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Contractors & trades / Stair & Stringer Calculator

Stair & Stringer Calculator

Work out stairs from the total rise: how many steps, the riser height, tread count, total run, stringer length and angle, with a check against the IRC riser and tread limits and a materials estimate.

Enter a number from 0 to 600

Finished floor (or ground) to finished floor or deck surface at the top. 9 ft = 108 in.

Enter a number from 4 to 12

7¾" is the most the model residential code (IRC) allows; check your local code.

Enter a number from 0 to 24

Run of each step, nosing to nosing. The IRC minimum is 10".

Usual for stairs to a floor or deck: one fewer tread than risers.

Whole number, 0–20

Wider stairs need more; follow your plans or local code.

$

Enter a number of at least 0

Usually a 2×12 long enough for the stringer length below.

$

Enter a number of at least 0

$

Enter a number of at least 0

0 for open risers.

Riser height
7.71 in
14risers (steps up)
13treads
11' 4 1/2"total run
Risers · 108" ÷ 7.75" max, rounded up14
Riser height · mark this on the stringer7.714" (7 11/16")
Treads · one fewer than risers; the top step is the floor13
Tread depth10.5" (10 1/2")
Total run · 13 × 10.5"136.5" (11' 4 1/2")
Stringer length · along the slope, approx.174.1" (14' 6 1/16")
Stringer board to buy · next 2-ft lumber length16 ft
Stair angle36.3°
Code check (model IRC) · add a ¾"–1¼" nosingMeets the IRC riser and tread limits
Materials · 3 stringers, 13 treads$460.00

Measure the rise at the stair's landing spot, finished surface to finished surface. Keep all risers within 3/8" of each other, and check headroom (6' 8" in the IRC) and your local code before cutting.

See breakdown

How the stair & stringer calculation works

  1. Measure the total rise

    From the finished floor or ground at the bottom to the finished floor or deck at the top. A 9-ft rise is 108 inches.

  2. Divide by the maximum riser and round up

    108 ÷ 7.75 is 13.9, so you need 14 risers. 108 ÷ 14 gives an even riser height of 7.71", about 7 11/16".

  3. Count treads and the total run

    With the top step at the deck, there are 13 treads. At 10½" each, the stairs run 136½", or 11' 4½".

  4. Size the stringers

    The stringer runs along the slope: √(136.5² + 108²) is about 174", 14' 6". Buy 16-ft boards. The stairs climb at about 36°.

Worked example. For a 108" total rise with a 7¾" maximum riser and 10½" treads, build 14 risers of 7.71" (7 11/16") and 13 treads for a total run of 11' 4½". Each stringer is about 14' 6" long, so buy 16-ft boards; the stairs climb at about 36°. Three stringers and 13 treads at $45 and $25 cost $460.

Stair & Stringer FAQ

How do I calculate stairs?

Divide the total rise by the maximum riser height and round up to get the number of risers. Divide the rise by that number for the exact riser height. Treads are usually one fewer than risers, and the total run is treads × tread depth.

What is the maximum riser height and minimum tread depth?

The model International Residential Code (IRC) allows risers up to 7¾" and treads at least 10" deep, with no more than 3/8" difference between the largest and smallest within a flight. Local codes can differ, so check yours.

How many stairs do I need for a 9-foot rise?

At a 7¾" maximum riser, 14 risers of about 7 11/16", with 13 treads if the top step is the upper floor.

How long should stair stringers be?

About the length of the slope: the square root of total run² + total rise². For a 108" rise and 136½" run that's about 174", so buy 16-ft boards.

Do stairs need a nosing?

Under the IRC, treads under 11" deep need a nosing that projects ¾" to 1¼". Treads 11" or deeper don't need one.

How much headroom do stairs need?

The IRC requires at least 6' 8", measured straight up from the slope line along the tread nosings.

Sources: ICC: 2024 International Residential Code, Chapter 3 (R318.7 Stairways) · ICC: 2021 International Residential Code, Chapter 3 (R311.7 Stairways)

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