Deck stair calculator

Enter how high your deck is and how wide you want the stairs. Treads are two boards deep, 11 1/4 inches, the most common deck stair build.

Steps (risers): –
Riser heighthow tall each step is: –
Framing squaretongue / blade, to mark the stringer: –
Treads: –
Stringersthe notched boards that carry the steps: –
Tread boardstwo deck boards per step: –
Total runhow far the stairs reach out: –
Stringer length: –

What to buy

Planning deck stairs

Deck stairs follow the same rule as any stair: equal risers no taller than 7 3/4 inches. The difference is the tread. Most decks use two deck boards per step, which gives an 11 1/4 inch tread and keeps the stairs matching the deck.

Stringers = (stair width − 1 1/2") ÷ spacing, rounded up, + 1

Worked example

A deck 30 inches off the ground: 30 ÷ 7.75 = 3.9, so 4 risers of 7 1/2 inches and 3 treads. For a 36 inch wide stair at 16 inch spacing you need 4 stringers and 6 tread boards cut to 36 inches. The stairs reach 2 feet 9 3/4 inches out from the deck.

Shopping list tips

Buy one extra tread board for mistakes and a riser board for each step if you want closed risers.
Precut stringers from the store fit a few standard heights only. Cut your own from 2x12 when your riser height doesn't match.
Set the bottom of the stringers on a concrete pad or pavers, not on soil.

Questions people ask

How many stringers do I need for deck stairs?

Space stringers no more than 16 inches apart for composite treads and 18 inches for wood. A 36 inch wide stair at 16 inches needs 4 stringers. Always check the tread maker's span limit.

How deep should deck stair treads be?

Two 5/4x6 or 2x6 boards with a small gap make an 11 1/4 inch tread, which clears the 10 inch minimum in the IRC with room for the nosing.

How do I measure deck height for stairs?

Measure from the top of the deck boards straight down to the ground or pad where the bottom step will land. If the ground slopes, measure at the landing spot, not at the deck post.

Do deck stairs need a handrail?

Under the IRC, stairs with four or more risers need a graspable handrail on at least one side. Your local code may differ.