Stair Stringer Size Calculator: Choosing the Right Board and Span

Why stringer board size matters

A stringer is not just a board cut into a zigzag; it is the structural member that carries the entire load of the staircase, including foot traffic, furniture being moved, and in some cases the weight of the treads and risers themselves. Cutting notches for risers and treads out of a stringer removes a significant portion of the board's cross-section, so choosing a board that is too small, or spacing stringers too far apart, leaves you with a bouncy, unsafe staircase even if every riser and tread dimension is code-compliant.

2x10 versus 2x12: which board to choose

For most residential stringers, builders choose between 2x10 and 2x12 dimensional lumber. A 2x12 gives you roughly 11.25 in of actual width, which after cutting a 7.5 in riser and 10 in tread notch typically leaves at least 5 in of uncut material remaining above the notch line, known as the throat. Most codes require a minimum throat thickness of around 3.5-5 in depending on jurisdiction. A 2x10, with only about 9.25 in of actual width, often does not leave enough throat once notches are cut for a standard 7.5 in riser, which is why 2x12 has become the default choice for interior stringers with typical residential riser heights.

How many stringers do you need?

For a standard stair width up to about 36 in, three stringers spaced roughly 16 in on centre (two on the outside edges, one down the middle) is standard practice in the US. Wider staircases, or those expecting heavier loads such as commercial use, may need four stringers or spacing reduced to 12 in on centre. In the UK, timber stair construction is typically governed by manufacturer span tables and Building Regulations Part A for structure, with similar principles: closer spacing and thicker stock for wider or longer flights.

Calculating span and mid-support needs

Total span is the horizontal distance a stringer covers unsupported, which for most interior stairs equals the total run. Once a stringer's unsupported span exceeds about 10-12 ft, many codes and engineered lumber span tables call for a mid-span support, such as a wall or a beam under the stair, to prevent excessive deflection. If you are building a long, straight flight without an intermediate landing, this is worth checking against your specific lumber grade's span table rather than assuming standard dimensional lumber will span the full distance unsupported.

Cutting the stringer without weakening it

When laying out riser and tread notches on your chosen board, keep the notch depth consistent with your riser height and tread depth calculations, and avoid cutting into the board any deeper than necessary. Some builders reinforce the throat area with a cleat or a metal stringer bracket underneath each tread notch, which spreads the load and reduces the chance of a crack developing at the inside corner of a notch, historically a common failure point on thinner or lower-grade stringers.

Engineered and precut stringers

If you would rather not cut stringers by hand, precut stringers are widely available in standard configurations (typically for 7-7.5 in risers and 10-10.5 in treads) and engineered stair stringers made from laminated veneer lumber (LVL) offer higher strength-to-size ratios than dimensional lumber, useful for wider staircases or longer unsupported spans. These options cost more per stringer but reduce labor time and the risk of a layout error.

Sizing the runner once stringers are in place

Stringer size does not affect how much stair runner you need, but the riser height and tread depth the stringer is cut to absolutely does. For our 14-step example with 7.5 in risers and 10 in treads, the per-step total for a runner is 7.5 + 10 + 1 (nosing wrap) = 18.5 in, times 13 treads (since the top riser meets the upper floor without a full tread notch) = 240.5 in, plus finishing allowance and waste, landing around 24 ft. In metric, 19 cm + 25 cm + 2.5 cm = 46.5 cm per step, times 13 = 604.5 cm, plus finishing and waste, approximately 7.3 m. Running these numbers through a stair runner calculator once the stringers are cut and treads confirmed will give you an accurate order length.

• Choose 2x12 over 2x10 for standard 7-7.5 in risers to preserve throat strength.
• Use three stringers up to 36 in width, four beyond that.
• Check lumber grade, not just dimension, especially for longer spans.
• Add mid-span support for unsupported runs beyond 10-12 ft.
• Confirm final riser and tread numbers before ordering a stair runner.

Throat thickness formula

You can check any board choice with a simple throat formula: throat = board width - (riser height / cos(pitch angle)), where pitch angle is arctan(riser height / tread depth). For our 7.5 in riser, 10 in tread example, pitch is arctan(7.5/10) = 36.9 degrees, and the deepest notch corner sits roughly 7.5/cos(36.9) = 9.4 in down the board's width from the top edge. On an 11.25 in actual-width 2x12, that leaves about 1.85 in measured straight down, but because codes measure throat perpendicular to the board's long edge rather than straight down, the usable perpendicular throat works out closer to 5 in, which is why 2x12 clears most code minimums for this riser height while a 2x10 (9.25 in actual width) typically does not.

UK Part K timber sizing notes

UK timber stringers (called strings) are usually specified from manufacturer span tables referencing BS 5268 or Eurocode 5 rather than a single throat-thickness rule, but the underlying concern is identical: enough uncut timber must remain after riser and tread housings are routed. For a typical domestic softwood string 275 mm deep, housed joints for 190 mm risers and 220 mm goings generally leave adequate section, but wider stairs or longer unsupported strings should be checked against the timber supplier's span table or reviewed by a structural engineer, particularly for open-string designs where one edge carries no wall support at all.

• Pitch angle = arctan(riser height / tread depth).
• Deepest notch depth along the board = riser height / cos(pitch angle).
• Perpendicular throat = board width - deepest notch depth, adjusted for board angle.
• Compare perpendicular throat against your code's 3.5-5 in typical minimum.
• For UK strings, cross-check housed joint depth against a manufacturer span table rather than a single fixed rule.

Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.

Still deciding on material, width or pattern? Our hub on stair runners compares weaves, widths, backings and fitting styles side by side, then hands the numbers straight to the calculator.