Deck stair rise and run
Legal stair geometry for every common deck height. No riser here exceeds the 7.75" maximum, and every riser in a flight is identical.
| Deck height | Risers | Rise each | Treads | Total run |
|---|---|---|---|---|
| 12" | 2 | 6" | 1 | 0.9′ |
| 18" | 3 | 6" | 2 | 1.9′ |
| 24" | 4 | 6" | 3 | 2.8′ |
| 30" | 4 | 7.5" | 3 | 2.8′ |
| 36" | 5 | 7.2" | 4 | 3.8′ |
| 42" | 6 | 7" | 5 | 4.7′ |
| 48" | 7 | 6.857" | 6 | 5.6′ |
| 54" | 7 | 7.714" | 6 | 5.6′ |
| 60" | 8 | 7.5" | 7 | 6.6′ |
| 66" | 9 | 7.333" | 8 | 7.5′ |
| 72" | 10 | 7.2" | 9 | 8.4′ |
| 78" | 11 | 7.091" | 10 | 9.4′ |
| 84" | 11 | 7.636" | 10 | 9.4′ |
| 90" | 12 | 7.5" | 11 | 10.3′ |
| 96" | 13 | 7.385" | 12 | 11.3′ |
| 102" | 14 | 7.286" | 13 | 12.2′ |
| 108" | 14 | 7.714" | 13 | 12.2′ |
| 114" | 15 | 7.6" | 14 | 13.1′ |
| 120" | 16 | 7.5" | 15 | 14.1′ |
Total run assumes 11.25" treads (two 5/4x6 boards plus the gap between them).
How the riser count is chosen
Divide the total height by a comfortable rise of about 7.5" and round to the nearest whole number. If that puts any riser above the 7.75" maximum in IRC R311.7.5.1, add one riser and divide again.
A 24" deck is the clearest example. Three risers gives exactly 8" each, which is over the limit. It has to be four risers at 6". Rounding to a convenient number instead of dividing the height evenly is how flights end up with an illegal step.
Every riser must match
The code allows no more than 3/8" variation between the tallest and shortest riser in a flight. That is why the rise column shows awkward figures like 6.857" rather than a tidy fraction — the height is divided evenly, and the builder works to that. The classic failure is making every step a round number and letting the bottom step absorb the remainder, which is both a trip hazard and a failed inspection.
Treads are one fewer than risers
The deck surface is the top tread. A flight with seven risers has six treads. Counting a tread for every riser leaves you a board over and a cut list that does not add up.
Check the run before you cut
Run grows faster than people expect. A deck 4 ft above grade needs around 5 ft 8 in of clear ground at the bottom of the stairs. If the yard slopes away, a path runs past, or a fence is close, that is a problem to find now rather than after the stringers are cut.
Minimum tread depth is 10" under IRC R311.7.5.2. The 11.25" used here is what two 5/4x6 boards and a gap actually measure, which is why it is the common choice.
Your deck
The deck stair calculator takes any height, not just the six-inch steps in this table, and adds the stringer count and tread stock for your stair width.