Key Takeaways
- The formula: Hole volume = radius² × π × depth (in metres), which gives you cubic metres of concrete needed. subtract the post’s own volume if you’re setting one into the hole.
- The quantity: A mix labelled to yield 0.01 m³ per 20kg bag needs its own yield checked, so a standard 450mm × 600mm hole needs 10 bags before any allowance for waste; use the table for other common sizes.
- The traps: Measure the actual hole, not the auger size, check depth on every hole on sloping ground, and follow the specified footing design before calculating concrete volume.
Here’s how to work out concrete for pier holes with one formula: get the hole’s radius and depth, and the rest is arithmetic. Guess instead, and you’ll either run short halfway through a row of fence posts or end up with three unused bags sitting in the shed.
A quick-reference table below means you don’t have to do the maths for every hole on the job.
How to work out concrete for pier holes
The volume of a cylindrical hole is radius squared, multiplied by pi, multiplied by depth. In plain terms: measure the hole’s diameter, halve it for the radius, then multiply radius × radius × 3.14 × depth. That gives you the volume in cubic metres, assuming your measurements are in metres.
Say you’ve augered a 450mm hole, 600mm deep. The radius is 0.225m. Squared, that’s 0.0506. Multiply by 3.14, then by 0.6m depth, and you land on roughly 0.095 cubic metres of concrete.
What if the post is going in the hole too?
If you’re setting a post rather than pouring a solid pier, subtract the post’s own volume from that figure first. A 90mm square timber post taking up 600mm of hole depth removes about 0.005 cubic metres, which barely changes the order for one hole but adds up across a full fence line.
How many bags of concrete do pier holes need?
For a product labelled to yield 0.01 m³ per 20kg bag, divide the concrete volume by 0.01. Easy Mix publishes this yield for its Concrete Mix; check the instructions for the product you choose because yields differ.
Take the 0.095 cubic metre hole from above and divide by 0.01, and you need around 9.5 bags. Round up to 10, because running one bag short mid-pour is worse than having one left over.
| Hole diameter | Depth | Approx. volume | Bags at 0.01 m³ yield each |
|---|---|---|---|
| 300mm | 450mm | 0.032 m³ | 4 |
| 300mm | 600mm | 0.042 m³ | 5 |
| 450mm | 450mm | 0.072 m³ | 8 |
| 450mm | 600mm | 0.095 m³ | 10 |
| 450mm | 900mm | 0.143 m³ | 15 |
These figures assume a solid cylindrical pour and a product yield of 0.01 m³ per bag. Subtract any embedded post volume, calculate the quantity using your chosen product’s stated yield, and round the result up to a whole bag. Allow separately for actual site dimensions and wastage.
Common mistakes that throw off your concrete order
Measuring the auger size instead of the actual hole is the most common one. Ground conditions mean a 450mm auger doesn’t always leave a perfectly uniform 450mm hole, especially in sandy or loose soil where the sides can slump slightly wider.
Forgetting depth variation across a job is another. A sloping yard means your holes won’t all be the same depth even if you’re aiming for consistency, so measure each one rather than multiplying your first hole’s figure by the total count.
Follow the specified footing and drainage design. Do not add a gravel or rubble layer, reduce the concrete depth or change the footing dimensions unless the responsible builder or engineer has specified it.
When digging the holes yourself isn’t worth the hassle
The formula works whether you’re digging by hand, with a post hole digger, or with an auger attachment. The limiting factor is usually the digging itself, not the concrete maths. Rocky ground, a long fence line, or more than a handful of holes can turn a weekend job into most of a week with a manual digger.
If that’s where the job’s landed, Diggermate can help with several parts of it:
- Post Hole Digging: excavator-mounted augers for fences, decks, pergolas and retaining walls, where the site and ground conditions suit it
- Fence Post Hole Digging: a dedicated service for boundary and acreage fence lines where several holes need digging to a consistent size
- Deck & Pergola Footing Holes: footing holes dug to your builder or engineer’s marked positions and specifications
Ready to get the holes done, not just the sums?
If the digging itself is the part you’d rather hand over, check the Mini Excavation and Earthworks With an Operator page for what’s involved, then get in touch with your suburb, photos of the access, and the number and size of holes so a local operator can confirm the right machine and auger for the job.
FAQs
How much concrete do I need for a 450mm pier hole?
For a 450mm diameter hole dug 600mm deep, you’re looking at 10 bags before any allowance for waste of standard 20kg premix concrete, before subtracting any volume taken up by the post itself.
Can I use regular premix concrete for pier holes, or do I need a different mix?
Use the concrete mix and strength specified for your project. Structural piers, decks, buildings and retaining walls may have engineering requirements; confirm suitability before choosing bagged premix.
Do pier holes need gravel at the bottom before pouring concrete?
Only include a base layer if it is part of the specified footing design. Confirm the footing dimensions and drainage detail with the responsible builder or engineer, rather than reducing the concrete depth by assumption.
