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Stair calculator: risers, treads, stringer length and code check

Enter the total rise and the run available to get the number of risers and treads, riser height, tread depth, stair angle and stringer length, checked against common residential code limits.

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Results

Number of risers

14 risers

Riser height

7.71 in

Tread depth (run per step)

10.00 in

Comfort check (2 × riser + tread)

25.43 in

Code compliant and comfortable Twice the riser plus the tread is close to the natural stride of 24–25 in.
Number of treads
13 treads
Stair angle
37.6 °
Stringer length (approx.)
13.68 ft
Stringers needed
4 stringers

Layout by target riser height

Number of risersRiser heightTread depth (run per step)Comfort check (2 × riser + tread)
6.50 in target current176.358.1320.83
7 in target current157.209.2923.69
7.25 in target current157.209.2923.69
7.50 in target current147.7110.0025.43
7.75 in target current147.7110.0025.43

Lay out a safe, comfortable staircase

Badly proportioned stairs are obvious at the first step: tall risers that leave you winded or shallow treads that force your foot sideways. Before you cut stringers or order a stair kit, this calculator works out the number of risers and treads, riser height, tread depth, stair angle and stringer length, and checks the result against the IRC limits and the 2R + T comfort rule.

How the layout is calculated

  1. Risers n = total rise ÷ target riser height, rounded to the nearest whole number.
  2. Actual riser height = total rise ÷ n.
  3. Treads = n − 1, because the top riser lands on the upper floor.
  4. Tread depth = total run ÷ (n − 1).
  5. Comfort = 2 × riser + tread, ideally 24 to 25 in.
  6. Angle = arctan(riser ÷ tread).

Worked example: 108 in of rise and 130 in of run

Target riser: 7.5 in.

  1. 108 ÷ 7.5 = 14.4, so 14 risers.
  2. Riser height: 108 ÷ 14 ≈ 7.71 in (under the 7¾ in limit).
  3. Treads: 13.
  4. Tread depth: 130 ÷ 13 = 10 in (the IRC minimum).
  5. Comfort: 2 × 7.71 + 10 ≈ 25.4 in.
  6. Angle: about 37.6°, on the steep side of comfortable.
  7. Stringer: about 13.7 ft, with 4 stringers for a 36 in wide stair.

Effect of the target riser height

Target Risers Riser height Tread depth 2R + T
6.5 in 17 6.35 in 8.12 in 20.83 in
7 in 15 7.20 in 9.29 in 23.69 in
7.5 in 14 7.71 in 10.00 in 25.43 in

With a fixed run, more risers mean shallower treads: 15 risers would leave 9.29 in treads, below code. To get both lower risers and deeper treads, you need more run.

Key IRC requirements

  • Riser height: 7¾ in maximum.
  • Tread depth: 10 in minimum, nosing to nosing.
  • Nosing: ¾ to 1¼ in when treads are less than 11 in deep.
  • Uniformity: risers and treads in a flight may not vary by more than ⅜ in.
  • Headroom: 6 ft 8 in minimum, measured from the nosing line.
  • Width: 36 in minimum, with a handrail 34 to 38 in high on at least one side when there are four or more risers.

Local codes may amend these rules, so check with your building department before you build.

When space is tight

An L-shaped stair with a landing or a U-shaped stair folds the run in half. Winder treads save even more space but must keep a 10 in tread depth at the walkline. Avoid fixing a short run with taller risers: above 7¾ in the stair is not code compliant, and steep stairs are the main source of falls at home.

Frequently asked questions

How do I calculate the number of stairs?

Divide the total rise by your target riser height and round to the nearest whole number: 108 in ÷ 7.5 in = 14.4, so 14 risers of 7.71 in. A straight stair has one fewer tread than risers, because the top riser lands on the upper floor.

What are the code limits for residential stairs?

The International Residential Code limits riser height to 7¾ in and requires a minimum tread depth of 10 in measured nosing to nosing, a clear width of at least 36 in and headroom of at least 6 ft 8 in. Within a flight, risers and treads may not vary by more than ⅜ in. Local codes can be stricter.

What is the 2R + T rule?

It is a comfort rule based on the average stride: twice the riser height plus the tread depth should be about 24 to 25 in. A 7.71 in riser with a 10 in tread gives 25.4 in, inside the range; a 7 in riser with an 11 in tread gives an ideal 25 in, while 7.75 in risers with 12 in treads (27.5 in) feel like an overly long stride.

How long should my stair stringers be?

Use the Pythagorean theorem on the rise and run along the nosings, then add a little for the cuts at the top and bottom. A stair with 108 in of rise and 130 in of run needs stringers of roughly 13.7 ft, so you would buy 14 ft or 16 ft boards depending on the cuts.

How much space does a straight staircase need?

With 14 risers and 10 in treads, the run is about 130 in, nearly 11 ft, plus a landing of at least 36 in at the bottom. If you are short on space, an L-shaped stair with a landing or winder treads can save several feet.

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Results are estimates provided for information purposes. They do not replace professional advice.

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