Calculator Group
Decks, Fences and Stairs
Published July 19, 2026
The 7 calculators in this group
- Deck Board CalculatorDeck boards from area, board width and gap, adjusted for picture-frame and diagonal layouts and checked against joist spacing.
- Deck Screw CalculatorDeck screws from the board-by-joist grid: how many boards cross how many joists, times the screws each crossing takes.
- Deck Footing Size CalculatorFooting diameter from the tributary area each post carries against your soil's bearing capacity, with the concrete each hole takes.
- Stair Rise & Run CalculatorRiser count and individual rise and run from total height, checked against the comfort ranges that make a stair feel right underfoot.
- Stair Stringer CalculatorStringer stock length, stringer count and the throat left after notching — the dimension that decides whether the stair carries the load.
- Fence CalculatorPosts, rails and pickets from fence run and post spacing, with the concrete each post hole needs cross-checked against its own tool.
- Concrete Post CalculatorBags per post hole from hole diameter and depth, less the volume the post itself displaces. A different shape of sum from a slab.
Nowhere else on this site does an early decision decide a later number as directly as it does here. Choose how far apart the posts on a deck or fence stand, and that single choice sets how much ground each post has to carry, which sets how big a footing that post needs, which sets how much concrete and how deep a hole the job requires — a chain running from a spacing decision made on paper to a volume of concrete poured in the ground, with nothing optional in between. The same shape shows up again in the boards themselves: how far apart they sit decides not just how many boards a deck needs but how many fasteners hold them down, because a fastener count follows the spacing grid rather than the board count alone.
That chain is why this group rewards working top to bottom rather than jumping straight to the material you can picture, like the decking itself. A board or picket choice made before the structure underneath it is settled is a choice that sometimes has to be unmade once the footing or post spacing comes back different than expected, and unmaking a decision after concrete has cured is a considerably worse afternoon than unmaking one on paper.
Post spacing decides the ground you have to dig, not just the material you buy
A footing does not carry the whole deck or fence — it carries a share of it, called the tributary area, roughly the portion of the structure closest to that particular post. Space posts farther apart and each one's tributary area grows, which means each footing has to be sized to carry more load, which usually means a wider or deeper hole. Space posts closer together and the reverse happens: more footings, but each one smaller, because the load is being split more ways.
That trade-off is why post spacing genuinely is a structural decision and not just a visual or a material-cost one, and why it belongs at the very start of planning a deck or a fence rather than at the end. A footing sized for the wrong tributary area is not a cosmetic mistake the way a wrong board color would be — it is the kind of error that shows up as movement in the structure well after the job is finished, which is exactly why footing sizing on this site carries the same explicit caution as anything else genuinely structural.
Board spacing decides the fastener count more than the board count does
It is tempting to assume fastener count follows board count in a simple ratio, and it does not, because fasteners are placed at every point a board crosses a joist, not once per board. Tighter joist spacing under the same deck means more crossing points per board, and therefore more screws, even though the board count itself hasn't changed at all. The deck board calculator and the fastener quantity that follows it are answering two different questions — how much decking covers the surface, and how many connection points that decking crosses underneath — and the second number is a function of the framing grid below, not of the boards above.
Gap width between boards matters here too, in a smaller way: a wider gap between boards means slightly less decking covers the same footprint, which nudges the board count down, but it does not touch the fastener count at all, since fasteners are still placed at the same joist crossings regardless of how wide the reveal between boards is. The two numbers move independently, which is worth knowing before assuming a wider gap saves money on both.
A stringer is not a plank, and a stair calculator answers a different question than a ramp would
Stair geometry is unforgiving in a way flat decking is not: rise and run have to stay consistent step to step, because a single stair that is even half an inch taller or shorter than its neighbors is a well-documented trip hazard, not a minor visual inconsistency. That is why the stair calculator works from a total rise and works backward to a consistent step count, rather than starting from a desired step count and hoping the rise divides evenly.
The stringer is a separate question again, because a stringer is not a plank cut to a stair-shaped outline in an obvious way — it needs enough material left below the deepest notch, the throat depth, to stay structurally sound, and running out of throat depth on a miscut stringer is not something a longer fastener or an extra brace reliably fixes. That is a cut-once, no-do-over material, which is worth confirming on paper before the saw ever touches the stock.
The order this group runs in: posts and footings first, then whatever fits on top of them
Posts and footings come first because everything else in this group is built to fit around them rather than the other way around. Joist span and decking layout are worked out from the post spacing that's already been set, not the reverse — deciding board direction or gap width before the substructure is settled means those choices might not survive contact with the actual post layout once footings, frost depth and tributary area have all been worked through.
Stairs come after the deck platform itself, since total rise is measured from the finished deck height down to the finished grade below it, and finished deck height is not always the number on an early plan once framing depth and decking thickness are both accounted for. Fasteners, for both decking and fencing, come last in the sequence, because a fastener count is derived from a spacing grid that has to already exist before there's anything to count crossings against. Working through a full deck build in order shows this chain end to end on one real project rather than as a list of separate rules.
A fence post and a deck post share a hole, not a load
Both get set in a concrete-filled hole, and it is easy to assume the same rules govern both, but the loads pulling on each are different in kind, not just in size. A deck post mostly carries weight straight down — people, furniture, snow load in some climates — which is why deck footing sizing is a tributary-area calculation built around vertical bearing. A fence post mostly resists a sideways push from wind against the panel or picket area above ground, which is a leverage problem more than a weight problem, and it is why a fence post typically needs to go deeper relative to its exposed height than a deck footing's bearing area alone would suggest.
That difference is also why a taller fence needs a disproportionately deeper post rather than a proportionally deeper one — wind leverage on the panel grows with height in a way that outpaces the post's own resistance to being pushed over, which the fence post spacing guide covers in more depth than belongs on this page.
FAQ
Do I need the footing calculator for a fence, or is that only for decks?
Fences generally use post-setting depth rather than a full engineered footing, since a fence carries far less vertical load than a deck does — mainly wind load pushing sideways on the panels rather than weight bearing down. The concrete post calculator covers the fence case; the dedicated footing calculator's tributary-area approach is built for the vertical load a deck actually carries.
Should I run the stair calculator or the stair stringer calculator first?
The stair calculator first, since it settles the rise and run for every step from the total height involved, and the stringer calculator then works from that settled rise-and-run figure to determine stock length and throat depth. Running the stringer numbers before rise and run are locked in risks cutting stock to a step geometry that changes once the full calculation is done.
My deck is attached to the house rather than freestanding. Does that change the footing calculation?
It changes which posts carry tributary load at all — an attached deck's house-side edge is typically supported by a ledger board fastened to the house structure rather than by posts and footings, so only the outer, freestanding edge needs the footing calculation this group covers. Confirm the ledger attachment itself is adequate for the load, since that connection carries real structural weight even without a footing under it.
Do deck screws and fence fasteners come from the same calculator?
No — deck screws are counted against a board-by-joist crossing grid, the pattern decking is actually fastened down with, while fence fasteners depend on the fence style instead, since a picket fence, a panel fence and a board-on-board fence all attach their vertical members to the rails in different patterns entirely. Run the fence calculator for the fence-specific fastener count rather than reusing the deck screw figure.