What this calculator does
Concrete per hole is the hole volume less the post volume, since the post occupies part of the space. A 300 mm hole 600 mm deep around a 100 mm post takes 0.036 cubic metres, against 0.042 for the empty hole.
Hole diameter drives the quantity far harder than depth does, because volume goes with the square of the radius. Going from 200 mm to 400 mm diameter multiplies the concrete by more than five, from 9 bags to 45 for six posts. Digging a wider hole than necessary is expensive in a way that digging deeper is not.
The formula
The hole volume is pi times the radius squared times the depth. The post cross-section times the depth is subtracted, since the post displaces that concrete. Multiplying by the number of posts gives the total, and the bag count assumes a 20 kg bag yields about 0.009 cubic metres.
| Term | Meaning |
|---|---|
| Displacement | The volume the post occupies, which is concrete you do not need. |
| Hole diameter | Usually three times the post width, which is the common rule. |
| Rapid-set concrete | Post mix that sets in minutes, often poured dry with water added to the hole. |
| Crown | Sloping the concrete away from the post at the top so water sheds rather than pools. |
The inputs explained
| Field | What to enter |
|---|---|
| Hole diameter (mm) | Hole diameter in millimetres. About three times the post width is the usual guide. |
| Hole depth (mm) | Hole depth. |
| Post width (square post) (mm) | Post width, assumed square. Its volume is deducted. |
| Number of posts | Number of posts. |
| Price per 20 kg bag ($) | Price per 20 kg bag, for the cost estimate. |
When to use it
Buying concrete for a fence
Post mix is bought by the bag, and a run of posts adds up faster than expected.
Comparing hole sizes
A wider hole gives more resistance to racking but costs disproportionately more concrete.
Setting deck posts
The same calculation applies to deck footings set in augered holes.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does hole diameter cost?
The same posts in a range of hole diameters.
| Hole diameter | Concrete needed per post | Total for 6 posts | 20 kg bags needed |
|---|---|---|---|
| 200 mm | 0.013 m³ | 0.077 m³ | 9 |
| 250 mm | 0.023 m³ | 0.141 m³ | 16 |
| 300 mm | 0.036 m³ | 0.218 m³ | 24 |
| 400 mm | 0.069 m³ | 0.416 m³ | 45 |
Questions
How wide should a post hole be?
About three times the post width is the common guide, so 300 mm for a 100 mm post. Wider gives more resistance to the post being pushed sideways, which matters for gates and corners. Deeper generally helps more per unit of concrete than wider does.
Should I deduct the post volume?
Yes, and this does so automatically. On a 300 mm hole around a 100 mm post the post occupies about 14% of the volume. Over a run of posts that is a bag or two, which is worth not buying.
Can I pour the mix in dry?
With rapid-set post mix, yes, which is the usual method: pour the dry mix around the post and add water to the hole. Read the product instructions, since the water quantity and whether to mix first vary between products.
Should the concrete come to the surface?
Slightly above it, crowned and sloped away from the post so water sheds rather than pooling against the timber. Finishing level or slightly below creates a basin that holds water at the exact point the post is most vulnerable to rot.
For how deep to dig, see the fence post depth calculator. For larger concrete pours, see the concrete calculator.