Enter deck stair details
Sample inputs and outputs
These examples illustrate typical stair math. Your local requirements may differ.
| Scenario | Total rise | Max riser | Tread depth | Nosing | Risers | Actual riser | Treads | Total run | Angle |
|---|---|---|---|---|---|---|---|---|---|
| Compact deck | 36 in | 7.75 in | 11 in | 1 in | 5 | 7.20 in | 4 | 40 in | 41.99° |
| Medium rise | 48 in | 7.50 in | 11 in | 1 in | 7 | 6.86 in | 6 | 60 in | 38.66° |
| Metric garden deck | 900 mm | 180 mm | 280 mm | 25 mm | 5 | 180 mm | 4 | 1020 mm | 41.42° |
How the calculator works
- Risers = ceil(total rise ÷ max riser) in auto mode.
- Actual riser = total rise ÷ risers
- Going = tread depth − nosing
- Treads = risers − 1 when the deck is the top step.
- Total run = going × treads
- Stringer length = √(rise² + run²)
- Angle = arctan(rise ÷ run) in degrees.
- Step diagonal = √(actual riser² + going²)
Outputs can be rounded to your chosen increment for easier marking.
Steps for accurate layout
- Measure the vertical height from ground to deck surface.
- Choose auto mode to cap riser height, or enter risers manually.
- Enter tread depth and nosing to compute the going.
- Keep “top step uses deck surface” checked for typical decks.
- Press Calculate, then review angle and stringer length.
- Download a CSV or PDF summary for your build notes.
Tip: If the result feels steep, increase tread going or add a landing. If it feels shallow, reduce going or adjust the riser count.
Riser planning for safer outdoor stairs
The calculator converts total rise into a practical riser count, then reports the resulting riser height. For exterior deck stairs, many builders target consistent risers within a narrow tolerance because small variations are easy to trip on. A typical target range is about 6.5–7.75 in (165–197 mm), but local rules and material choices can push that range. Use the “auto” mode to keep risers under your preferred cap.
Tread depth, nosing, and effective going
Your board depth is not the same as the going. The going is the nosing‑to‑nosing distance, which the tool computes as tread depth minus nosing overhang. Outdoor treads often land around 10–11 in (254–279 mm) of board depth with 0.75–1.25 in (19–32 mm) nosing, producing a comfortable going for most users. If going is small, stair angle increases quickly.
Total run, angle, and comfort checks
Total run equals going multiplied by tread count, and it directly controls pitch. Many comfortable exterior stairs fall near 30–40°; above that, steps feel steep and handrail use becomes more important. The calculator flags very steep or very shallow results as warnings so you can revise tread depth, add a landing, or increase the riser count. Small changes in going multiply across every step.
Stringer length and cut allowance
Stringer length is the diagonal of the rise‑run triangle: √(rise² + run²). The reported “with allowance” value adds a trimming margin for layout, end cuts, and minor field adjustments. If you are buying stock lumber, compare the allowance‑inclusive length to common board lengths (8 ft, 10 ft, 12 ft; or 2.4 m, 3.0 m, 3.6 m) to reduce waste.
Stringer count and spacing considerations
The tool accepts your planned stringer count and suggests a minimum of two. For typical deck stair widths, builders commonly space stringers around 12–16 in (300–400 mm) on center, especially under composite or thinner treads. Wider stairs, heavier treads, or high traffic may justify tighter spacing. Always match spacing to your tread manufacturer guidance and hardware capacity.
How do I measure total rise accurately?
Measure from finished ground level to the top of the finished deck surface. If you will add pavers, gravel, or a landing pad, include that final height in the measurement.
Why are treads sometimes one less than risers?
If the deck surface acts as the final “step,” you climb the last riser onto the deck without needing an additional tread. That makes treads equal to risers minus one.
What does the allowance do for stringer length?
Allowance adds extra length beyond the geometric minimum. It helps with trimming, end cuts, and fine‑tuning on site. Keep it small but realistic for your tools and layout method.
Should I use fractions or decimals for inches?
Use fractions when you mark with a tape measure and framing square. Use decimals when you need precise comparisons or you are transferring values to digital plans. Both use the same math.
My stair angle warning shows “steep.” What should I change?
Increase total run by using deeper treads or larger going, or increase the number of risers to reduce riser height. Adding a landing can also reduce steepness over long climbs.
How many stringers do I need for a wider stair?
Add stringers as width and tread flexibility increase. Many builds use 12–16 in (300–400 mm) spacing, so a 48 in (1220 mm) stair often needs four or more. Confirm with your tread material guidance.