For a standard 75mm or 100mm timber fence post, allow around 1.5–2 bags of 20kg Postmix per post. If you want the practical Wern-Wood answer, buy 2 bags per post and aim to use them.
Some holes will take a little less and some a little more. Across a normal fence run, that usually averages out. Larger posts, concrete fence posts, gate posts, sandy or waterlogged ground, and exposed sites generally need more.
Postmix is also commonly searched for as Postcrete or Post Crete. The quantity guidance below is based on the rapid-set 20kg Postmix we sell and the fencing jobs we discuss with customers at the yard.
Postmix quantity guide
| Post or job | Practical allowance |
|---|---|
| 75mm timber fence post | 1.5–2 × 20kg bags. We would normally buy 2. |
| 100mm timber fence post | 1.5–2 × 20kg bags. We would normally buy 2. |
| Concrete fence post | Usually more than a timber post; often around twice the standard timber allowance because the footing needs more mass to resist tipping and leaning. |
| 4×4 garden gate post | Usually around 3–4 × 20kg bags. |
| 6×6 gate post | Usually around 4–6 × 20kg bags. |
These are practical allowances, not exact measurements. Hole size, post size, ground conditions and how important it is to prevent movement all affect the final amount.
Is 1.5 or 2 bags better for a normal timber fence post?
If cost is the main concern, 1.5 bags per standard 75mm or 100mm timber post is a sensible working allowance for a normal fence. If we were buying the material for our own job, however, we would normally allow 2 bags per post.
That does not mean every hole will take exactly 40kg. One post might use 1.75 bags and the next might use 2.25. The hole you actually dig decides the final amount.
For normal garden fencing, the footing does not always need to finish flush with the surface. Once the post is secure, the top of the hole can be backfilled with soil where appropriate so the fence blends back into the garden.
Is one bag of Postmix enough per fence post?
We would not normally plan a standard fence around one 20kg bag per post. Customers who allow one bag for every post are some of the people we most often see back at the yard a few hours later buying more.
Too little Postmix is a warning sign because the footing needs enough mass around the post to resist movement once the rails, boards or panels are fitted and the wind starts loading the fence.
It is normally cheaper and easier to buy enough at the start than to stop the job and make another trip. As we tell customers: save the fuel and buy enough to start with.
How much Postmix do concrete fence posts need?
Concrete fence posts usually need more Postmix than an equivalent timber post. The post itself is heavier and creates more of a counterbalance problem, so the footing needs enough volume and weight to stop the post tipping or leaning.
As a practical starting point, we often allow around twice the amount used for a comparable standard timber-post installation. The exact quantity still depends on the size of the concrete post, the hole and the ground.
If you are still choosing the post itself, browse our Fence Posts before working out the final Postmix quantity.
How much Postmix do gate posts need?
Gate posts are different from ordinary intermediate fence posts because movement matters much more. The post is carrying the gate and repeatedly taking leverage as the gate opens, closes and moves in the wind.
For a typical 4×4 garden gate post, we normally think in terms of around 3–4 bags. For a 6×6 gate post, around 4–6 bags is a more realistic allowance.
The heavier the gate and the more serious the consequences of the post moving, the more substantial the installation needs to be. With larger gate posts, we generally recommend filling the footing to the surface, or slightly above where appropriate, rather than deliberately leaving a large section to backfill with soil. The aim is to reduce movement and keep the post stable.
Do I need spare bags for a whole fence run?
Normally, no. We would rather calculate a sensible allowance across the run than add arbitrary spare bags.
If you work on 1.5–2 bags per standard timber post, the variation between holes usually balances itself out. A slightly smaller hole might leave some of a bag for the next post, while another hole may take a little over 2 bags.
This is also why a precise Postmix calculator based on hole volume is not especially useful for a normal garden fence. In the real world, people do not dig a row of identical laboratory-sized holes. A practical per-post allowance is usually the better way to buy.
Ground conditions can increase the amount you need
The standard allowance assumes a normal fencing installation. Difficult ground or a highly exposed fence can justify more Postmix.
- Sandy ground: this is one of the worst conditions for keeping fence posts stable. We normally recommend a bell-shaped hole so the surrounding soil can help hold the set footing down.
- Waterlogged or flood-prone ground: using more material can help keep the post stable when the surrounding soil becomes saturated.
- Very windy or exposed sites: fences on hills or open ground can take much higher wind loading, so we would normally be more generous with the footing.
- Large posts: a larger post usually means a more substantial installation and therefore more Postmix.
The hole matters as much as the bag count
There are two easy ways to create problems: dig the hole far too large, or do not dig it deep enough.
An oversized hole simply swallows Postmix and can turn a sensible material allowance into a much more expensive job. A hole that is too shallow is worse: it can leave the post prone to leaning however much attention was paid to the rest of the fence.
This page is about quantity rather than post depth. For the depth side of the job, use our separate fence post depth guide.
A quick water guide when using Postmix
We do not normally work to a fixed litre ratio for a fence-post hole because the hole and the amount of Postmix vary. A simple practical method is to use the water in the hole and the condition of the mix as your guide.
- Fill the hole roughly one-third with water once the post is level and supported.
- Evenly pour in the first 20kg bag of Postmix.
- Evenly pour in the second bag until the water has been absorbed.
- If dry Postmix is still visible, add a little more water carefully. Avoid adding so much that water starts puddling.
- If free water remains, add enough Postmix to absorb it, without leaving dry mix behind.
The finished mix should be loose enough to trowel smooth, with no puddling water and no dry Postmix left visible. That gives a much more useful on-site check than trying to measure an exact water quantity for every different hole.
This is our quick practical guide. Always follow the instructions on the specific bag of rapid-set Postmix you are using.
Have the post ready before you pour
Rapid-set Postmix gives you very little time to change your mind. In our day-to-day experience, you want the post plumb, supported and ready before the material goes in; you may only have around 3–5 minutes to make useful adjustments once the process starts.
Do not assume you can pour first and straighten the post later. Always follow the instructions on the bag for the specific Postmix you are using.
Frequently asked questions
How many 20kg bags of Postmix do I need per fence post?
For a standard 75mm or 100mm timber fence post, allow around 1.5–2 bags. We normally recommend buying 2 bags per post and letting the small differences between holes balance out across the fence run.
Is 40kg of Postmix enough for a fence post?
For a typical 75mm or 100mm timber fence post in normal ground, 40kg — two 20kg bags — is our normal ideal allowance. Larger posts, concrete posts, gates and difficult ground can need more.
How much Postmix do I need for a concrete fence post?
Concrete posts generally need more material because of their weight and the amount of footing needed to resist tipping. We often allow around twice the quantity used for a comparable standard timber post, then adjust for the actual hole and ground conditions.
How much Postmix do I need for a gate post?
A typical 4×4 garden gate post is usually around 3–4 bags. A 6×6 gate post is more likely to need around 4–6 bags. The gate size, exposure and consequences of movement all matter.
Should I buy extra Postmix just in case?
You normally do not need dedicated spare bags if you have allowed 1.5–2 bags for each standard timber fence post. Some holes use a little less and some a little more, so the quantity tends to average out across the run.
The Wern-Wood rule of thumb
For standard timber fencing, buy 2 × 20kg bags per post if the budget allows. You may average closer to 1.5–2 bags once the whole run is complete, but starting with enough material avoids the common mid-job trip back for more.
Use more for concrete posts, gate posts, larger posts and difficult or exposed ground. Dig the hole to the right depth, avoid making it unnecessarily wide, and have the post level and supported before you start pouring.
Shop 20kg Rapid-Set Postmix or browse our Fence Posts to plan the complete job.
