How much concrete for a round form?
Round concrete forms are calculated as cylinders. Use inside diameter, height or depth, and count, then convert the total volume to cubic yards or bags.
Diameter has a big effect
Cylinder volume uses radius squared, so a small increase in diameter can noticeably increase concrete quantity. Measure the inside diameter of the form or hole.
Tubes, piers, columns, and holes
Tube forms, piers, columns, and round holes use similar volume math, but their structural requirements can be very different. This page estimates material quantity only.
Before you calculate
- Count only the holes that will receive concrete.
- Measure hole diameter in inches and hole depth in feet before calculating bag count.
- Use the yield printed on the concrete bag, because bag sizes and mixes can vary.
Common mistakes
- Using post size instead of actual hole diameter.
- Forgetting corner, gate, brace, or terminal posts.
- Ignoring loose soil or over-dug holes that increase concrete volume.
Formula
cubic yards = count * pi * (diameter / 24)^2 * height / 27 * (1 + wastePercent / 100)
Assumptions
- The holes are treated as cylinders.
- This estimates volume only; post size and footing depth are separate decisions.
- Loose soil and overdig can increase concrete usage.
Example
Estimated post-hole concrete volume: 0.48 cubic yards
How to use this round concrete calculator
- Count the matching tubes, piers, columns, or round forms in the project.
- Enter the inside diameter in inches, then enter the concrete-filled height or depth in feet.
- Use separate estimates when round forms have different diameters or depths.
- Review cubic feet, cubic yards, 60 lb bags, and 80 lb bags before comparing bagged concrete with ready-mix.
- Confirm form bracing, frost depth, reinforcement, and inspection requirements outside the calculator.
Before you buy materials
- Round bag quantities up, because partial bags are rarely useful for final ordering.
- Check fence layout and local frost-depth guidance before setting final hole depth.
FAQ
What is the example post hole volume cubic-yard result?
Use diameter or radius, height or depth, count, and waste, then calculate the planning result. In the default example, the result is 0.48 cubic yards.
How do I calculate concrete for a sonotube or tube form?
Calculate the cylinder volume from diameter and height, multiply by count, add waste, then convert to cubic yards or bags.
Do I use inside or outside diameter?
Use inside diameter because that is the space the concrete fills.
How do I calculate multiple piers?
Calculate one pier, then multiply by the number of identical piers. Calculate different pier sizes separately.
Does this design a structural pier or column?
No. It estimates concrete quantity only. Structural sizing and reinforcement require qualified guidance.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.