Postcrete Calculator UK
Work out how many bags of Postcrete (or post-mix) you need for your fence posts. Enter hole diameter, depth and post size and get a bag count for any number of posts.
Post and hole details
Total posts you are setting
Typical: 200–300mm
Typical: 600mm minimum
Width/diameter of post
Default 0.010 m³. Check your product – yields vary.
Covers cuts, breakages and uneven ground. 10% is a typical starting point.
Postcrete estimate
Enter number of posts and hole dimensions
How this Postcrete calculator works
The hole volume is calculated as a cylinder: π × (hole radius)² × depth. The post volume is subtracted (using the post dimensions and shape you enter) to give the net concrete volume per hole.
That net volume is divided by the bag yield (typically 0.010 m³ or 10 litres per 20kg bag - but check your specific product) to give bag count. The result is rounded up and a wastage allowance added.
Real post holes vary in shape - hand-dug holes are rarely perfect cylinders. Always buy at least one or two extra bags to be safe, particularly in loose or crumbly soil.
Common post hole sizes
| Post size | Typical hole diameter | Typical hole depth | Approx. bags (20kg) |
|---|---|---|---|
| 75×75mm | 200mm | 600mm | 1 bag |
| 100×100mm | 250mm | 600mm | 1–2 bags |
| 100×100mm | 250mm | 750mm | 2 bags |
| 125×125mm | 300mm | 750mm | 2–3 bags |
Approximate figures only - use the calculator above for your specific dimensions.
Related calculators
Calculate how many fence panels, posts, gravel boards and Postcrete bags you need for a garden fence.
Work out concrete requirements for fence posts - bags, volume or ready-mix.
Work out how much concrete you need for a shed base, garage base or garden room.
Work out gravel in tonnes and bulk bags for driveways, paths and garden borders.
Frequently asked questions
Planning estimates only
These results are estimates for planning purposes only. Actual material quantities can vary based on site conditions, compaction, wastage, product size, installation method and supplier guidance. For structural, safety-critical or regulated work, always consult a qualified professional.