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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)
Enter your supplier price for a cost estimate
Filled hole-volume per 20kg bag (Blue Circle Postcrete PDS, Dec 2023 ≈ 0.018 m³)
Per-bag yield scales with this (default 20kg). Use 25kg for a heavier product
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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. 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
Frequently Asked Questions
The current Blue Circle Postcrete Product Data Sheet (December 2023) bags-required table gives 2 bags of 20kg postcrete for a 100mm post in a standard 300mm × 600mm hole, so allow about 2 to 3 bags per post once you add a little spare. Smaller 200–250mm holes need only 1 to 1½ bags; a deeper 300mm × 750mm hole for an exposed site or a gate post needs about 2½ bags. Always buy a spare bag for the job — this calculator sizes the bags from that same Blue Circle table.
Postcrete (fast-setting post mix) is much quicker — it sets in 5-10 minutes versus 24-48 hours for standard concrete. It is ideal for domestic fence posts and saves significant time. However, for very heavy gate posts or commercial installations, hand-mixed concrete (a 1:2:4 by-volume mix, about C20) provides a stronger, more economical solution for large quantities.
The standard rule is to bury one-third of the total post length, with a minimum of 600mm (2ft). For a standard 6ft (1.83m) fence, posts should be buried 600mm deep. For exposed or windy locations, 750mm is recommended. Gate posts should be buried at least 750mm deep for extra stability.
A 300mm (12 inch) diameter hole is standard for most domestic fence posts. This allows enough room for concrete around a 75-100mm post. For gate posts or very exposed sites, use a 350-400mm hole. You can dig holes with a manual post hole digger (clamshell type) or a petrol-powered auger for larger jobs.
While it is possible to set posts in compacted hardcore or post-fix foam, concrete or postcrete provides the most secure and long-lasting fixing. In firm clay soil, posts can be driven in with a post driver, but this only works for pointed timber posts and is not suitable for panel fencing with slotted concrete posts. See our Fence Panel Calculator or Post and Rail Calculator for full material lists including posts.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.