Postcrete calculator: bags per fence post
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Number of posts | 10 |
| Post shape | Square |
| Post width | 100 mm |
| Hole shape | Round |
| Hole diameter | 300 mm |
| Hole depth, from the post-fix maker's instructions | 600 mm |
| Yield per bag | 14 L |
Order: 27 bags of post-fix (3 per post if you mix whole bags per hole: 30)
| Result | Value |
|---|---|
| Bags in total, sharing bags between holes | 27 |
| Bags per post (a whole number per hole) | 3 |
| Bags in total, whole bags per hole | 30 |
| Fill per post | 36.41 L |
| Fill for all posts | 364.12 L |
Show the working
- Hole: π × (300 mm ÷ 2)² = 0.071 m²; post 0.01 m²
- Fill per post: (0.071 − 0.01) × 0.6 m = 0.036 m³ = 36.4 L
- All posts: 36.4 L × 10 = 364.1 L
- Per post: 36.4 L ÷ 14 L = 2.6, rounded up = 3 (30 for 10 posts)
- Sharing bags: 364.1 L ÷ 14 L = 26.01, rounded up = 27
How it works
- Fill per post (m³) = (hole area − post area) × fill depth, where fill depth = hole depth − the gap left at the top
- Round area = π × (diameter ÷ 2)²; square area = width²
- Bags per post = fill per post (L) ÷ yield per bag, rounded up; total whole bags = posts × bags per post
- Sharing bags between holes: total fill (L) ÷ yield, rounded up
Hole size and depth come from the post-fix maker's instructions and the post height: follow them for your fence.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Post width | 0.1 m | Common UK size Not yet checked against the source | From your product 100 mm post. |