Fence Calculator

Fence quantities all follow from one decision: the post spacing. Set the posts and everything else is determined — the number of bays, the panels, the rails, the pickets, and the concrete for the holes. Change the spacing and every number changes with it.

This calculator handles panel fences, picket fences, post-and-rail and chain link. It also works out the concrete for the post holes, which is the quantity people most often forget and then guess at, usually low.

Post spacing sets everything else

Most timber and panel fences use bays of 8 feet, or about 2.4 metres. Chain link often stretches to 10 feet because the fabric is tensioned between posts rather than supported by them. Some decorative or lightweight fence styles go to 6 feet, which adds posts and cost but makes the fence noticeably more rigid.

The end post matters as much as the spacings. A run of 100 feet at 8 foot bays divides exactly into 12½ bays, so you need 14 posts — 12 full bays plus a shorter final bay, and an extra post to close it. Dividing the run by the spacing and rounding down will leave you one post short every time, which is why the calculator always adds the closing post.

Angles and ends need more substantial posts than the ones in between. A run of posts in a straight line only has to hold the fence up; an end post has to resist the entire fence pulling on it, and a corner post has to resist it from two directions. By tradition these are set deeper and frequently braced.

Post holes, depth and the one-third rule

A common rule is that a third of the post's total length goes into the ground. An 8 foot post becomes 2 feet 8 inches in the ground with 5 foot 4 inches above. At least 24 inches is usual for a 6 foot fence, and more in sandy or loose ground.

The depth should also reach below the frost line where ground freezing is a factor. Frost heave lifts an inadequately set post a little each winter, and after a few seasons the fence is visibly leaning. In cold climates this can mean a hole deeper than the one-third rule suggests — check your local building guidance.

Hole diameter should be about three times the post's width. For a 4 inch post that is a 12 inch hole, which is what the calculator defaults to. A hole that is too tight cannot take enough concrete to hold the post; a hole that is too wide simply uses more material for the same result.

Worked example

A 100 foot fence run at 8 foot spacing needs 14 posts and creates 13 bays. With two rails per bay that is 200 linear feet of rail — two runs of 100 feet, one per rail line.

Note that this is not 13 bays × 8 feet × 2, which would give 208 feet. The run is 100 feet, so the last bay is 4 feet rather than 8, and rails are cut to the run, not to the nominal bay width. Estimating rails from bays × spacing over-orders by 4% on this run, and by more whenever the run divides awkwardly by the post spacing. As a panel fence it needs 13 panels.

Post holes at 12 inches diameter and 24 inches deep, with a 4 inch post. Each hole is π × 0.5² × 2 = 1.571 cubic feet, less the 0.222 cubic feet occupied by the post, so 1.349 cubic feet of concrete — 0.038 cubic metres per post, or 0.535 cubic metres across all 14 posts. At a bag yield of 0.011 cubic metres per 25 kilogram bag, that is 49 bags.

That last figure surprises people. Setting 14 posts properly takes the best part of a pallet of post-mix, and discovering it halfway through is the difference between finishing the job and making a second trip.

Post-mix, dry mix and what actually holds a post

Fast-setting post-mix is the convenient option: pour it dry into the hole around the post, add water, and leave it. It is more expensive per kilogram than mixing your own, and its advantage is speed and no mess. Where you have many posts, a 1 : 4 cement-to-ballast mix is considerably cheaper.

Whatever you use, the concrete's job is not to grip the post — it is to be a heavy, wide lump of mass that the post cannot move through. That is why the hole needs both depth and width, and why a wider hole with more concrete resists lateral load far better than a narrow one. Pack the concrete well as you fill; air pockets reduce the mass you are relying on.

Crown the concrete slightly above ground level so water runs away from the post rather than pooling at its base. And use posts rated for ground contact — timber that is only treated for above-ground use will rot at the soil line long before the rest of the fence fails.

Fence height, planing permission and neighbours

The default height is 6 feet, which is the common limit for a domestic boundary fence without planning permission in many jurisdictions — but the rules differ, and they differ again for a fence at the front of a property, on a corner, or next to a highway. Check locally before building. In many places a 6 foot fence is permitted at the rear but only 3 or 4 feet at the front.

Wind load rises steeply with height, and a solid 6 foot fence presents a great deal of it. In exposed locations, posts that are simply planted 2 feet into ordinary soil will not survive. Options are deeper holes, more posts, or a design that lets some wind through.

Finally, if the fence is on a boundary, talk to the neighbour first. It is cheaper and considerably more pleasant than a dispute, and in many places the fence becomes a shared asset with shared obligations.