Post holes
Post mix (Postcrete) bags per hole
- where
- hole volume (litres)
- hole diameter (mm)
- depth of concrete in the hole (mm)
- volume taken by the post (litres)
- post cross-section (mm²): side² for square, πp²/4 for round
- fill volume per hole (litres)
- bags per hole
- hole volume filled per bag (litres)
- total bags
- number of posts
Explain this equation
Works out how many bags of post mix are needed to set fence or gate posts.
The hole is a cylinder: π d²/4 times depth. The post takes up its own cross-section over the same depth, and the rest is filled with mix. With dimensions in millimetres, dividing by a million gives litres. Dividing by the litres each bag fills gives bags per hole, and the total for all posts is rounded up.
Cylinder volume minus post volume, divided by yield per bag.
Sense check: a 300 mm hole 750 mm deep round a 100 mm square post needs about 45 litres, about 3.5 bags at 13 litres a bag; ten posts need 35 bags.
Watch out: holes are rarely the exact diameter; a slightly bigger hole uses a lot more mix. Check the bag's stated yield.
Works out how many 20 kg bags of rapid-setting post mix fill the space round each post, and in total.
Enter your values to see the result.
Good to know
The default of 13 litres of hole per 20 kg bag is derived from the current Tarmac Postcrete bags-per-hole table (for example a 100 mm square post in a 300 × 750 mm hole takes 3½ bags). Older Blue Circle tables imply nearer 18 litres, so 13 is on the safe side. Manufacturers suggest a hole about three times the post width and a depth of about a third of the fence height above ground. For big holes it is cheaper to use a general-purpose concrete mix.
Common mistake
Assuming one bag per post whatever the hole size: a 300 mm wide hole 750 mm deep needs three or four bags.
Limits
Round holes with straight sides. The litres-per-bag figure varies by product; check the bag. Gravel boards, concrete spurs and heavy gates may need a designed footing.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.