Concrete Post Mix Calculator
Instantly work out how many bags of post mix or postcrete you need to set your fence posts, based on hole size, post size, and number of posts.
Calculate Your Post Mix Bags
Enter your hole dimensions, post size, and number of posts below to get an instant estimate of how many bags of post mix concrete you’ll need.
Post Mix Calculator
Enter your dimensions to see the estimated bags needed
Measure across the widest point of the dug hole
Typically one third of the post’s above-ground height
Standard square timber posts are usually 75mm or 100mm
Check your product packaging for the exact yield
Estimated Bags Needed
Calculated from your hole and post dimensions
Post Mix Bags Per Post Reference Chart
Approximate number of 20kg post mix bags needed per hole, based on a 100mm square timber post and standard hole sizes.
| Hole Diameter | Hole Depth | Approx. Volume Needed | 20kg Bags Per Post |
|---|---|---|---|
| 250mm | 450mm | ~0.017 m³ | 2-3 bags |
| 300mm | 600mm | ~0.033 m³ | 4-5 bags |
| 350mm | 600mm | ~0.048 m³ | 6-7 bags |
| 400mm | 750mm | ~0.084 m³ | 10-12 bags |
Post Mix Concrete FAQ
Everything you need to know about calculating and using post mix concrete for fence posts.
For a standard fence post hole of around 300mm diameter and 600mm deep, you typically need 1 to 2 bags of 20kg post mix concrete per post, depending on the post size and how deep the hole is.
As a general rule, bury the post to roughly one third of its total above-ground height, with a minimum depth of 450-600mm for standard garden fence posts. Taller fences and exposed, windy sites need deeper holes.
Most fast-setting post mix products are poured dry directly around the post and then soaked with water, which triggers the setting reaction. Always check the manufacturer’s instructions, as some traditional concrete mixes need to be mixed with water before pouring.
Fast-setting post mix typically sets firm enough to hold a post upright within 10 to 30 minutes, though it can take a further 24 to 48 hours to fully cure before attaching fence panels or rails.