How deck stairs are calculated
- Risers. risers = total rise ÷ max riser, rounded up. Every riser is the same height: total rise ÷ risers.
- Treads. One fewer than risers — the deck surface is the top step.
- Total run. treads × tread depth: how far the stairs reach out from the deck.
- Stringer length. The diagonal of rise and run, plus one tread so the stringer bears at the bottom.
- Stringers. Stair width ÷ stringer spacing, rounded up, plus one.
Example: a 30" high deck
With a 30" rise, a 7-3/4" maximum riser, two-board treads (11-3/16") and a 4 ft stair at 12" stringer spacing:
- 4 risers of 7-1/2" and 3 treads.
- A total run of 2' 9-5/8" and stringers about 4' 5-7/8" long — cut from 8 ft boards.
- 5 stringers and 6 tread boards 4 ft long.
Measure the rise at the spot where the bottom step will land — ground often slopes away from the house, and an inch or two changes the riser height.
What this calculator doesn't cover
It sizes the steps and counts materials. It doesn't design stringer attachment, landing pads, handrails or guards, which have their own code requirements — handrails are generally required once a stair has four or more risers. Check with your building department before you build.
Frequently asked questions
How many steps do I need for my deck?
Divide the total rise (ground to deck surface) by the tallest riser you’re allowed, and round up. With the common 7-3/4" maximum, a 30" rise needs 4 risers of 7-1/2". Because the deck surface is the top step, you build one tread fewer than risers.
What are the code limits for deck stairs?
Most US jurisdictions use the International Residential Code: risers no taller than 7-3/4" and treads at least 10" deep, with riser heights varying by no more than 3/8" within a flight (IRC R311.7.5). Local amendments exist, so check with your building department.
How far apart should stair stringers be?
It depends on the tread material. Wood treads are often supported at 16" or less; many composite manufacturers require closer spacing — Fiberon calls for 12" for solid boards and 9" for scalloped ones, and TimberTech lists 9"–10" for several lines. Always use your tread’s installation guide.
How many boards make a stair tread?
Two 5-1/2" boards with a gap make a tread just over 11" deep, which clears the 10" minimum. The calculator works out boards per tread from your tread depth, board width and gap.