Stair Stringers Calculator: Sizing, Spacing and Material

Two calculations, one job

The word stringer covers two distinct calculations that a good calculator handles separately before combining them. The first is the profile calculation: given total rise and total run, what riser height and tread depth does each step need, and therefore what shape does each stringer's sawtooth cut take. The second is the count and spacing calculation: given the width of the staircase and the material the treads are made from, how many stringers do you need, spaced how far apart, so that a person's full weight on the centre of a tread does not produce a springy or unsafe amount of deflection.

Most DIY guides only cover the first calculation because it is the more visually interesting one — the sawtooth cutting diagram — but the second calculation is the one that actually determines whether the finished stair feels solid underfoot or bounces alarmingly on every step. Both US and UK residential codes address stringer spacing indirectly through general structural provisions and manufacturer span tables rather than a single fixed number, which is why a calculator that only outputs riser and tread numbers is giving you half the answer.

How many stringers do you need

For a standard 36 in (91 cm) wide interior staircase, three stringers — one at each side and one down the centre — is the common minimum for solid wood treads over about 1 in (25 mm) thick. Wider staircases, thinner tread material, or heavy use (a main household stair versus an occasional loft stair) push that number up. As a rough working rule, stringers spaced no more than about 16-18 in (400-450 mm) apart keep deflection within a comfortable range for standard 1-1.25 in (25-32 mm) solid tread board; go wider than that and the tread needs to be thicker, engineered, or supported by an additional stringer.

Material and size for the stringer itself

The stringer board needs to be deep enough that, after the sawtooth profile is cut out, there is still a solid section of timber left to carry the load without flexing or splitting along the grain at the narrowest point of each notch. For most residential interior stairs, a nominal 2x12 (actual roughly 1.5 x 11.25 in, or 38 x 285 mm) board in a structural grade of pine, fir or spruce is the standard starting point, cut so that at least 3.5 in (89 mm) of solid material remains at the narrowest point after the notches are cut. Outdoor and deck stringers typically use pressure-treated lumber of the same nominal size, and engineered LVL stringers are increasingly common where a longer clear span is needed without intermediate support.

Spacing for special cases

Open-riser stairs, where there is no vertical riser board between treads, put more stress on individual treads because there is nothing tying the assembly together vertically, so spacing tends to be tighter — many designs use four stringers on a standard-width flight rather than three. Winders and curved sections need their own stringer layout for each tapered tread, and calculators built specifically for winder or curved stairs typically ask for the inner and outer radius separately rather than a single tread depth, since a winder tread's depth varies dramatically across its width.

A landing partway up a flight also changes the stringer calculation, since the stringers on each side of the landing are shorter and each needs its own riser count that, combined, matches the total rise, while the landing framing itself needs to be sized as a small floor structure in its own right, not simply as an extension of the stringer.

A worked spacing example

Take a 42 in (107 cm) wide staircase with 1.25 in (32 mm) solid oak treads. At the upper end of the recommended 12-16 in spacing range, four stringers spaced roughly 14 in (355 mm) apart across the 42 in width comfortably support the tread without excessive deflection. Reduce the tread thickness to 1 in (25 mm) standard pine and the same width would be better served by moving to five stringers or tightening spacing toward 12 in (300 mm), since thinner tread stock flexes more between supports.

1. Confirm total rise and run and calculate riser height and tread depth first, since stringer profile depends on these.
2. Measure the finished width of the staircase between walls, strings or balustrade posts.
3. Decide on tread material and thickness, since this drives how far apart stringers can safely sit.
4. Use the spacing table or a manufacturer span table for engineered tread material to fix the number of stringers.
5. Check the remaining solid timber depth at the narrowest notch point on your chosen stringer board size.
6. Mark and cut one stringer as a template, test-fit it, then use it to mark the rest identically.

Cutting efficiency and material waste

Once you know how many stringers you need and their length, a calculator can also flag how efficiently that length nests onto standard lumber lengths — a stringer needing 14 ft of board length wastes very little off a 14 ft length but wastes nearly 2 ft off a 16 ft length, which matters when you are buying three, four or five of them. For longer flights or where the calculated stringer length falls awkwardly between standard board lengths, it is often cheaper to source LVL or engineered stringer stock cut to length rather than force-fitting dimensional lumber.

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.