Deckwright
Stairs · IRC R311.7

Deck Stair Calculator

Enter the total rise and get an even riser height, tread count, total run, stringer length and the stock to buy, with every code check shown.

1 Rise & run
in
Top of decking to landing
in
Nosing to nosing; 10″ minimum
in
2 Stringers & treads
Stringer spacing
Composite treads often need 12″ or less. Check the product.
in
in
Risers
5
7 3/16″ each
Treads
4
10 1/4″ run
Total run
3′ 5″
35.1° slope
Stringers
5
2x12 × 8′ (5′ 2 5/8″ long)
5 risers: the IRC requires a handrail 34–38″ high on at least one side (R311.7.8).
More than 30″ of rise: the open sides of the stair need guards (R312.1).

Comfort check: 2 × riser + run = 24.7″ (24–25″ is the traditional sweet spot). Throat after notching: 5.36″. Tread boards: 8 pieces at 4′ 2″ (11.19″ deep per tread).

Stringer layout

side view
decklanding7 3/16″10 1/4″36″3′ 5″ total run

Checks

  • ✓Riser heightIRC R311.7.5.1: 7¾ in. maximum
  • ✓Tread depthIRC R311.7.5.2: 10 in. minimum, nosing to nosing
  • ✓Stair widthIRC R311.7.1: 36 in. minimum clear width
  • ✓Stringer spacingAWC DCA 6: 18 in. maximum for cut stringers; many tread products require 12 in. or less
  • ✓Stringer throatAWC DCA 6: at least 5 in. left after notching

Measure the total rise correctly

Measure from the top of the finished decking to the surface the bottom step will land on, not to the ground at the deck edge. If the ground slopes away, set a level or a straight board out from the deck to where the stair will end, and measure down from it there. An error of an inch here makes every riser wrong.

How the numbers work

  1. Risers = total rise ÷ maximum riser, rounded up. Every riser is then the same height: total rise ÷ risers.
  2. Treads = risers − 1. The deck surface is the top step.
  3. Total run = treads × unit run. This is how far the stair projects from the deck. Make sure the landing fits.
  4. Stringer length is roughly the diagonal of the rise and run plus one more tread for the bottom seat. Buy the next stock length up.
  5. Throat is the solid wood left below the notches. AWC DCA 6 calls for at least 5″, which is why cut stringers are almost always 2x12s.

Two treads of 5½″ decking with a 3/16″ gap give 11 3/16″ of depth. With a 1″ nosing past the riser, that is a 10 3/16″ run, just over the IRC’s 10″ minimum. That is why two-board treads are standard on decks. For all the stair rules on one page, see deck stair rules in plain English.

Questions people ask

How many steps do I need for my deck?
Divide the total rise (top of the decking to the landing) by the maximum riser height, 7¾″ under the IRC, and round up. That gives the number of risers. There is one fewer tread than risers, because the deck itself is the top step. A 36″ rise needs 5 risers of 7.2″ and 4 treads.
How long should deck stair stringers be?
Each stringer is the hypotenuse of the total rise and the total run, plus enough to seat the bottom cut. For 5 risers and 4 treads of 10¼″ on a 36″ rise, a stringer is about 5′-3″ long, so it comes out of an 8 ft 2x12.
How far apart should stair stringers be?
The American Wood Council’s DCA 6 deck guide sets 18″ on center as the maximum for cut stringers. Many composite tread products require 12″ or less, and some require 9″. Follow whichever is closer.
What is the maximum riser height for deck stairs?
The IRC allows 7¾″ maximum and requires a 10″ minimum tread depth. Within one flight, riser heights and tread depths may not vary by more than ⅜″. Some states amend the riser limit, so check locally.
Do deck stairs need a handrail?
Under the IRC, stairs with four or more risers need a handrail 34–38″ high on at least one side. If the stair is more than 30″ above grade, its open sides also need guards.

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