The number that decides how the fence looks
A privacy fence is built in bays, and the arithmetic that matters happens inside one bay. Post centres give you a nominal width, but boards do not fit between post centres. They fit between post faces, so the clear width is the centre spacing in inches minus the actual post width. Eight foot centres with 4x4 posts leave 92.5 inches of clear space, not 96.
Into that clear width go boards of a real width, separated by a gap you chose. The count is a floor division: the number of boards is the largest whole number where the boards and the gaps between them still fit. Written out, it is the clear width plus one gap, divided by one board plus one gap, rounded down. The plus-one-gap in the numerator is there because a bay with boards tight to both posts has one fewer gap than it has boards.
The result is almost never a clean fit. Fifteen boards at 5.5 inches with 3/8 inch gaps occupy 87.75 inches of a 92.5 inch bay. Four and three quarter inches are left. If you start at one post and work across at the gap you chose, all of that leftover lands in one place: a final gap nearly five inches wide, against a post, at the end of every bay, repeated down the whole fence. Spread the same leftover across all fourteen gaps instead and each gap becomes about 11/16 of an inch. Nobody looking at the fence can tell that from 3/8. Everybody can tell a five inch hole.
Which end the mistake shows up on
There is a second version of this problem, one bay wide, that people meet only after the fence is built. If you divide the run into full-width bays and let the last one be whatever is left, that last bay might be eighteen inches wide. It takes three boards where every other bay takes fifteen, and it reads as a mistake even though it is dimensionally correct. Dividing the run into equal bays instead — take the run, divide by the maximum spacing you are willing to use, round up, and divide the run by that count — gives bays that are all slightly under the maximum and all identical. That is the default here.
The equal-bay approach costs nothing except that your rails are no longer exactly eight feet. It usually gains a post, sometimes not. Where it matters most is on a fence that turns corners, because each side gets divided on its own and a run of three short end-bays around a yard is far more visible than one.
Rails, and why the third one is not optional above five feet
Two rails hold boards. Three rails hold boards straight. A six foot board fixed only at the top and bottom is free to cup and twist across its middle, and treated softwood fence boards do exactly that in their first summer. The middle rail removes the free length. It is the cheapest part of the fence and the one most often left out.
Rails are cut to the clear span between posts, not to the centre spacing, whenever they butt into the post faces. If instead you run rails past the posts and lap them, you need the centre dimension plus the lap. This calculator reports the clear span and the lineal footage at centres so you have both.
| Board and gap | Boards in a 92.5 in bay | Gap you get |
|---|---|---|
| 5.5 in board, 0 gap | 16 | about 5/16 in, unavoidably |
| 5.5 in board, 3/8 in gap | 15 | about 23/32 in |
| 5.5 in board, 1/2 in gap | 15 | about 23/32 in |
| 3.5 in board, 3/8 in gap | 23 | about 17/32 in |
Two of those rows land on the same answer, which is the point. A range of target gaps collapses onto one board count, and inside that range the gap you actually get is fixed by the count rather than by your intention. Choosing the gap is really choosing the board count.
What this calculator will not settle
It gives you material, not permission and not a boundary. Where a fence may stand, how tall it may be, which side the finished face must show and whether anything at all is allowed are set locally and vary between neighbouring towns. Nothing here should be read as telling you what is permitted where you live.
The boundary itself is the more expensive question. An existing fence, a hedge, a line of stakes left by someone else and a remembered agreement are all evidence of nothing. Only a survey establishes where a property line runs. A fence built over a line is a legal problem rather than a construction one, and the remedy can be moving it, which means pulling posts set in concrete. Find the line before you buy material.
Once the layout is settled, the fence post calculator covers post count and the concrete each hole takes, and the gate sag and brace calculator handles the opening this page removed from the run. If the ground falls across the run, work the layout out first with the fence on a slope calculator, because the bay drop changes the post lengths. For overlapping rather than spaced boards, the shadowbox and board-on-board calculator uses different arithmetic entirely, and when the fence is up the fence stain calculator counts the surface area including the picket edges.
Questions people ask
Why is the adjusted gap wider than the gap I asked for?
Because the boards never fill the bay exactly, and the leftover has to go somewhere. The calculator fits the largest whole number of boards that will hold your target gap or better, then redistributes everything left over into the gaps evenly. That always makes the gaps the same size or larger than the target, never smaller, and it is why a 3/8 inch target frequently comes out around 11/16. The alternative is to hold the target exactly and rip the two outer boards narrower to absorb the difference, which the results show as a second option when the leftover is big enough to be worth the extra cuts.
Should there be a gap between the end board and the post?
It depends on how you are covering the post. If the fence runs board to board with the posts visible on one side, tight to the post looks intentional and stops the eye at a clean line. If you are cladding over the posts so the fence reads as one continuous surface, a gap at the post keeps the spacing rhythm unbroken across the joint, which is what the second option here calculates. The practical argument for a small gap either way is that end grain and post faces both hold water where they meet, and a fence that never dries at that junction rots there first.
Do I calculate boards on post centres or post faces?
Faces. This is the single most common error in a fence order and it is worth stating plainly: post centres describe where the holes go, not how much space the boards have. Eight foot centres with a 4x4 post give 92.5 inches of clear width, and with a 6x6 post only 90.5. Two inches sounds trivial until it is the difference between fifteen boards and sixteen across twelve bays, which is twelve boards you either did not buy or cannot use. Enter the post size honestly, using the actual dimension rather than the nominal one.
How much extra should I order for waste?
Less than you would for sheet goods, but not nothing. Boards are used whole in a spaced fence, so there is no cutting waste in the ordinary sense. What you do lose is stock you reject at the pile: crooked boards, boards with a wane along one edge, boards that have already started to cup. On treated softwood, five percent is thin and ten percent is realistic if you are picking through what is delivered rather than selecting at the yard yourself. Order the rejects into the count rather than planning a second trip for eleven boards.
Does the gap size change how private the fence is?
A single plane of boards with any gap at all is see-through from an angle, and the narrower the gap the steeper the angle you need before you can see anything. What the gap really buys is wind. A solid fence takes the full wind load on its face and transfers it into the posts and their footings, and a solid six foot fence is a substantial sail. Gaps let air through and reduce the load, which is why spaced fences survive in windy places where solid ones lean. If you want privacy and wind relief together, an overlapping arrangement gives both, and the shadowbox calculator handles that geometry.