How it’s calculated
- Risers = ⌈total rise ÷ max riser⌉; every riser is the same height: total rise ÷ risers.
- Treads = risers − 1 (the deck surface is the top step). Total run = treads × tread run.
- Stringer length = √(total rise² + total run²); stock adds 12 in for the top and bottom cuts.
- Stringers = ⌈stair width ÷ spacing⌉ + 1. Tread boards per step = ⌈(run + nosing) ÷ (board width + ¼ in gap)⌉.
- Landing pad = stair width × 36 in deep × thickness, in 80 lb bags at 0.60 cu ft each.
Deck stair chart (computed by this calculator)
7¾ in max riser, 10.5 in tread run, 36 in wide, 16 in stringer spacing.
| Total rise | Risers | Riser height | Treads | Total run | Stringer |
|---|---|---|---|---|---|
| 18 in | 3 | 6.00 in | 2 | 21 in | 27.7 in (8 ft) |
| 24 in | 4 | 6.00 in | 3 | 31.5 in | 39.6 in (8 ft) |
| 30 in | 4 | 7.50 in | 3 | 31.5 in | 43.5 in (8 ft) |
| 36 in | 5 | 7.20 in | 4 | 42 in | 55.3 in (8 ft) |
| 42 in | 6 | 7.00 in | 5 | 52.5 in | 67.2 in (8 ft) |
| 48 in | 7 | 6.86 in | 6 | 63 in | 79.2 in (8 ft) |
| 60 in | 8 | 7.50 in | 7 | 73.5 in | 94.9 in (10 ft) |
Common mistakes
- Measuring rise at the deck edge on sloped ground. Measure where the bottom step will land.
- Making the bottom or top step a different height. The IRC allows only 3/8 in variation between risers.
- Forgetting the deck board on top changes the first riser if the stringer is hung below the rim.
- Setting the stringers on soil instead of a pad or footing.
Frequently asked questions
How many steps do I need for my deck?
Divide the total rise by the maximum riser height (7¾ in under the IRC) and round up. That gives the number of risers; the number of treads is one less because the deck is the top step. A 36 in rise needs 5 risers of 7.2 in and 4 treads.
What is the maximum riser height for deck stairs?
The 2021 IRC limits risers to 7¾ in (R311.7.5.1) and requires treads at least 10 in deep (R311.7.5.2). Many local codes adopt these numbers; confirm with your building department.
How long should deck stair stringers be?
The stringer is the diagonal of the total rise and total run. For a 36 in rise with four 10.5 in treads, that is √(36² + 42²) = 55.3 in. Buy stock with room for the top and bottom cuts, so an 8 ft 2x12.
How many stringers do deck stairs need?
Divide the stair width by the stringer spacing and add one. A 36 in stair at 16 in spacing needs 4 stringers. Check the maximum span of your tread boards; many composite treads need closer spacing.
When does a deck stair need a handrail?
The IRC requires a handrail on stairs with four or more risers (R311.7.8).