Framing Calc

Deck Stairs Calculator

From deck height and ground slope to stringers, landing and a board list.

Deck height top of decking to ground at the deck edge
Ground drops in per ft out
Stair width
Tread run cut on stringer
More: boards per tread, board stock length

Riser options

Riser height–
Treads–
Rise at landing–
Bottom step lands–
Stringers–
Stringer length–
Tread depth / nosing–
Stair angle–
Landing–

Estimate only. "Meets" means it meets the IRC 2021 figures we check, not that the stair is approved or safe. Most decks and deck stairs need a permit and inspection; for spans, footings and ledgers use DCA 6 or a licensed builder/engineer. Local codes differ.

Board list

Stringer cut layout

    Stringers and layout tools

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    How deck stairs are worked out

    Deck stairs use the same rule as any stair: divide the rise by 7-3/4" and round up for the fewest risers, then share the rise equally. The deck surface is the top step, so there is one fewer tread than risers.

    Sloping ground changes the rise. If the yard drops away from the deck, the bottom step lands lower than the ground at the deck edge, so the rise grows, which can add a riser, which moves the landing further out. The calculator repeats that loop until the count settles. Enter the drop in inches per foot; use ± if the ground rises away from the deck. It's always best to set a string line level from the deck and measure the real rise where the landing will go.

    Tread depth is the boards side by side (5-1/2" each with a 1/8" gap) and the nosing is how far that overhangs the stringer cut. Under 11" deep, the IRC wants a 3/4" to 1-1/4" nosing.

    Stringer spacing: AWC's DCA 6 is commonly cited for cut stringers at most 18" apart with a 5" minimum throat and no more than about 6' of horizontal span without a support. Composite and 5/4 tread makers often ask for 12" spacing; we use 12" for those, but the product's install guide decides.

    Questions

    How many steps do I need for a 3 foot high deck?

    A 36" rise needs at least 5 risers (36 ÷ 7.75 rounds up to 5) of 7-3/16" each, with 4 treads. If the ground slopes away, measure the rise where the bottom step will land.

    How many stringers do deck stairs need?

    At 18" spacing: 3 stringers for 36" wide stairs, 4 for 48". At 12" spacing for composite: 4 for 36", 5 for 48".

    Do deck stairs need a landing at the bottom?

    The IRC (R311.7.6) calls for a landing at the top and bottom of each stairway, at least as wide as the stairs and 36" deep. Many decks use a concrete pad or compacted gravel; check what your building department accepts.

    Indoor stairs or just the stringer? Use the stair stringer calculator. Counting deck boards and joists? Try the deck board calculator.

    Safety

    This is geometry and a check against a few IRC 2021 figures, not a structural design or code approval. Most decks need a permit and inspection. For spans, footings, ledgers and stringer attachment use DCA 6 or a licensed builder or engineer, and check your local code.

    Call 811 before digging footings or a landing pad.