Concrete for Fence Posts Calculator — Postcrete & Mix Quantities

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. Timber and concrete posts are square, so we subtract the square post: a 100mm post = 0.1 × 0.1 × 0.6 = 0.0060 m³. Concrete needed per hole = 0.0424 − 0.0060 = 0.0364 m³.

Postcrete vs hand-mixed concrete

• Postcrete (20kg bags): Pour in dry around a water-filled hole, sets in 5–10 minutes. We size bags from the current Blue Circle Postcrete Product Data Sheet (December 2023), whose “bags required” table works out at roughly 0.018 m³ of filled hole-volume per 20kg bag — e.g. a 100mm post in a 300mm × 600mm hole = 2 bags
• Hand-mixed concrete: 1:2:4 by volume (cement:sharp sand:20mm aggregate), about a C20 mix — roughly 13 × 25kg cement bags, 0.70 t sharp sand and 1.48 t aggregate per cubic metre (standard absolute-volume method, matching our Concrete Mix Calculator). 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