This deck stair calculator works out the full stair math and shopping list — risers, treads, total run, stringer length and count, tread boards and screws — from your deck height. No sign-up. The working is shown under every number.
Quick example: stairs up to a 48 in high deck need 7 risers at 6.86 in each, 6 treads, a 66 in run and 4 stringers — $203.00 of materials at the example prices.
| Item | Buy | Cost |
|---|---|---|
| Estimated total | $0.00 |
Risers are divided equally across the flight, the way code requires — never one short step at the bottom. Stringer length includes a 12 in trimming allowance.
Estimate only — not a guarantee. Mistakes are possible, so always check the quantities with your supplier or a builder before you buy or build.
Divide the total rise (deck height) by the maximum riser height and round UP — that is your number of risers. For a 48 in deck with the code maximum 7¾ in riser: 48 ÷ 7.75 = 6.19, so 7 risers, and each riser actually works out at 48 ÷ 7 = 6.86 in. There is always one fewer tread than risers (the deck itself is the top tread), so 6 treads at 11 in deep give a total run of 66 in. These are exactly the steps this calculator follows.
The stringer is the hypotenuse of the stair triangle: √(48² + 66²) = 81.6 in of sloped length. Add about 12 in for trimming the ends and leveling the bottom, and the board to buy is 93.6 in — an 8 ft 2 × 12. Published stair examples give the same 81.6 in figure for a 48 in rise with 11 in treads.
Stringers are usually spaced no more than 16 in apart, so a 36 in wide stair needs 4 stringers (one at each edge and two between, evenly spaced) — the count published worked examples give for a 36 in stair. Wider stairs or composite treads may need 12 in spacing; set it in the calculator.
The common US residential code (IRC) limits riser height to a maximum of 7¾ in and tread depth to a minimum of 10 in, plus a nosing on most treads. Every riser in one flight must be within ⅜ in of the others, which is why this calculator divides the total rise equally instead of using one short step. This calculator plans materials only — your local code has the final say.
| Deck height | Risers | Actual riser | Treads | Total run | Stringer board |
|---|---|---|---|---|---|
| 36 in | 5 | 7.20 in | 4 | 44 in | 8 ft 2 × 12 |
| 42 in | 6 | 7.00 in | 5 | 55 in | 8 ft 2 × 12 |
| 48 in | 7 | 6.86 in | 6 | 66 in | 8 ft 2 × 12 |
| 60 in | 8 | 7.50 in | 7 | 77 in | 10 ft 2 × 12 |
Stringer board length is the sloped stringer length plus a 12 in trimming allowance, rounded up to a standard board length (8, 10, 12, 14, 16 or 20 ft). Stringers counted at 16 in spacing for a 36 in wide stair (4 stringers).
These figures are planning estimates only — not a guarantee and not professional advice. Mistakes are possible, including through an error in this calculator. Always check the quantities, sizes, spacings and depths with your supplier, a licensed builder, and your local building code before you buy materials or start work. The owner of this site accepts no responsibility or liability for any loss, cost, damage, injury, wasted material or failed inspection that results from using these estimates. Prices are examples to start from, not live prices or quotes.