Stair Length Calculator: Total Run, Stringer Length and Runner Length

Three meanings of "stair length"

Total run

Total run = number of treads × tread depth. For 13 risers (12 treads) at 10in: 12 × 10 = 120in, or 10ft. This is the number to check against your available floor space before committing to a straight flight — if the stairwell opening is shorter than this, you need a turn, a shorter tread run (within code limits), or fewer/steeper steps.

Stringer (pitch) length

The stringer follows the diagonal hypotenuse of the total rise and total run, not the horizontal run alone. Using the Pythagorean theorem: stringer length = √(total rise² + total run²). For 97.5in total rise (13 × 7.5in) and 120in total run: √(97.5² + 120²) = √(9506.25 + 14400) = √23906.25 = about 154.6in. Add a margin at the top and bottom for the horizontal cuts (typically 6-12in extra stock) when ordering board length.

Runner/carpet length

Runner length follows the tread-plus-riser path up every step: length = steps × (tread depth + riser height), plus nosing wrap if wanted, plus a waste allowance. For 13 steps at 10in tread and 7.5in riser: 13 × (10 + 7.5) = 13 × 17.5 = 227.5in. Add a typical 10% waste margin: 227.5 × 1.10 = 250.25in, or about 6.36m / 20ft 10in.

Comparing the three for the same flight

Metric version

For a 2700mm rise, 170mm riser, 280mm tread flight (16 risers, 15 treads): total run = 15 × 280 = 4200mm. Stringer length = √(2700² + 4200²) = √(7,290,000 + 17,640,000) = √24,930,000 ≈ 4993mm. Runner length = 16 × (280+170) × 1.10 = 16 × 450 × 1.10 = 7920mm ≈ 7.92m.

Which calculator to use for which question

• Checking if the stair fits the available floor space: total run, via the stair calculator.
• Ordering stringer board stock: pitch length, via the stair stringer calculator.
• Ordering carpet or a runner: runner length, via the stair runner calculator.
• All three: they share the same underlying rise/run numbers, so calculate riser and tread once and feed the same figures into each.

Whichever length you're solving for, always add a margin: 6-12in of extra stringer stock for the horizontal end cuts, and 10-15% waste on runner or carpet length to allow for pattern matching, trimming and mistakes on site.

How much runner do I need?

This is the calculation I get asked about most, because runner length is the one figure a joiner's tape measure can't give you directly - you need the step count and the tread/riser dimensions first, then the arithmetic above. On a 13-step US flight with a 7.5in riser and 10in tread, running the numbers through a stair runner calculator gives 227.5in of core coverage before waste, which is exactly the runner/carpet length worked example above. Add the usual 10% and you're ordering just over 250in, or about 21ft, of stair runner carpet.

In metric, a 17-step UK flight with a 190mm riser and 250mm going (so 17 risers, 16 treads) needs 17 × (190+250) = 17 × 440 = 7480mm of core length. Add a 10% waste allowance and you're ordering 8228mm, call it 8.3m, of runner. If you want a deeper waterfall wrap over a bullnose tread, add roughly another 20-30mm per step on top of that figure, since the fabric has to travel around the extra curve rather than sitting flat.

Ordering length for a stair with a landing or half-turn

A flight that turns at a half-landing is really two shorter flights plus a flat section, and each part needs its own length calculation before you add them together. Say the lower flight has 7 steps at 7.5in/10in (7 x 17.5 = 122.5in), the landing itself is 40in of flat runner if you're carpeting across it, and the upper flight has 6 steps at the same dimensions (6 x 17.5 = 105in). Total core length = 122.5 + 40 + 105 = 267.5in, then add waste on top: 267.5 x 1.10 = 294.25in, about 24.5ft.

1. Measure and calculate each flight either side of the landing separately - never assume the two flights are identical even if they look it from the bottom.
2. Add the flat landing length once, as its own segment, rather than folding it into either flight's step count.
3. Sum the segments first, then apply the waste allowance to the total, not to each segment individually, or you'll over-order.
4. If the runner has a directional pattern, add extra at the landing to allow the pattern to be re-matched after the turn.

Why the same flight gives different lengths on different calculators

If you've run the same staircase through two different stair length calculators and got two different runner figures, the usual culprit is waste percentage - some tools default to 5%, others to 15%, and that alone accounts for a 10-percentage-point swing on the final number. The second most common cause is whether nosing wrap is included by default; a calculator that adds 1in of wrap per step on top of the flat tread-plus-riser sum will always return a longer figure than one that doesn't. Check both settings before comparing two tools' outputs directly, and remember that stair runners bought from different suppliers can also differ slightly in usable width once bound edges are accounted for.

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.