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.
Concrete bag planning table
Uses common planning yields: 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag. Verify the product label.
| Concrete volume | Cubic feet | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 0.25 cu yd | 6.75 cu ft | 12 bags | 15 bags |
| 0.50 cu yd | 13.5 cu ft | 23 bags | 30 bags |
| 1.00 cu yd | 27 cu ft | 45 bags | 60 bags |
| 2.00 cu yd | 54 cu ft | 90 bags | 120 bags |
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 a tube forms post-hole example.
- Hole diameter and depth should match the actual dug holes, not just the post size.
- Post sizing, uplift, bracing, frost depth, and inspection 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 Sonotube Bag 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 sonotube bag?
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 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.