Quick answer: for a standard 75mm or 100mm timber fence post, allow around 1.5-2 bags of 20kg Postmix per post. For a full run, we normally buy 2 bags per standard post and let the small differences between holes balance out.
People often search this as how much Postcrete per post or how many bags of Postcrete per fence post. The figures below are practical guidance for the 20kg Wern-Wood Postmix we sell.
Postmix bags per post
| Post or job | Practical allowance |
|---|---|
| 75mm timber fence post | About 1.5-2 x 20kg bags |
| 100mm timber fence post | About 1.5-2 x 20kg bags |
| Concrete fence post | Often around 2 x 20kg bags, depending on the hole |
| DuraPost / slim metal post | Often similar to a standard post because the footing still needs enough mass |
| Gate post | Use a larger footing; see the gate-post rule below |
For a normal fence run, buy 2 bags per post
Real post holes are never identical. One may use a little under two bags and the next may use slightly more. Buying two bags per standard post is easier than trying to make every hole use the minimum possible amount.
Example: for a run with 8 standard timber posts, we would normally buy around 16 bags. If the ground is awkward, the holes are oversized or gate posts are included, allow extra.
Gate posts need much more support
We avoid pretending there is one exact number for every gate because gate size, post section, ground conditions and leverage all matter.
A useful rough rule for gate posts is one 20kg bag for every inch of post section. That means roughly:
- 4in gate post: around 4 bags
- 6in gate post: around 6 bags
That is a starting point, not a structural formula. The honest answer is: use enough to support the post and the gate. The hinge-side post normally needs more support than the latch-side post because it carries the gate weight and repeated leverage.
How deep should the post hole be?
Quantity depends on the hole as much as the post. For normal 6ft fencing, we often work around a 600mm post depth, but ground conditions, exposure and the fence design matter.
For depth guidance, read How Deep Should Fence Posts Be?.
An oversized hole can use a lot more Postmix
If the hole has been dug too wide, do not assume you should throw loose rubble into the sides and hope for the best. You have three practical options:
- fill the hole back in, pack the sides firmly and dig it again to the correct size;
- use scrap plywood or similar as a simple form to contain the Postmix where appropriate;
- or accept that the larger hole needs an extra bag.
The problem with filling a wide hole with hardcore is that it is easy to overdo it and make the hole too small around the post.
What about water per bag?
Follow the instructions on the bag supplied. In practice, we add clean water to the hole first, pour the Postmix evenly around the post, then add small amounts only where needed so there are no dry pockets and no puddling water left on top.
For the full method, read How to Use Postcrete / Postmix.
The Wern-Wood rule
Standard 75mm or 100mm fence post: buy 2 bags. Concrete post: often around 2 bags. Gate post: allow a much larger footing and use the inch-of-post rule only as a practical starting point.
Need the product? Shop Wern-Wood 20kg Rapid-Set Postmix.
