Concrete for Fence Posts Calculator — Postcrete & Mix Quantities
Calculate how much postcrete or concrete you need to set fence posts. Works out bags per post, or cement, sand and aggregate for hand-mixing.
Total fence posts to concrete in
Standard depth is 600mm (2ft)
Standard diameter is 300mm (12in)
Postcrete is faster; hand-mixed concrete is cheaper for large jobs
Enter your supplier price for a cost estimate
How We Calculate This
Our concrete for fence posts calculator uses the cylinder volume formula to work out the exact amount of concrete needed for each post hole, minus the volume occupied by the post itself.
The formula
Hole volume = π × r² × depth
Concrete per hole = Hole volume − Post volume in hole
Total concrete = Concrete per hole × Number of posts
For example, a 300mm diameter hole at 600mm deep: π × 0.15² × 0.6 = 0.0424 m³. Minus a 100mm post: π × 0.05² × 0.6 = 0.0047 m³. Concrete needed per hole = 0.0377 m³.
Postcrete vs hand-mixed concrete
- Postcrete (20kg bags): Pour in dry, add water, sets in 5-10 minutes. One bag fills approximately 0.0085 m³
- Hand-mixed concrete: 1:2:3 ratio (cement:sharp sand:20mm aggregate). Stronger and cheaper for large jobs but takes 24-48 hours to set
Tips for setting fence posts
- Always use a spirit level to check the post is plumb before the concrete sets
- Slope the top of the concrete away from the post so water drains off
- In winter, avoid setting posts in frozen ground or when frost is expected overnight
- For postcrete, do not add too much water — follow the bag instructions exactly
Frequently Asked Questions
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Last updated: February 2026
All calculations are estimates. Verify with your supplier.