Fence Post and Materials Calculator: Posts and Concrete
Fence Post and Materials Calculator
Fence run to post count, panel count and bags of concrete, with the actual post spacing worked out rather than assumed. Gates are subtracted from the run and add their own posts.
Post spacing is a maximum, not a target: the run divides into equal sections at or under the limit, which is why 30 metres at a 2.4 metre maximum gives 13 sections at 2.31 metres rather than 12 at 2.4 plus an awkward stub. Hole depth should reach below the local frost line, commonly 1.2 metres or more in cold regions, and a nominal 4 by 4 post actually measures 89 mm, which is what the concrete volume assumes.
How to use this tool
Enter the run length and the maximum post spacing, add any gates and their width, and set the hole geometry. The tool returns posts, sections, the actual spacing it settled on, and the bags of concrete for the holes.
Spacing is a ceiling, not a target
This is the part most estimates get wrong. A maximum spacing of 2.4 metres does not mean posts every 2.4 metres with a leftover stub at the end. It means the run divides into equal sections none of which exceeds 2.4 metres. A 30 metre run needs 13 sections, which come out at 2.31 metres each, and 14 posts. Twelve sections would be 2.5 metres and over the limit; a stub section at the end looks wrong and is structurally the weakest bay in the fence.
The post count follows from the sections: a continuous run has one more post than it has sections, because both ends need one. Gates add their own posts and remove their width from the fenced run, which is why the tool subtracts them before dividing.
A worked check you can repeat on paper
Thirty metres, 2.4 metre maximum spacing, one 1 metre gate. Fenced run is 29 metres, which needs 13 sections at 2.23 metres, giving 14 line posts plus 1 for the gate. Each hole is 300 millimetres across and 900 deep: the cylinder is 0.0636 cubic metres, minus a 89 millimetre post occupying 0.0071, leaving 56.5 litres of concrete per hole. At 25 litres a bag that is 3 bags per hole, and it is worth noticing that the naive calculation, forgetting to subtract the post, overstates by about 12 percent.
Mistakes that catch people out
Dividing length by spacing and calling it the post count. That gives sections, and it drops the closing post. Fix: posts equal sections plus one per continuous run.
Leaving a stub bay. Fix: equal sections at or under the maximum, which is what the tool computes.
Digging above the frost line. A shallow post heaves and the fence goes crooked in the first hard winter. Fix: depth below the local frost line, commonly 1.2 metres or more in cold regions; set the depth accordingly.
Using nominal post dimensions. A 4 by 4 is 89 millimetres, not 100. Fix: the default is the real size.
How this was checked
Section and post counts were verified on both an inexact run, 30 metres at 2.4 giving 13 sections and 14 posts at 2.31 metre actual spacing, and an exact one, 24 metres at 2.4 giving exactly 10 sections and 11 posts with no spurious extra. The concrete volume was computed independently from the cylinder and post geometry and agreed to the sixth decimal.
Related Calculators and Reference
The concrete itself, if mixed rather than bagged, is the Concrete Mix Volume and Quantity Estimator, and post hole volumes in other shapes are the 3D Volume Calculator. A deck built off the same posts is the Deck Board Calculator.