How to Plan a Deck or Fence
Spans before boards, posts before panels — the order that stops a deck being rebuilt, and the half of a stain job everyone underestimates.
The tools in this guide
All free, no signup. Each one has a printable sheet too.
- Deck Board CalculatorHow many deck boards a deck needs, allowing for the gap between them, plus the screws to fix them down.
- Deck Joist Span CalculatorLooks up the AWC DCA 6 prescriptive span table — a Southern Pine 2x10 at 16 in centres spans 14 ft. Your local building department has the final say.
- Deck Stain CalculatorThe floor is the easy half. Railings and steps are nearly 49% of a typical deck — and old, weathered wood drinks more stain than new wood, not less.
- Fence Panel & Post CalculatorHow many fence panels and posts a run needs, allowing for gates, plus the rails and the concrete holes to dig.
- Post Hole Concrete CalculatorBags of concrete for fence posts, with the post's own volume subtracted - forget that and you over-order by about a seventh.
A deck and a fence are the same job in different proportions: set posts in concrete, span something between them, then finish it. The mistakes are the same too, and they all come from estimating the visible part before the structural part.
Structure first, always
The Deck Joist Span Calculator looks up the AWC DCA 6 prescriptive span table — for example, a Southern Pine 2×10 at 16-inch centres spans 14 ft.
Do this before you buy a single board. Span is what decides your joist size and spacing, and joist spacing is what decides how many boards you need and how they can run. Working the other way round means picking a board layout and then discovering the structure underneath it has to change.
Your local building department has the final say. A prescriptive table is a national starting point, not a permit. Check before you build, not after.
Then the deck surface
The Deck Board Calculator gives board count allowing for the gap between them, plus the screws.
The gap is not a detail. Boards are spaced deliberately for drainage and movement, and across a whole deck that spacing changes the count. Estimating by area alone quietly ignores it.
Fences: posts and gates, then panels
The Fence Panel & Post Calculator handles a run, allowing for gates, and gives you rails and the concrete holes to dig.
Gates are the part that breaks a simple calculation. A gate is not a panel — it changes post positions on both sides of itself, and a run with two gates is not the same as a run of panels minus two.
Both jobs need concrete in the ground
The Post Hole Concrete Calculator appears in this group and in the concrete group, because it is genuinely the overlap between them.
The thing it does that mental arithmetic does not: it subtracts the post's own volume from the hole. Miss that and you over-order by about a seventh — trivial on one post, real across a fence line.
Staining: the floor is the easy half
The Deck Stain Calculator carries the number most people get wrong by a factor approaching two.
Railings and steps are nearly 49% of a typical deck. Almost half the job is the parts that are not the floor — every baluster has four faces, every step has a tread and a riser, and none of it shows up if you estimate the deck as a rectangle.
The second thing it accounts for: old, weathered wood drinks more stain than new wood, not less. That is the opposite of most people's intuition, and it is why a re-stain often needs more product than the original.
The short version
- Check the span before buying anything — and check with your building department.
- Board count comes after joist spacing, not before.
- On a fence, place gates and posts before counting panels.
- Subtract the post from the post hole.
- Budget stain for the whole deck — railings and steps are nearly half of it.
- Weathered wood needs more, not less.
Each tool has a printable, and the span sheet in particular is worth taking to the building department rather than working from memory.
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