How much concrete per post hole?
Post-hole concrete depends on hole diameter, hole depth, and the number of holes. The calculator treats each hole as a cylinder and converts the volume into bags using the yield you enter.
Hole diameter matters
A small increase in diameter can add a lot of concrete because round-hole volume is based on radius squared. Measure the actual hole diameter, not only the post size.
Fence posts, deck posts, and soil conditions
Loose soil, over-dug holes, corner posts, gate posts, and brace posts can change the final bag count. Use the result as a planning number and verify local depth requirements.
Before you calculate
- Use the yield printed on the bag you plan to buy, because 40 lb, 50 lb, 60 lb, and 80 lb products can differ.
- Measure project area and thickness before converting the volume into bags.
- Keep waste visible for over-excavation, uneven base, mixing loss, and final top-off.
Common mistakes
- Assuming every bag yields the same cubic feet.
- Buying by bag weight without checking volume yield.
- Forgetting that the final answer must round up to whole bags.
Formula
bags = ceil((holeCount * pi * (diameter / 24)^2 * depth * (1 + wastePercent / 100)) / yieldPerBag)
Assumptions
- Defaults represent bollards.
- Use actual hole diameter and concrete-filled depth, not only the post size.
- Post sizing, frost depth, bracing, uplift, and local requirements are separate.
Example
Estimated concrete bags needed: 18 bags
How to use this calculator
- Enter post holes in count.
- Enter hole diameter in in.
- Enter hole depth in ft.
- Enter yield per bag in cu ft.
- Enter waste in %.
- Review the live estimate and compare it with the example result.
- Check the formula, assumptions, product labels, and site conditions before using the Bollard Concrete Calculator result to plan materials.
Before you buy materials
- Round up to full bags and compare the total bag count with ready-mix if the quantity gets large.
- Check the product label for yield, water instructions, working time, and intended use.
FAQ
How many bags do I need for bollard concrete?
Use hole diameter, hole depth, post count, bag yield, and waste, then round up to the buying unit when the result is sold as whole items. In the default example, the result is 18 bags.
How do I calculate concrete for post holes?
Calculate each hole as a cylinder using diameter and depth, multiply by the number of holes, add waste, then divide by bag yield.
Should I use post size or hole diameter?
Use actual hole diameter. The concrete fills the hole around the post, so post size alone is not enough.
How many bags per fence post?
It depends on hole diameter, depth, and bag yield. Larger or deeper holes can require several bags per post.
Does frost depth affect this estimate?
Frost depth can affect required hole depth in some areas. This calculator estimates quantity only; verify local guidance before setting posts.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.