How to use the fence post calculator calculator
- 1
Arrange utility location first
Post holes are excavation. Follow the utility-location procedure that applies where you are before digging, since buried services run through gardens more often than people expect.
- 2
Measure the fence run
Measure each straight run separately. A fence that turns a corner is two runs, and corner posts belong to one of them, so treating the whole perimeter as one length undercounts.
- 3
Enter your maximum post spacing
Use the spacing your fence plan calls for, or the panel width if you are using prefabricated panels. This calculator does not recommend a spacing.
- 4
Enter the hole size
Enter the hole diameter and the depth that will actually be concrete. If gravel is going in the bottom, subtract it from the hole depth first.
- 5
Decide about post displacement
Enter the post width to subtract what the post occupies, or leave it blank to fill the whole hole, which is the more conservative figure.
Formula
Posts = ceil(run(ft) / spacing(ft)) + 1
Concrete per hole is pi times the radius squared times the depth, less the post displacement. Multiply by the number of holes.
Worked example
A 100 foot fence run with a maximum post spacing of 8 feet
- 1Bays: 100 / 8 = 12.5, which rounds up to 13.
- 2Posts: 13 + 1 = 14.
- 3Actual spacing if divided evenly: 100 / 13 = 7.69 feet.
The run takes 14 posts. Spacing them evenly at 7.69 feet is usually neater than 12 bays at 8 feet plus one short 4 foot bay at the end.
Worked examples
Concrete for those holes
Fourteen holes, each 12 inches across and 30 inches deep, holding a post 3.5 inches square
- 1Hole volume: pi x (0.5 ft)^2 x 2.5 ft = 1.96 cubic feet.
- 2Post displacement: (3.5 / 12)^2 x 2.5 = 0.21 cubic feet.
- 3Concrete per hole: 1.96 - 0.21 = 1.75 cubic feet.
- 4Total for fourteen: 1.75 x 14 = 24.51 cubic feet.
The fence needs about 24.51 cubic feet of concrete, or 0.91 cubic yards. Filling the holes completely without subtracting the posts would be 27.49 cubic feet.
Why spacing matters more than it looks
The same 100 foot run at 6 foot spacing instead of 8
- 1Bays: 100 / 6 = 16.67, which rounds up to 17.
- 2Posts: 17 + 1 = 18.
- 3That is 4 more posts than at 8 foot spacing.
- 4Concrete at 1.75 cubic feet per hole: 31.52 cubic feet, up from 24.51.
Tightening the spacing by 2 feet adds 4 posts and about 7 cubic feet of concrete on a 100 foot run. Both quantities move together.
Why bays round up rather than down
A fence run has to reach the end. If a 100 foot run is divided at 8 foot spacing, twelve bays cover only 96 feet, leaving 4 feet unfenced, so you need thirteen bays with the last one short.
That is the opposite of a wall stud layout, where the run stops at the last full bay and the arithmetic rounds down. It is worth noticing, because the same looking calculation behaves differently depending on whether the run must be fully spanned.
In practice most people then redistribute the spacing so the bays are equal, which is why the calculator shows the actual spacing if divided evenly alongside the maximum you entered. Thirteen bays across 100 feet is 7.69 feet each, which looks better than twelve at 8 feet and one at 4.
Runs, corners, and the posts the count misses
The count assumes one straight run with a post at each end. Real fences are rarely one run, and every extra corner or end adds posts the simple formula does not capture.
A rectangular enclosure is four runs, and if you calculate each separately you will count each corner post twice, once for each run that meets there. A four-sided enclosure needs four corner posts, not eight, so subtract one post per corner from the total of the separate runs.
Gates complicate it further, because a gate opening needs a post on each side, and those are usually heavier than line posts. Terminal posts where a fence meets a building are similar. None of that follows from a length and a spacing, so count them from your layout and add them.
How the concrete side works
A post hole is close to a cylinder, so its volume is pi times the radius squared times the depth. The important consequence is that volume grows with the square of the diameter: a 12 inch hole holds four times what a 6 inch hole holds at the same depth.
The post itself occupies part of the hole, and subtracting it gives the tighter figure. Whether that is worth doing depends on the ratio: a 3.5 inch post in a 12 inch hole takes up about 11 percent of the volume, while the same post in an 8 inch hole takes up around a quarter.
Leaving the post size blank gives the conservative answer that assumes the hole is filled completely. On a small fence, buying that little extra is often simpler than working to the tighter number, and unused bags keep.
Common mistakes
Where fence post estimates go wrong:
- Rounding bays down, which leaves the end of the run unfenced.
- Treating a perimeter as one run and undercounting corner posts.
- Adding separate runs together without subtracting the double-counted corner posts.
- Forgetting gate posts and terminal posts, which the spacing formula never produces.
- Using the post width as the hole diameter, which understates the concrete badly.
- Entering the full hole depth when part of it will be gravel.
- Digging before arranging utility location.
Sources and methodology
Every figure on this page comes from geometry, not from a product or materials database. Areas use the standard area formulas, volumes are area multiplied by depth, cylinders use pi times the radius squared times the height, and unit conversions use exact factors: 12 inches to a foot, 27 cubic feet to a cubic yard, and 9 square feet to a square yard.
YardCalc publishes no material density, coverage rate, product dimension, soil swell factor, compaction factor, or price. Those vary by product, by supplier, by soil, and by region, and a figure that is right in one case is wrong in another, so the calculator asks you for the values that apply to your job. Weight estimates appear only when you supply a density.
Nothing here is engineering, safety, or code guidance. Results are planning estimates. Read how every value on the site is derived and checked in the YardCalc calculation methodology.
Post counts by run length and spacing
Posts in one straight run, with bays rounded up and one closing post, excluding gate, corner, and terminal posts. The spacings shown are calculation examples, not recommendations.
| Fence run | 4 ft spacing | 6 ft spacing | 8 ft spacing | 10 ft spacing |
|---|---|---|---|---|
| 20 ft | 6 | 5 | 4 | 3 |
| 40 ft | 11 | 8 | 6 | 5 |
| 50 ft | 14 | 10 | 8 | 6 |
| 75 ft | 20 | 14 | 11 | 9 |
| 100 ft | 26 | 18 | 14 | 11 |
| 150 ft | 39 | 26 | 20 | 16 |
| 200 ft | 51 | 35 | 26 | 21 |
| 300 ft | 76 | 51 | 39 | 31 |
Post hole concrete by hole size
Full hole volume at each size, before any post is subtracted, with the number of holes one cubic yard would fill. These sizes demonstrate the arithmetic and are not recommended hole dimensions.
| Diameter x depth | Cubic feet each | Cubic yards each | Number that fit in 1 cubic yard |
|---|---|---|---|
| 8 in x 24 in | 0.698 | 0.0259 | 38 |
| 10 in x 24 in | 1.09 | 0.0404 | 24 |
| 10 in x 30 in | 1.36 | 0.0505 | 19 |
| 12 in x 30 in | 1.96 | 0.0727 | 13 |
| 12 in x 36 in | 2.36 | 0.0873 | 11 |
| 16 in x 42 in | 4.89 | 0.181 | 5 |
Frequently asked questions
- How many fence posts do I need?
- Divide the run by your maximum spacing, round up for the bays, then add one for the closing post. A 100 foot run at 8 foot maximum spacing needs 14 posts, before gate, corner, and terminal posts.
- How many posts for a 100 foot fence?
- At 8 foot spacing, 14 posts. At 6 foot spacing, 18. At 10 foot spacing, 11. Add posts for corners, gates, and terminal points from your layout.
- How much concrete per fence post?
- It depends on the hole size. A 12 inch hole 30 inches deep holds 1.96 cubic feet filled completely, or about 1.75 cubic feet with a 3.5 inch post in it. Enter your own hole dimensions above.
- How far apart should fence posts be?
- This calculator does not decide that. Spacing depends on the fence type, the panel width, the wind loads, and local requirements, so take it from the manufacturer's instructions or your plan.
- How deep should a post hole be?
- Not something this page determines. Depth depends on the post height, the loads, the soil, and the frost depth, and it may be set by local code. Take it from the manufacturer's instructions or your building department.
- Should I subtract the post from the hole volume?
- It depends how much of the hole the post fills. A 3.5 inch post in a 12 inch hole takes up about 11 percent; in an 8 inch hole it takes about a quarter. Leaving the post size blank gives the conservative figure.
Post counts round bays up so the run is fully spanned, then add the closing post. Hole concrete uses the exact cylinder formula less any post displacement you specify. No spacing, hole size, embedment depth, bag yield, or price is assumed, and gate, corner, and terminal posts are not included. Quantities come from exact geometry using the dimensions you enter. Confirm product dimensions, installation requirements, local codes, site conditions, safety requirements, and final order quantities with the appropriate manufacturer, supplier, contractor, or professional.
Results are planning estimates. Last reviewed 2026-07-28.