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CalcBelt

Stair calculator

Enter the total rise to get the number of risers, the riser height, the total run and the stringer length, checked against the IRC's limits for home stairs.

Units
Height

Finished floor to finished floor, or deck surface to the ground. 9' 2" works. Where to measure

IRC maximum: 7 3/4 in. Lower it for gentler steps.

Treads and width

Nosing to nosing. IRC minimum: 10 in.

IRC minimum: 36 in. Trex composite treads need closer stringers: 9 or 12 in apart.

Lowest point above the stairs, measured straight up from the line of the nosings. IRC minimum: 6' 8".

Limits shown are the International Residential Code's (R311.7). Your state or town may change them, so check with your building department before you cut stringers.

Stairs

14 risers

7 11/16 in each (7.71 in)

Riser height
7 11/16 inIRC maximum 7 3/4 in
Treads
13 × 10 inIRC minimum 10 in
Total run
10 ft 10 inLevel distance, bottom riser to top riser
Stringer length
14 ft 1 inDiagonal of the total rise and run; the board is longer
Stringers
3At most 18 in apart, at least 3 (AWC DCA 6)
Stair angle
39.7°
  • 14 risers: a handrail 34 to 38 in high is required on at least one side (4 or more risers, R311.7.8).
  • Treads under 11 in with solid risers need a nosing of 3/4 to 1 1/4 in (R311.7.5.3).
  • Risers in one flight may differ by at most 3/8 in, and so may treads, so cut every step to the same size.
How it's calculated
  1. Risers: 108 in ÷ 7 3/4 in = 13.94, rounded up to 14
  2. Each riser: 108 in ÷ 14 = 7.714 in
  3. Treads: 14 − 1 = 13 (the top step is the upper floor)
  4. Total run: 13 × 10 in = 130 in
  5. Stringers: 36 in ÷ 18 in = 2, rounded up, + 1 = 3 (at least 3)
  6. Stringer: √(108² + 130²) = 169 in

Estimates use manufacturer data sheets and industry guides (sources). Check local codes and your supplier's figures before you order.

How to use the stair calculator

  1. Measure the total rise: from the finished floor or ground at the bottom to the finished floor or deck surface at the top.
  2. Keep the tallest riser at 7 3/4 in, the IRC limit, or lower it for an easier climb.
  3. Enter the tread depth, nosing to nosing, and the stair width. Add the headroom if a ceiling or beam is above the stairs.
  4. Read the risers, treads, total run and stringer length, and fix anything flagged in the warnings.

How to calculate stairs

Start from the total rise, the height the stairs have to climb, and the tallest riser you’ll accept:

Risers = total rise ÷ tallest riser, rounded up
Riser height = total rise ÷ risers
Treads = risers − 1
Total run = treads × tread depth
Stringer = √(total rise² + total run²)
Stringers = width ÷ 18 in, rounded up, + 1 (at least 3)

Example: a floor 108 in above the one below, with 10 in treads. 108 ÷ 7.75 = 13.94, so 14 risers, each 108 ÷ 14 = 7.71 in (just under 7 3/4 in). That gives 13 treads and a total run of 13 × 10 = 130 in, or 10 ft 10 in. The stringer line is √(108² + 130²) = 169 in, about 14 ft 1 in, at an angle of 39.7°. A 36 in wide stair needs 36 ÷ 18 = 2, plus 1 = 3 stringers.

Stair calculator diagram: a side view of a staircase showing the total rise from finished floor to finished floor, the total run from the bottom riser to the top riser, one riser height, one tread depth from nosing to nosing, and the stringer length as the diagonal of the total rise and total run.
Where to measure: the total rise from finished floor to finished floor. The calculator works out the risers, the treads, the total run and the stringer line from it.

Measure the total rise between finished surfaces. If flooring is still to go on at the top or the bottom, allow for its thickness, or the first or last step will come out a different height from the rest.

Steps for common heights

With the IRC’s 7 3/4 in riser limit and 10 in treads:

Total rise Risers Riser height Total run Stringer line
24 in 4 6 in 30 in 38.4 in
36 in 5 7 3/16 in 40 in 53.8 in
48 in 7 6 7/8 in 60 in 76.8 in
60 in 8 7 1/2 in 70 in 92.2 in
72 in 10 7 3/16 in 90 in 115.3 in
84 in 11 7 5/8 in 100 in 130.6 in
96 in 13 7 3/8 in 120 in 153.7 in
108 in 14 7 11/16 in 130 in 169.0 in
120 in 16 7 1/2 in 150 in 192.1 in

Riser heights are rounded to the nearest 1/16 in. Deeper treads lengthen the run and the stringer.

IRC limits for home stairs

Rule Limit Section
Riser height 7 3/4 in maximum R311.7.5.1
Tread depth 10 in minimum R311.7.5.2
Difference within a flight 3/8 in for risers and for treads R311.7.5.1, R311.7.5.2
Nosing 3/4 to 1 1/4 in with solid risers, unless treads are 11 in or deeper R311.7.5.3
Headroom 6 ft 8 in R311.7.2
Width 36 in above the handrail R311.7.1
Rise of one flight 151 in (12 ft 7 in) between floors or landings R311.7.3
Handrail One side, 34 to 38 in high, from 4 risers R311.7.8

For deck stairs, the American Wood Council’s DCA 6 guide requires at least three stringers when only cut stringers are used, spaced no more than 18 in on center on stairs wider than 36 in, and an intermediate landing once a stairway rises more than 12 ft. Trex composite treads need closer support: its 2025 guide gives a maximum stringer spacing of 9 or 12 in depending on the board, 4 stringers for a 36 in wide stair and 5 for a wider one.

These are the model code’s figures, taken from Knox County, Tennessee’s 2018 IRC handout and Gilroy, California’s handout. States and towns adopt the code with their own changes, so check with your building department.

Questions and answers

How do I calculate the number of stairs?

Divide the total rise by the tallest riser you allow and round up. That's the number of risers. For a 108 in rise: 108 ÷ 7.75 = 13.9, so 14 risers of 108 ÷ 14 = 7.71 in. There is one fewer tread than risers, 13 here, because the top step is the upper floor.

What is the maximum riser height and minimum tread depth?

Under the International Residential Code, risers can be at most 7 3/4 in tall (R311.7.5.1) and treads at least 10 in deep (R311.7.5.2). Within a flight, the tallest riser can't exceed the shortest by more than 3/8 in, and the same goes for treads.

How long is a stair stringer?

The calculator gives the diagonal from the foot of the bottom riser to the top of the top riser: the square root of the total rise squared plus the total run squared. With a 108 in rise and 13 treads of 10 in (130 in run), it's √(108² + 130²) = 169 in, about 14 ft 1 in. The board you cut the stringer from has to be longer to fit the cuts at each end.

How many stringers do I need?

The American Wood Council's deck guide (DCA 6) requires at least three stringers when only cut stringers are used, and spacing of no more than 18 in on center for stairs wider than 36 in. The calculator combines the two: a 36 in wide stair needs 3 and a 48 in stair 4. Trex composite treads need them closer: 9 or 12 in apart depending on the board, 4 stringers on a 36 in stair and 5 on a wider one.

How much headroom do stairs need?

At least 6 ft 8 in, measured straight up from the sloped line that joins the tread nosings (IRC R311.7.2).

When do stairs need a handrail?

When a flight has four or more risers. The IRC (R311.7.8) requires a handrail on at least one side, 34 to 38 in above the nosings.

How wide do stairs have to be?

At least 36 in of clear width above the handrail (IRC R311.7.1), and a flight can rise at most 151 in, 12 ft 7 in, before it needs a landing (R311.7.3).

Do these rules apply to deck stairs?

Yes, and the American Wood Council's deck guide (DCA 6) adds one stricter rule: a deck stairway that rises more than 12 ft (144 in) needs an intermediate landing, where the IRC allows 151 in. DCA 6 applies the IRC's stair section (R311.7), including the 36 in width and the handrail from four risers. Composite treads add the maker's own rules: Trex's stair detail, for example, shows treads at least 11 in deep.

Sources

Figures checked October 2026.