Turning a finished stair design into a board order
This tool assumes you already have a stringer design — total rise, total run, and a stair width. If you're still deciding those numbers, plan them first, then come back here to order lumber.
Enter total rise and total run
These two numbers set the straight-line stringer length via the Pythagorean triangle. If you already used a rise-and-run calculator, carry those same figures over here.
Enter your stair width
Clear width between finished walls or the outer edges of the treads. This decides how many stringers you need at your chosen on-center spacing.
Check the advanced spacing and notch fields
16" on-center spacing and a 14" cut allowance cover most residential jobs. Adjust the riser and tread in the notch-depth panel to confirm the board still holds enough wood after the step cuts.
Read the results and compliance chips
The board length is already rounded up to a stocked length (8'–20'). The chips flag whether the remaining notch depth needs a closer look and whether your stringer fits a single standard board.
The five formulas behind every result
Nothing here is a black box. This is exactly what runs when you change a number.
Stringer Length
The straight-line hypotenuse between the top and bottom of the stair — the minimum length any single board has to span before notching.
Board Length Needed
Add the birdsmouth cut and notch waste to the stringer length, then round up to the next length actually stocked at a yard: 8', 10', 12', 14', 16', 18' or 20'.
Stringers Required
One stringer at each edge plus however many interior ones keep every span within your on-center spacing target.
Remaining Depth at the Notch
The perpendicular distance from the inside corner of a step notch to the board's sloped bottom edge — the thinnest, weakest point on a cut stringer.
Board Feet per Board
For nominal 2x12 stock, thickness(2") × width(12") ÷ 12 simplifies to 2 board-feet per linear foot of board.
Walking through a real 108" rise, 130" run stair
Same five formulas, filled in step by step, for a basement stair climbing 108" of rise over 130" of run at a 36" width — the default this calculator loads with.
Stringer length
√(108² + 130²) = √(11,664 + 16,900) = √28,564 ≈ 169.0". That's the straight-line board length before any notching or waste is added.
Board length needed
169.0" + a 14" cut allowance = 183.0" raw need. The next stocked length at or above that is 192" (16'), so the calculator rounds up to a 16' board per stringer.
Stringers and boards to buy
ceil(36 ÷ 16) + 1 = ceil(2.25) + 1 = 4 stringers. A 36" stair needs 3 interior spans no wider than 16" each, which takes 4 boards, not the 3 people often assume from habit. Add 1 spare and the order is 5 boards.
Remaining notch depth
With a 7.5" riser and 10.5" tread: 11.25 − (7.5 × 10.5) ÷ √(7.5² + 10.5²) = 11.25 − 78.75 ÷ 12.90 ≈ 11.25 − 6.10 = 5.15" of wood remaining below each notch — comfortably above the commonly cited minimum, so the chip reads as a pass once you confirm the figure locally.
Notice the stringer count jumped straight from 3 to 4 rather than landing on the "one stringer every 18 inches" gut estimate many people start with — that's the spacing rule rounding up, not down, once a span would exceed the on-center target.
Standard 2x12 lengths and what they support
Every yard stocks roughly the same set of lengths. This shows how much usable stringer length each one leaves after a typical 14" cut allowance, and roughly how much total rise that supports at a common 7.5"/10.5" pitch.
| Stocked Length | Length (in) | Usable After 14" Allowance | Approx. Max Rise Supported |
|---|---|---|---|
| 8' | 96" | 82" | ≈48" |
| 10' | 120" | 106" | ≈62" |
| 12' | 144" | 130" | ≈76" |
| 14' | 168" | 154" | ≈90" |
| 16' | 192" | 178" | ≈103" |
| 18' | 216" | 202" | ≈117" |
| 20' | 240" | 226" | ≈131" |
"Approx. Max Rise" assumes a typical 7.5" riser / 10.5" tread pitch. Your actual stringer length depends on total rise and total run together — use the calculator above for your exact numbers, not this table.
2x12 stringer quick reference
This calculator checks your result against the figures below. Confirm the flagged ones against your local adopted code before cutting.
| Item | Typical Figure | Notes |
|---|---|---|
| Nominal size / actual size | 2x12 / 1.5"×11.25" | Standard cut-stringer stock; narrower sizes leave less margin at the notch. |
| Min. remaining notch depth | [VERIFY: 3.5"–5", jurisdiction-dependent] | Most commonly cited residential minimum; some jurisdictions require more. |
| Max stringer spacing, o.c. | [VERIFY: commonly 16"–18"] | Depends on tread material and thickness; wood treads under load typically want the tighter end. |
| Riser variance across a flight | 3/8" | Matches the IRC residential figure used across this site. |
| Max residential riser height | 7.75" | Same IRC figure referenced by the Stair Rise and Run Calculator. |
| Min residential tread depth | 10" | Same IRC figure referenced by the Stair Rise and Run Calculator. |
| Lumber treatment | [VERIFY: PT required for exterior/ground contact] | Exterior and deck stringers typically require pressure-treated lumber rated for the exposure. |
Reference only — always confirm against your local jurisdiction's adopted code before building.
Remaining depth, splicing & treatment
A board that's the right length can still fail if the notches leave too little wood, if the run is too long for a single stocked board, or if it's the wrong lumber for the exposure.
Why remaining depth matters
Every step notch removes a triangular bite from the top of the board. The inside corner of that bite is the closest point to the sloped bottom edge — that distance is the thinnest cross-section the stringer has, and it's what actually carries the load. It's constant along a straight-run stringer with uniform risers and treads.
When you need to splice or upsize
A single 2x12 tops out around 20' in most yards. If your raw board need exceeds that — a tall, shallow-angle run — you're looking at an engineered LVL stringer, a mid-span landing to split the flight, or a scarfed/spliced stringer designed by an engineer, not a straight lumber-yard order.
Treated lumber & ground contact
Exterior and deck stringers are typically required to be pressure-treated, and stringers that touch or sit near soil usually need a ground-contact rating rather than the lighter above-ground rating. Interior basement stringers on a dry, framed opening are usually fine in untreated lumber, but confirm locally.
Why order a spare board
A mis-marked birdsmouth cut, a knot that lands right at the notch corner, or a miscut angle ruins a board outright — there's no fixing a stringer that's been cut short. One spare board is cheap insurance against a wasted trip back to the yard mid-build.
Common mistakes when ordering stringer boards
Most wasted lumber and mid-project trips to the yard trace back to one of these five planning errors.
Using nominal 12" instead of actual 11.25"
A 2x12 is 11.25" deep, not 12". Running the remaining-depth check against the nominal size overstates how much wood is actually left after notching.
Forgetting the cut allowance when ordering
The stringer length from a rise-and-run calculator is a straight-line hypotenuse, not a board length. Order to that number and the birdsmouth cut at the top has nowhere to come from.
Assuming three stringers covers any 36" width
Three stringers on a 36" stair leaves 18" spans — wider than a typical 16" on-center target. Check the spacing math instead of defaulting to three out of habit.
Skipping the remaining-depth check on a steep stair
A tall riser paired with a shallow tread narrows the remaining wood at the notch fast. Two designs with the same stringer length can have very different remaining depth.
Buying untreated lumber for an exterior stair
Interior-rated 2x12 doesn't hold up to weather or ground contact. Confirm the treatment rating your application needs before the boards are cut and installed.
When to use a 2x12 stringer board calculator
This tool picks up right where stair-layout planning ends — turning a finished design into an actual lumber order.
Basement Stair Rebuild
Ordering replacement stringers for a known rise and run before demo day.
Deck Stair Build
Sizing treated 2x12 stock for an outdoor stair off a raised deck.
Exterior Entry Steps
Front or back-door steps where ground contact and treatment matter.
New Construction Framing
Generating a lumber take-off for stringers straight from the plan set.
DIY Stringer Replacement
Swapping a single failed or squeaky stringer without re-planning the whole stair.
Lumber Yard Order Sheet
Getting an exact board count and length before calling in or visiting the yard.