Trades & DIY
Concrete & Post-crete Bag Calculator
Work out bagged concrete for slabs and footings, or post-crete for fence posts.
- Volume needed
- 0.792 m³
- 25kg bags to buy
- 73
- Estimated cost
- £474.50
How the bag calculation works
A standard 25kg bag of concrete mix yields roughly 0.011 m³ once mixed — about 90 bags per cubic metre. The calculator works out your volume, adds your chosen allowance for spillage and uneven excavation, then divides by the bag yield and rounds up.
For posts, the hole volume is a cylinder (π × r² × depth) minus the volume the post itself takes up, multiplied by the number of posts.
How deep should a fence post hole be?
The rule of thumb is one third of the post height in the ground, plus around 100mm of gravel at the bottom for drainage. A 1.8m (6ft) fence usually means a 600mm deep hole. In exposed or soft ground, go deeper.
Typical bag counts
| Job | Volume | 25kg bags |
|---|---|---|
| 1 post hole (300 × 600mm, 100mm post) | 0.041 m³ | ~4 |
| Shed base 2.4 × 1.8m × 75mm | 0.36 m³ | ~33 |
| Path 3 × 1m × 100mm | 0.33 m³ | ~31 |
| Small slab 3 × 2.4m × 100mm | 0.79 m³ | ~73 |
When to order ready-mix instead
Above about 0.5 m³ (roughly 45 bags), mixing by hand or with a small mixer becomes slow and hard to keep consistent. Consider a mini-mix delivery or a volumetric lorry — often cheaper per cubic metre and far less effort.