About this calculator
How many bags of fast-setting concrete you need to set fence posts, mailbox posts or pergola legs. The post’s own volume is subtracted.
How it’s calculated
Concrete per hole = (hole area − post area) × (hole depth − gravel depth). Multiply by the number of posts, add waste, and divide by the bag yield.
Worked example
With these inputs:
- Number of posts
- 10
- Hole diameter
- 10 in
- Hole depth
- 48 in
- Post shape
- Square
- Post size
- 3.5 in
- Gravel at base
- 4 in
- Product
- Fast-setting concrete 30 kg
- Waste allowance
- 10%
the result is:
- Bags needed
- 38 bags
- Concrete per hole
- 1.7 cu ft
- Total concrete
- 0.53 m³
- Gravel for bases
- 1.8 cu ft
Tips from the trade
- Hole diameter is usually about three times the post width.
- Bury about one third of the post’s total length, and go deeper on exposed or windy sites.
- Gravel in the bottom of the hole lets water drain away from the post end.
- Slope the top of the concrete away from the post so water runs off.
Common questions
How many bags per fence post?
A 4×4 post in a 10" × 48" hole, with 4" of gravel in the bottom, takes about four 30 kg bags. Posts in Canada usually go about 4 ft deep (deeper in Ottawa, Montreal and on the Prairies), below the frost, so they take more concrete than US figures suggest.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the building code adopted in your province before ordering.