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
- 30 in
- Post shape
- Square
- Post size
- 3.5 in
- Gravel at base
- 4 in
- Product
- Fast-setting 50 lb (0.375 cu ft)
- Waste allowance
- 10%
the result is:
- Bags needed
- 30 bags
- Concrete per hole
- 1 cu ft
- Total concrete
- 0.41 cu yd
- 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" × 30" hole takes about 1.5 to 2 bags of 50 lb fast-setting mix.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and local building codes before ordering.