Stair Stringer Calculator for Deck Stairs (Rise, Run and Stringer Length)

Quick answer: for a deck 30 inches above the ground, you need 4 risers of 7-1/2" and 3 treads of 10", a total run of 30". Each stringer is about 42-7/16" long along the slope, so one 2x12 x 6 ft board is enough per stringer. Enter your own height below.

Stair stringer calculator

Total rise: deck surface to ground (in)

Maximum riser height (in)

Tread depth (in)

Stair width (in)

Stringer board

Based on the IRC stair rules for homes: riser no more than 7-3/4", tread at least 10", risers within 3/8" of each other, stairs at least 36" wide. Cut stringers are checked for at least 5" of solid wood left below the cuts and spaced no more than 18" apart, following the AWC DCA 6 deck guide. Local rules may differ.

Steps needed by deck height

Using risers of 7-3/4" or less and 10" treads.

How to lay out a stringer

Set a framing square on the 2x12 with the riser height on one arm and the tread depth on the other, both on the same edge of the board.
Trace the notch, slide the square along and repeat for each step.
At the bottom, cut off one tread thickness from the first riser so the first step is the same height as the others.
Cut one stringer, test it in place, then use it as the template for the rest.

Common questions

How many stairs for a 3 ft (36 in) deck?

5 risers of 7-3/16" and 4 treads. With 10" treads that is 40" of total run.

How far apart should stair stringers be?

No more than 18" center to center for cut 2x12 stringers. A 36" wide stair needs 3 stringers.

Does the top step count as a tread?

When the stairs end at the deck surface, the deck itself is the top step, so there is one fewer tread than risers. That is what this calculator assumes.

Checked by: Engr. Abdul Latif, civil and structural engineer. Limits follow IRC Section R311.7 and the AWC DCA 6 deck guide. Estimates for planning only.

Stair Stringer Calculator for Deck Stairs (Rise, Run and Stringer Length)