Post-Hole Concrete Calculator
Work out the concrete for fence posts, deck posts and gate posts, with the space taken up by the post itself subtracted from each hole.
Results
Enter your project measurements to see your results.
What the post-hole calculator does
Fence runs punish small errors, because whatever you get wrong on one hole you get wrong on every hole. This tool works out the concrete for a single typical hole, subtracts the space the post occupies, then multiplies across the whole run before applying your allowance.
How to measure a post hole
- Measure the hole as dug, not as planned: augers and spades rarely produce exact sizes.
- Take the depth from the finished ground level down to the bottom of the hole.
- Measure the post across the face, and again across the side if it is not square.
- Note how far the post actually goes down. If it stands on a pad of concrete, the embedded part is shorter than the hole is deep.
The displacement formula in plain language
Start with the volume of the hole. Then work out the cross-section of the post — width × depth for a timber post, π × radius² for a round one — and multiply that by how far the post sits into the hole. Take the second figure away from the first and what remains is the space concrete has to fill.
Metric example
A 300 mm diameter hole 600 mm deep holds about 42.4 litres. A 100 mm × 100 mm post embedded the full 600 mm displaces 6 litres, leaving 36.4 litres of concrete per hole. Across eight posts that is 291 litres, or 0.291 m³, and a 10% allowance brings it to 0.32 m³.
Imperial example
A 12 in diameter hole 24 in deep is 1.57 cubic feet. A 4 in × 4 in post embedded 24 in takes up 0.22 cubic feet, leaving 1.35 cubic feet per hole. Ten holes need 13.5 cubic feet, which is exactly half a cubic yard before waste.
Common mistakes
- Ignoring displacement and over-ordering by a bag or two across a long run.
- Measuring the post's nominal size rather than its actual finished size.
- Using the hole depth as the embedded depth when the post sits on a pad.
- Reusing a yield figure from a different product to the one being bought.
Depth, width and local requirements
Suitable hole dimensions depend on the post, what it carries, the soil type, frost conditions, wind exposure and the rules in your area. Gate posts, retaining posts and structural deck posts are not the same problem as a light garden fence. Check local guidance, manufacturer instructions and, where the structure matters, a qualified professional before you dig.
Frequently asked questions
- How deep and wide should a post hole be?
- There is no universal answer, and this calculator will not give you one. Suitable dimensions depend on the post size, what it supports, the soil, frost depth, wind exposure and the rules where you live. Ask a local professional or your building authority, then enter those dimensions here.
- Why does the post reduce the amount of concrete?
- Because the post physically occupies part of the hole. A 100 mm square post embedded 600 mm takes up six litres that concrete cannot fill. Over a run of fence posts that adds up to whole bags, which is why displacement is subtracted by default.
- When should I turn post displacement off?
- Turn it off when there is no post in the hole while you pour — for example when you are casting a plain concrete pad or setting a post base plate afterwards — or when you deliberately want the gross hole volume.
- What is the concrete below the post for?
- Many installations place a layer of concrete or compacted material under the post foot rather than standing the post on bare soil. If you plan that, enter the depth so the calculator knows the post only displaces concrete above it.
- Are Post Mix and Postcrete products calculated the same way?
- The volume maths is identical, but their stated yields differ from general-purpose mixes and from each other, so enter the yield from the specific product you are buying rather than reusing a figure from another bag.
- Can I calculate a whole fence at once?
- Yes. Enter the dimensions for one typical hole and set the number of posts. The calculator reports the volume per hole and the total for every post, with waste applied to the combined figure.