Deck stair calculator
Risers, treads, stringers and total run — with every riser held inside the 7.75 in code limit.
- The stair needs 45.0 in (3.8 ft) of clear run at the bottom. Check it fits before you build.
Stairs
| Item | Qty | Unit |
|---|---|---|
| 2x12 stringers Stringers no more than 16 in apart across a 36 in wide stair: ceil(36 / 16) + 1 = 4. | 4 | each |
| Tread stock (2 boards per tread) 4 treads x 2 boards x 3.0 ft wide = 24.0 linear ft. | 24 | linear ft |
How this calculator works
Riser count is decided by code, not preference
Start from a comfortable rise of about 7.5 in, then divide it into your total height. If the result puts any riser above the 7.75 in maximum in IRC R311.7.5.1, you add a riser and recalculate. A 24 in drop looks like three risers at 8 in each — which is illegal. It has to be four risers at exactly 6 in.
Every riser must be the same
The code allows no more than 3/8 in variation between the tallest and shortest riser in a flight. That is why the rise is total height divided by riser count, and never rounded to a convenient fraction. An uneven bottom step is both a trip hazard and a failed inspection.
Treads are one fewer than risers
The deck surface itself acts as the top tread, so a flight with four risers needs three treads. Counting a tread for every riser is a common off-by-one that leaves you with a spare board and a confusing cut list.
Check the run before you build
Total run is tread count times tread depth, and it grows fast. A deck 48 in above grade needs seven risers, six treads, and around 68 in of clear ground at the bottom. If your yard slopes away or a path is in the way, you need to know that before the stringers are cut.
What we will not guess
Joist and beam span limits are structural-safety data. Until our span tables are verified line by line against the published AWC DCA6 tables, this site withholds them and says so, rather than printing a number that looks authoritative. Footing depth must reach below your local frost line, which varies from nothing in the far south to over eight feet in the far north. Confirm both with your local building department — see our methodology.
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- Deck framing calculator Joists, blocking, rim, ledger and hangers — counted from the frame geometry rather than a rule of thumb.
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