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Deck Stair Calculator

Lay out stairs from a deck down to the ground: enter the total rise to get the number of steps, the exact riser height, the total run, how long each stringer is and how many stringers you need. Riser, tread and width are checked against the IRC limits for homes.

Units

From the top of the deck boards straight down to the ground or landing where the stairs end.

7¾ in is the IRC maximum for homes. Use less for gentler stairs.

At least 10 in (IRC). Two 5.5 in boards with a gap give about 11¼ in.

36 in minimum (IRC).

Stringers and treads

16 in is common; DCA 6 allows up to 18 in with 2× treads. Composite treads: often 12 in.

For buying tread boards: two 5.5 in boards per step is typical.

Cost estimate (optional)

Prices vary by region and season. Enter your local prices; nothing is pre-filled.

Enter your local price.

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Risers (steps)

5

Each riser
7.2in
Treads
4
Total run
42in
Stringer length
55.32in
Stringers
4
Stair angle
34.4°

Lumber

Stringer boards at least
8ft
Tread boards
8

Cost estimate

Enter your local prices under Cost estimate to see totals.

  • 5 risers: a handrail is required on at least one side (IRC R311.7.8, 4 or more risers).
  • Stringers are usually 2×12 lumber. Your local code and inspector have the final say.
deck36 inrun 42 instringer 55.3 in

Safety: Riser and tread limits follow the International Residential Code (R311.7); local codes can differ, so check with your building department before you build.

How to use the deck stair calculator

  1. Measure the total rise: from the top of the deck boards straight down to the ground or pad where the stairs will land. If the ground slopes, measure at the spot where the bottom step will sit.
  2. Leave the maximum riser at 7¾ in (the IRC limit) or lower it for gentler stairs.
  3. Enter the tread depth (at least 10 in) and the stair width (at least 36 in).
  4. Set the stringer spacing: 16 in is common, up to 18 in under DCA 6 with 2× lumber treads, and often 12 in for composite treads.
  5. Read the layout and use Print to take the riser and tread numbers to the saw.

How deck stairs are calculated

  • Risers = total rise ÷ maximum riser, rounded up
  • Riser height = total rise ÷ risers (every riser the same)
  • Treads = risers − 1, because the deck itself is the top step
  • Total run = treads × tread depth
  • Stringer length = √(total rise² + total run²)
  • Stringers = stair width ÷ spacing, rounded up, + 1 (and never fewer than three)

All risers must be equal: the IRC allows no more than ⅜ in difference between the tallest and shortest riser in a flight, which is why the calculator divides the rise evenly instead of using 7¾ in steps with a short one at the bottom.

Worked example: 36 in of total rise

This is the stair loaded above.

  1. Risers: 36 ÷ 7.75 = 4.6, rounded up to 5.
  2. Riser height: 36 ÷ 5 = 7.2 in, comfortably under the 7¾ in limit.
  3. Treads: 5 − 1 = 4.
  4. Total run: 4 × 10.5 = 42 in, so the stairs reach 3½ ft out from the deck.
  5. Stringer length: √(36² + 42²) = 55.32 in. A 2×12 at least 8 ft long leaves room for the top and bottom cuts.
  6. Stringers: 36 ÷ 16 = 2.25, rounded up to 3, + 1 = 4 stringers.
  7. Angle: about 34.4 °.

The IRC rules the calculator checks

Section R311.7 of the International Residential Code sets the basics for stairs at homes: risers of no more than 7¾ in, treads of at least 10 in and a clear width of at least 36 in, with a handrail on at least one side once a flight has four or more risers. The American Wood Council's DCA 6 deck guide adds that stringers should be at least 2×12, cut stringers no more than 18 in apart (at least three of them) and that a stair with more than 12 ft of total rise needs an intermediate landing. The calculator flags any input that breaks these limits. Local codes can be stricter, so confirm with your building department.

Stair layout by total rise

Total riseRisersRiser heightTreadsTotal run (10.5 in treads)Stringer length
18 in36 in221 in27.7 in
24 in46 in331.5 in39.6 in
30 in47.5 in331.5 in43.5 in
36 in57.2 in442 in55.3 in
42 in67 in552.5 in67.2 in
48 in76.86 in663 in79.2 in
60 in87.5 in773.5 in94.9 in
72 in107.2 in994.5 in118.8 in
96 in137.38 in12126 in158.4 in

7¾ in maximum riser (IRC R311.7.5). The deck surface is the top landing, so there is one fewer tread than risers.

Stair building tips

  • Measure the rise at the landing spot. If the ground slopes, the rise measured at the deck edge can be off by an inch or more where the bottom step lands. Measure, then adjust.
  • Land on something solid. DCA 6 says stringers must not bear on a patio or pad that isn't founded below the frost line or 12 in, whichever is deeper; posts or a proper footing are used instead.
  • Account for the tread thickness. When you mark the stringer, the bottom riser is cut shorter by one tread thickness so every step ends up the same height.
  • Composite treads need closer stringers. Many composite decking makers require stringers 12 in apart or less; check the installation guide.

Frequently asked questions

How many steps do I need for a 36 inch rise?

Five risers of 7.2 in each, with four treads. With 10.5 in treads the stairs run 42 in out from the deck and each stringer is about 55.3 in long.

What is the maximum riser height for deck stairs?

7¾ in under the International Residential Code (R311.7.5), with treads at least 10 in deep. Some local codes differ, so check yours.

How long should stair stringers be?

Use the Pythagorean theorem: √(total rise² + total run²). For a 36 in rise and a 42 in run that is 55.3 in. Buy boards a little longer to allow for the top and bottom cuts.

How many stringers do I need for deck stairs?

Divide the stair width by the stringer spacing, round up and add one. A 36 in wide stair with stringers 16 in apart needs four. DCA 6 calls for at least three cut stringers, no more than 18 in apart.

Do deck stairs need a handrail?

Under the IRC, yes once there are four or more risers: a handrail on at least one side, 34 to 38 in above the tread nosings.

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