When I replaced the old panel fence along our back garden boundary last summer, I made the classic mistake of underestimating the Postcrete. I had done a rough calculation in my head, picked up 20 bags for 10 posts, and confidently started digging. By post six I was already running low. I had to stop, make an emergency run to the builder's merchant, and come back in the afternoon to finish off.
Twenty minutes of proper calculation would have saved me an hour and a half of wasted time. That is what this guide is for.
What Postcrete actually is
Postcrete is a brand of fast-setting dry concrete compound, but the name has become so common that most people use it to mean any quick-set post-fix product (similar to Hoover being used for all vacuum cleaners). Competing products include Post Fix, PostSet and various own-brand equivalents from builder's merchants.
The working principle is the same across all of them: you set the post in position, pour the dry compound around it, then slowly pour water over the top. The material sets in approximately 10-15 minutes, holding the post firm and straight.
The key figure for calculating bags is the bag yield - how much set volume one bag produces. A standard 20kg bag of Postcrete yields approximately 0.010 m3 (10 litres). This can vary slightly between products, so always check the bag - some 20kg bags yield slightly more or less.
How to calculate bags per post
This is the calculation most people skip, and it is why they run short.
Step 1: Calculate hole volume (for a round hole)
Volume = pi x radius squared x depth
For a 250mm diameter hole (radius = 0.125m), 600mm deep:
- Volume = 3.14159 x (0.125)2 x 0.6 = 0.0295 m3
Step 2: Subtract the post volume
For a 100mm square post, 600mm deep:
- Post volume = 0.1 x 0.1 x 0.6 = 0.006 m3
Step 3: Net concrete volume per hole
0.0295 - 0.006 = 0.0235 m3 of concrete needed
Step 4: Number of bags
0.0235 / 0.010 = 2.35 bags - round up to 3 bags per post
For 10 posts, that is 30 bags - not 20. That is where I went wrong.
Use the Postcrete Calculator to get the exact figure for your project across all posts in one calculation.
Bags needed for common hole and post combinations
At 600mm depth, 100mm square post:
| Hole diameter | Net volume per hole | 20kg bags per post |
|---|---|---|
| 150mm | 0.011 m3 | 2 bags |
| 200mm | 0.018 m3 | 2 bags |
| 250mm | 0.024 m3 | 3 bags |
| 300mm | 0.035 m3 | 4 bags |
At 750mm depth (for taller fences), 100mm square post:
| Hole diameter | Net volume per hole | 20kg bags per post |
|---|---|---|
| 200mm | 0.023 m3 | 3 bags |
| 250mm | 0.029 m3 | 3 bags |
| 300mm | 0.044 m3 | 5 bags |
Hole depth: how deep do fence posts need to go?
The widely accepted rule of thumb is one third of the total post length in the ground:
| Fence height | Total post length | Minimum hole depth |
|---|---|---|
| 0.9m (3ft) | 1.5m | 450-500mm |
| 1.2m (4ft) | 1.8m | 500-600mm |
| 1.5m (5ft) | 2.1m | 600mm |
| 1.8m (6ft) | 2.4m | 600-750mm |
| 2.1m (7ft) | 2.7m | 750-900mm |
Go deeper than this minimum on exposed sites, in sandy soil, or if the fence is going to bear a significant wind load. A post set to the minimum depth in a coastal location will not thank you in its first winter storm.
Hole diameter: the overlooked dimension
People often undersize the holes. A hole that is only barely wider than the post does not give the concrete enough contact area with the ground to hold the post firmly.
The hole should be approximately 2.5 to 3 times the post width in diameter:
| Post size | Recommended hole diameter |
|---|---|
| 75mm post | 200mm |
| 100mm post | 250-300mm |
| 125mm post | 300-350mm |
A wider hole uses more Postcrete but creates a much more stable post. I have pulled out and reset posts that were concreted into narrow holes - the concrete is a thin cylinder that the post can lever out of over time. Width matters.
Round post vs square post: does it change the calculation?
Yes, slightly. A round post displaces less volume than a square post of the same nominal dimension. For a 100mm round post versus a 100mm square post, the difference in displaced volume is about 21%. This means a round post requires slightly more concrete to fill the same hole.
In practice the difference is one bag every few holes - worth accounting for on large projects, less significant for a small fence run. Our Postcrete Calculator handles both shapes accurately.
My tips for setting fence posts with Postcrete
After setting somewhere around 50 fence posts over the years, here is what I have learned:
Get the post plumb before you pour. Use a spirit level and temporary bracing (a couple of lengths of scrap timber nailed to the post and pegged into the ground) to hold the post perfectly upright. Once Postcrete goes in and you add water, you have about 5-10 minutes to make any last adjustments. After that, it is set and moving it means digging out.
One post at a time. Work through one hole completely - set, pour, water, check plumb - before moving to the next. Rushing between multiple holes means you come back to find a post slightly off vertical and the Postcrete already hardening.
Do not flood it with water. The instructions say to add water slowly. This is not just fussiness - too much water weakens the mix significantly. Pour the water gradually and evenly over the top of the dry compound, let it absorb, and resist the urge to add more.
Mound the top. When the Postcrete is set and before you backfill, check that the surface around the post is slightly mounded rather than flat or dished. A slight mound sheds rainwater away from the post base, which is where most fence posts start to rot.
Give it an hour before loading. The packet says set in 10 minutes, but 10 minutes is the initial set - the point where the post is held and will not move. Full working strength takes longer. Do not hang gates or attach fence panels under tension until the Postcrete has had at least an hour.
For a full project calculation including all posts and wastage, use the Postcrete Calculator.