Concrete Holes Calculator Guide - Round Holes & Bags
Calculate concrete for round holes, post holes, pier holes, and footing holes by diameter, depth, hole count, gravel depth, bags, yards, and cost.
A concrete holes calculator estimates the cylinder volume inside round holes: post holes, pier holes, small caisson holes, sign posts, gate posts, and deck support holes. The key is to use the actual dug diameter and concrete-filled depth, not only the auger size printed on a plan.
Use the Concrete Post Hole Calculator for round hole inputs. If the hole is a structural pier or deck footing, use the result as a planning check and confirm the design with a qualified local professional.
Competitor pages such as ConcreteCalculatorMax's post hole calculator and ConcreteCalculator.pro's fence post calculator cover post-hole quantity intent. This guide handles the broader "concrete holes calculator" query and keeps post, pier, and gravel assumptions visible.
Quick answer
For one round hole:
cubic feet = 3.1416 x radius ft x radius ft x depth ft
If diameter is measured in inches:
radius ft = diameter in / 24
A 12 in diameter hole that is 36 in deep needs about 2.36 ft3 before waste, or 2.59 ft3 with 10% waste. That rounds to about 5 common 80 lb bags.
Common round hole examples
These examples use 10% waste and common bag yields.
| Hole size | Concrete with waste | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 8 in dia x 24 in deep | 0.77 ft3 | 2 | 2 | 3 |
| 10 in dia x 30 in deep | 1.50 ft3 | 3 | 4 | 5 |
| 12 in dia x 36 in deep | 2.59 ft3 | 5 | 6 | 9 |
| 14 in dia x 42 in deep | 4.12 ft3 | 7 | 10 | 14 |
| 16 in dia x 48 in deep | 6.14 ft3 | 11 | 14 | 21 |
For multiple holes, multiply the concrete-filled volume by hole count before rounding up to whole bags.
What to enter
| Input | Use this value |
|---|---|
| Diameter | The actual dug diameter after soil breakaway, not only planned auger size. |
| Depth | The concrete-filled depth, subtracting gravel if gravel is used. |
| Hole count | Group similar holes together. |
| Waste | 10% is a useful early planning buffer. |
| Bag yield | Use the yield printed on the exact bag. |
| Larger holes | Calculate gate, corner, deck, or pier holes separately. |
For fence projects, see the Fence Post Concrete Calculator Guide. For deck piers, see the Deck Footing Contractor Quote Checklist.
Post displacement
Some calculators subtract the post volume from the hole. That can help on a large repeated project, but over-digging and loose soil can matter more than post displacement. For early buying, many DIY estimates use the full hole volume, then round to whole bags.
If the estimate is tight, calculate both:
net concrete = hole volume - post volume
Then compare the result with the actual dug hole and a practical waste factor.
FAQ
How many bags do I need for a concrete hole?
Calculate cylinder volume from diameter and depth, add waste, then divide by bag yield. A 12 in x 36 in hole with 10% waste is about 5 common 80 lb bags.
Should I subtract gravel depth?
Yes, if gravel replaces concrete at the bottom. Use only the concrete-filled depth in the concrete volume calculation.
Should I subtract the post from the hole?
You can, but for rough buying the full hole volume is often safer because real holes are oversized, uneven, or partly collapsed.
Is a post hole the same as a pier hole?
The volume formula may be similar, but structural requirements are different. Pier size, depth, reinforcement, frost, and loads need review by a qualified local professional.
Can I use ready-mix for many holes?
For many large holes, ready-mix may beat bag labor. Compare the total cubic yards, truck access, chute reach, wheelbarrow distance, and timing.
Next step
Use the Concrete Post Hole Calculator for the volume, then compare bags, ready-mix, and installed bids in the Concrete Quote Reviewer. Contractors can write the scope in the Concrete Proposal Kit.
Quote planning next step
Turn this guide into a concrete buying check
Run the matching calculator, then compare ready-mix, bagged concrete, delivery fees, access needs, and quote gaps before you buy materials or approve a contractor number.