1 Your measurements

Type feet and inches the way you'd write them, like 12' 6" or 30 3/8. Decimals work too.

in
Measure from the landing spot to the top of the finished deck/floor. Use a level and straightedge if the ground slopes.
in
IRC maximum is 7¾". 7–7½" feels comfortable.
in
IRC minimum is 10". Two 5/4×6 deck boards give 11¼" with a gap.
in
Used to drop the stringer so the first step equals the rest.
in

2 What to buy

Risers
4 × 7 1/2"
7.5" each
Treads
3 × 10 1/2"
total run 2' 7 1/2"
Stringer board
2×12 × 8 ft
3 stringers · 35.5° slope
1234 Total run 2' 7 1/2" Rise 2' 6"
Side view, to scale. Step numbers count up from the bottom.

These dimensions meet the common IRC limits. Always confirm with your local building department.

  • ✓ Riser height 7.50" ≤ 7.75" IRC maximum
  • ✓ Tread depth 10.5" ≥ 10" IRC minimum
  • ✓ Comfort rule (2 × riser + tread = 25.5") is within the 24–25½" sweet spot
  • ✓ Remaining stringer throat 5.15" ≥ 5" commonly recommended (some inspectors accept 3½")
  • ✓ Stair width 36" ≥ 36" IRC minimum
Riser height (exact)7.5" · 7 1/2" · 19.05 cm
Tread depth (run per step)10 1/2" · 26.7 cm
Bottom riser cut (drop for tread)6 1/2". Trim the bottom of the stringer so the first step equals the rest once treads are on
Step diagonal (layout increment)12 7/8"
Stringer length needed4' 8 3/8" → buy 8 ft boards
Throat remaining5 1/8"
HandrailRequired (4 or more risers)

Layout marks along the stringer

Measure from the bottom end of the board along its top edge. Each mark is where a riser line meets the edge.

Step 11' 7/8" (32.8 cm)
Step 22' 1 13/16" (65.5 cm)
Step 33' 2 11/16" (98.3 cm)
Step 44' 3 5/8" (131.1 cm)

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