Deck stair calculator

Enter the total rise from the ground to the top of the deck. You get the number of steps, the exact riser height, how far the stairs run out, and what to buy for stringers and treads.

Steps

risers of 7 3/16 in

Side view, to scale.

Riser height7.2 in (about 7 3/16 in)
Treads4 treads at 11 1/4 in
Total run45 in (3 ft 9 in)
Stringer length4 ft 9 5/8 in
Stringers4 × 2x12, 8 ft
Tread boards8 pieces, from 2 × 16 ft
Stair angle33°

Code check

Riser 7 3/16 inWithin 7 3/4 in
Tread 11 1/4 inAt least 10 in
Width 48 inAt least 36 in
HandrailRequired

How to calculate deck stairs

Measure the total rise, from where the bottom of the stairs will land to the top of the deck boards. Say it's 36 in.
Divide by your target riser height and round up: 36 ÷ 7.5 = 4.8, so 5 risers.
Divide the rise by the risers for the exact height: 36 ÷ 5 = 7.2 in, about 7 3/16 in.
There's one tread fewer than risers, because the deck is the top step: 4 treads.
Multiply treads by tread depth for the run: 4 × 11 1/4 in = 45 in.
The stringer is the diagonal: √(36² + 45²) = 57.63 in, about 4 ft 9 5/8 in. A 2x12 × 8 ft covers it with room for the cuts.

IRC stair rules for decks

Risers: 7 3/4 in maximum (Section R311.7.5.1).
Treads: 10 in minimum depth (Section R311.7.5.2).
Consistency: in one flight, the tallest and shortest risers can differ by 3/8 in at most, and the same goes for tread depths. Equal risers, as this calculator gives, are the goal.
Width: 36 in minimum (Section R311.7.1).
Handrail: required on stairs with four or more risers, 34 to 38 in above the tread nosings (Section R311.7.8).
Open risers: where the stairs are more than 30 in above the ground, the gaps between treads can't let a 4 in ball through.

Many towns adopt the IRC with changes, so check your local code.

Steps by deck height

Total rise	Risers	Riser height	Treads	Total run	Stringer
12 in	2	6 in	1	0 ft 11 1/4 in	1 ft 4 7/16 in
18 in	3	6 in	2	1 ft 10 1/2 in	2 ft 4 13/16 in
24 in	4	6 in	3	2 ft 9 3/4 in	3 ft 5 7/16 in
30 in	4	7 1/2 in	3	2 ft 9 3/4 in	3 ft 9 1/8 in
36 in	5	7 3/16 in	4	3 ft 9 in	4 ft 9 5/8 in
42 in	6	7 in	5	4 ft 8 1/4 in	5 ft 10 3/16 in
48 in	7	6 7/8 in	6	5 ft 7 1/2 in	6 ft 10 13/16 in
54 in	8	6 3/4 in	7	6 ft 6 3/4 in	7 ft 11 1/2 in
60 in	8	7 1/2 in	7	6 ft 6 3/4 in	8 ft 3 in
72 in	10	7 3/16 in	9	8 ft 5 1/4 in	10 ft 4 1/4 in
84 in	12	7 in	11	10 ft 3 3/4 in	12 ft 5 9/16 in
96 in	13	7 3/8 in	12	11 ft 3 in	13 ft 9 5/8 in

Risers up to 7 1/2 in, treads 11 1/4 in deep.

Stringers and treads

Stringers are usually cut from 2x12s. Space them no more than 16 in apart for wood treads; most composite treads need 12 in. Each tread here is two deck boards with a small gap, cut to the stair width, so a 4 ft wide stair gets four tread pieces from every 16 ft board.

Questions

How many steps do I need for a 30 inch deck?

4 risers of 7 1/2 in, with 3 treads between them; the deck itself is the top step. With 11 1/4 in treads the stairs run 2 ft 9 3/4 in out from the deck.

How many steps for a 4 ft high deck?

A 48 in rise takes 7 risers of about 6 7/8 in and 6 treads, running 5 ft 7 1/2 in. With four or more risers, the stairs need a handrail.

How do I measure the total rise?

From the top of the deck boards straight down to where the bottom of the stairs will land. If the ground slopes, measure at the landing spot, not at the deck, or the bottom step will come out the wrong height.

How far apart should stair stringers be?

For wood treads, no more than 16 in apart. Composite treads usually need 12 in or closer, and some brands ask for more support; check the decking maker's install guide.

Do deck stairs need a landing at the bottom?

The stairs need a firm, level spot to land on, such as a concrete pad or compacted gravel, and the IRC requires a landing at the bottom of the stairs. Very tall stairs also need a landing partway up; check your local code.

Stairs

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