Deck Stair Calculator: Risers, Treads, Stringers and Board Count
A deck stair calculator has one job: turn a total rise — the vertical distance from the lower surface to the finished deck — into a riser count, a tread count, a run, and a stringer length you can cut to.
This one does that, and then does the thing the others do not: it works out how many stringers the stair needs to support the tread, using the support spacing our boards are rated for. On every common stair width that comes out higher than the three stringers a timber plan assumes.
Deck Stair Calculator
Measure the total rise first — floor surface to finished deck surface, not to the framing. Everything updates as you type.
Ground to finished deck surface
Clear width between the outer stringers
Start From the Total Rise, Not the Number of Steps
Everything on a stair follows from one measurement, and it is the one people take carelessly: the total rise, from the finished lower surface to the finished deck surface. Not to the framing, not to the top of the joist — to the surface people will actually step onto, which means the deck boards have to be on, or their thickness has to be in the number.
From there the arithmetic is fixed. Divide the total rise by the maximum riser height you are working to and round up; that is your riser count. Divide the total rise by that count and you get the actual riser height, which will be a little under your maximum and — critically — identical for every step.
Equal risers are not a nicety, they are the whole point. Most US jurisdictions allow no more than 3/8 in of variation between the tallest and shortest riser on a flight. People do not look at stairs; they calibrate to the first step and then move on autopilot. A single riser that is half an inch out is the one somebody catches a toe on, and it is the most common reason a stair fails inspection.
The number of treads is one fewer than the number of risers, because the deck surface itself serves as the top tread. That single off-by-one is behind a lot of ordering mistakes.
How Many Stringers a Composite Tread Needs
Here is the part no other stair calculator will tell you, because most of them are written for timber.
A timber stair tread is a 2× plank that happily spans a 36 in stair on three stringers — one at each edge and one in the middle. That is the default build, it is what the plans show, and it is what a framer will do without being asked.
Our boards have a maximum support spacing of 350 mm. Three stringers on a 36 in stair puts them 18 in apart — 457 mm, over our figure by more than 100 mm. It is not a marginal call.
Read this table as a product spec, not as code. The 350 mm figure is ours, for our boards, and it is what the stringer counts below are calculated against. Every composite manufacturer publishes its own support spacing and they are not interchangeable — some are wider than ours, some tighter. If you are buying somebody else's board, get their figure in writing and re-run the arithmetic against it. Nothing here is a building-code requirement; the code governs the stringer, the riser and the rail, not the board.
| Stair width | 3 stringers gives | Against our 350 mm | Stringers needed | Resulting spacing |
|---|---|---|---|---|
| 30 in · 762 mm | 381 mm | Over | 4 | 254 mm |
| 36 in · 914 mm | 457 mm | Over | 4 | 305 mm |
| 42 in · 1067 mm | 533 mm | Over | 5 | 267 mm |
| 48 in · 1219 mm | 610 mm | Over | 5 | 305 mm |
| 60 in · 1524 mm | 762 mm | Over | 6 | 305 mm |
| 72 in · 1829 mm | 914 mm | Over | 7 | 305 mm |
Read the middle column again: against our 350 mm figure there is no common stair width where three stringers works. On a deck the equivalent mistake — 16 in on centre — at least has a correct alternative at 12 in. On a stair, with our boards, the standard timber build is over the limit at every width.
The cost of getting it right is one or two extra stringers at the framing stage, which is cheap. The cost of getting it wrong is that every tread flexes underfoot, and fixing it means pulling the treads off a finished stair. A bouncy tread is also worse than a bouncy deck board, because it is underfoot at the exact moment somebody is committing their weight to a step.
Building a Tread From Composite Boards
Composite decking comes in board widths, not in tread widths, so a tread is made up rather than cut from one piece. With our 140 mm boards at a 145 mm cover width:
- Two boards = 290 mm of tread depth, which is 11.4 in. That clears the 10 in minimum most US jurisdictions adopt with room to spare, and it is the normal choice.
- Three boards = 435 mm, 17.1 in. Used where the stair doubles as seating or a transition landing.
One board is not an option — 145 mm is 5.7 in, nowhere near a legal tread.
The 5 mm gap between the boards is not optional either. It drains, and it gives the boards somewhere to go when they expand. On a stair it does one more thing: it stops water sitting on the tread surface, which on an exterior stair is the difference between a surface you can use in the rain and one you cannot.
What This Calculator Will Not Tell You
Stringer material and thickness, fixing specification, footing at the base, handrail height and graspability, guard requirements alongside the flight — none of it is here.
Those come from the IRC and your local amendments, and stairs are the part of a deck that inspectors look at hardest, because they are where people get hurt. The figures this page does use — a 7.75 in maximum riser, a 10 in minimum tread, 3/8 in maximum variation — are the ones most US jurisdictions adopt, but yours governs, and they do vary. Confirm before you cut a stringer.
What we can tell you, and have: the board cover width, our maximum support spacing, and the weight. Those three are ours to publish, they are product figures rather than code figures, and they are what decide the stringer layout and the order.
Worked Examples
All at a 7.75 in maximum riser and two boards per tread, on a 36 in wide stair.
| Total rise | Risers | Riser height | Treads | Total run | Stringer length | Tread boards |
|---|---|---|---|---|---|---|
| 24 in · 610 mm | 4 | 152 mm · 6.00 in | 3 | 870 mm | 1062 mm | 6 |
| 36 in · 914 mm | 5 | 183 mm · 7.20 in | 4 | 1160 mm | 1477 mm | 8 |
| 48 in · 1219 mm | 7 | 174 mm · 6.86 in | 6 | 1740 mm | 2124 mm | 12 |
| 60 in · 1524 mm | 8 | 191 mm · 7.50 in | 7 | 2030 mm | 2538 mm | 14 |
| 84 in · 2134 mm | 11 | 194 mm · 7.64 in | 10 | 2900 mm | 3600 mm | 20 |
| 96 in · 2438 mm | 13 | 188 mm · 7.38 in | 12 | 3480 mm | 4249 mm | 24 |
Note how the run grows: a 96 in rise needs nearly 11½ ft of horizontal space before the stair even reaches the ground. Stairs are the thing that does not fit, far more often than the deck is.
Frequently Asked Questions
How do I calculate deck stairs?
Measure the total rise from the finished lower surface to the finished deck surface. Divide by your maximum riser height and round up — that is the riser count. Divide the total rise by that count for the actual riser height, which will be equal on every step. Treads are one fewer than risers. Multiply treads by tread depth for the total run, then the stringer length is the diagonal: the square root of rise squared plus run squared.
How many stringers does a deck stair need?
For a timber tread, usually three on a 36 in stair. For our composite boards, four — because our maximum support spacing is 350 mm and three stringers on a 36 in stair sits at 457 mm. Wider stairs need more: five at 42 and 48 in, six at 60 in, seven at 72 in. These counts are calculated against our figure and do not transfer to another manufacturer's board — ask for theirs in writing. The calculator above works it out from the width you enter.
What is the maximum riser height for deck stairs?
Most US jurisdictions adopt 7.75 in from the IRC for residential stairs, with no more than 3/8 in of variation between risers on a flight. Commercial work and some states are tighter. Confirm with your building department — this is the number inspectors check first.
How deep should a deck stair tread be?
10 in is the minimum most US jurisdictions adopt. Two of our 140 mm boards give 290 mm, which is 11.4 in, so a standard two-board tread clears it. Three boards give 435 mm where you want a deeper, slower stair.
How long does a stair stringer need to be?
The diagonal of the rise and the run: for a 96 in rise with a 3480 mm run, the stringer is about 4249 mm before notching. Buy longer than the calculated length — notching consumes material and a mis-cut stringer is scrap.
How many boards do I need for deck stairs?
Treads multiplied by boards per tread, each cut to the stair width. A 96 in rise on a 36 in stair with two boards per tread needs 24 pieces at 914 mm — about 22 linear metres net, or 24 with a cutting allowance. The calculator gives this for your own dimensions.
Doing the deck as well as the stair? The deck calculator handles the deck surface, and the same 350 mm figure applies there — 12 in on centre works, 16 in does not. For the boards themselves, see the WPC decking range.
Building more than one of these? Send us the stair dimensions and the deck area together and we will price the whole job per container. Samples are free; you cover the courier and we credit it to your first order.
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