the throat is what remains to carry the load. Its depth equals the board's actual depth minus the perpendicular depth of the notch, which is (riser height × tread depth) ÷ the step's diagonal. Keeping the throat at 5 inches or more is what prevents a cut stringer from splitting under load.
How long a board do I need for a stair stringer?
Buy a board at least as long as the stringer's diagonal length plus a few inches for the top and bottom end cuts. The stringer runs the hypotenuse of the stair, so it is longer than either the total rise or the total run — a typical 9-foot-rise stair needs a stringer around 14 to 15 feet, so you buy 16-foot boards. The calculator gives the exact diagonal, rounds it up to the next stocked lumber length, and multiplies by the number of stringers for your total lineal footage.
How do you lay out a stair stringer with a framing square?
Set the framing square so the riser height falls on the tongue (short arm) and the tread depth on the blade (long arm), then trace the corner to mark each step. You march that same setting up the board once per step, so a 15-riser stair is stepped off 14 times for the treads. Using stair gauges (small clamps on the square) to lock the rise and run numbers keeps every step identical. The calculator gives you the exact rise and run marks to set on the square.
Do you subtract the tread thickness from the bottom stringer?
Yes. The bottom riser is cut shorter by one tread thickness so that every finished step ends up the same height once the treads are installed. If you skip this drop, the first step sits a full tread-thickness too tall and the top step ends up too short, which fails the code rule that risers stay within 3/8 inch of each other. This adjustment applies to the bottom cut of every stringer in the set.
How far can a stair stringer span without support?
Prescriptive deck guidance (AWC's DCA6) recommends a cut stringer span no more than about 6 feet horizontally between its top and bottom supports. This limit is about the stair's horizontal run, not the board depth — a 2x12 doesn't span farther than a 2x10 here, since the throat that carries the load is the same once both are notched. If the run is longer, add a mid-span support beam or intermediate footing rather than a bigger board, or split the stair into flights with a landing. Always confirm against your locally adopted code.
How do I calculate stringers for stairs with a landing?
Treat each flight as its own staircase. Measure the total rise from the floor up to the landing for the lower flight, and from the landing up to the next floor for the upper flight, then run each rise through the calculator separately for its own set of stringers. The landing is a level surface, not an extra step crammed into one stringer, so its height counts as a riser in whichever flight lands on it. Splitting a tall rise this way also keeps each flight under the unsupported-span limit.
Are deck stair stringers different from interior stringers?
The layout math is identical, but deck stringers use pressure-treated 2x12 stock, tighter spacing, and a concrete bearing at the bottom. NADRA recommends spacing deck stringers 12 inches on center (code allows up to about 16 to 18) because exterior treads and 5/4 decking flex more than solid interior treads. The stringers should land on a concrete pad or footing with roughly a 12-inch bearing rather than on soil, so the base doesn't settle and pull the riser heights out of tolerance.
Related calculators
Sources
- 2024 International Residential Code — Section R311.7 (Stairways)International Code Council (ICC)Source for the residential stair limits the stringer layout is checked against: max 7.75" riser height and min 10" tread depth, plus the requirement that risers within a flight stay uniform.
- Stair Calculator — stringer formulas and cut detailsmycarpentry.comSource for the notched cut stringer needing at least 5" of remaining uncut throat, the 2x12 as the standard cut-stringer board, and the stringer length as the diagonal (hypotenuse) of total run and total rise.
- How to Lay Out Deck Stair StringersDecks.com (Trex)Source for deck stringer specifics: pressure-treated 2x12 stock, ~12" on-center spacing for deck treads, and roughly a 12" concrete bearing at the stringer base. The ~6 ft max unsupported cut-stringer span reflects AWC DCA6 prescriptive deck guidance.