How Many Bags of Concrete for a Post Hole?
Estimate how many bags of concrete a post hole needs by hole diameter, depth, post size, bag yield, waste, and fence or deck post scope.
The number of concrete bags for a post hole depends on hole diameter, depth, post size, bag yield, and waste. A larger hole can use far more concrete than a small fence post hole, so it is worth calculating before buying bags.
Use the Post Hole Concrete Calculator for the exact count. If the post holes are part of a contractor quote, review the scope in the Concrete Quote Reviewer.
Competitor pages such as ConcreteCalculator.pro's fence post calculator and ConcreteCalculatorMax's post hole calculator listing show that post-hole bag intent is a common search path. This page answers the bag-count question directly.
Quick answer
A 10 inch diameter hole, 30 inches deep, around a 4x4 post often needs about 2 bags of 80 lb concrete or about 3 bags of 60 lb concrete after rounding. A 12 inch diameter hole at the same depth can need about 3 bags of 80 lb mix.
| Hole size | Approx. concrete after post displacement | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 8 in dia x 24 in deep, 4x4 post | 0.64 ft3 | 2 | 2 |
| 10 in dia x 30 in deep, 4x4 post | 1.15 ft3 | 3 | 2 |
| 12 in dia x 30 in deep, 4x4 post | 1.94 ft3 | 5 | 4 |
| 12 in dia x 36 in deep, 6x6 post | 2.37 ft3 | 6 | 4 |
These are planning counts. Post depth, frost, soil, wind load, fence height, deck loads, and local code can change the correct hole size. Confirm critical posts with a qualified local professional.
Post hole bag formula
hole volume ft3 = 3.1416 x radius ft x radius ft x depth ft
post displacement ft3 = post width ft x post depth ft x buried depth ft
concrete ft3 = hole volume - post displacement
bags = concrete ft3 / bag yield ft3
Then add waste and round up to whole bags.
Common planning yields:
| Bag size | Common yield |
|---|---|
| 40 lb | 0.30 ft3 |
| 60 lb | 0.45 ft3 |
| 80 lb | 0.60 ft3 |
For yield details, see the Concrete Bag Yield Chart.
Example: 10 inch hole, 30 inches deep
For a 10 inch diameter hole, 30 inches deep, with a 4x4 post:
| Step | Result |
|---|---|
| Hole volume | about 1.36 ft3 |
| 4x4 post displacement | about 0.21 ft3 |
| Concrete volume | about 1.15 ft3 |
| 80 lb bags before waste | 1.92 bags |
| Planning purchase | 2 bags |
If the hole is irregular, over-dug, or bell-shaped, buy extra or calculate the larger shape.
Buying checklist
- Confirm hole diameter and depth.
- Confirm post actual size.
- Check the yield printed on the bag.
- Keep bags dry before use.
- Do not rely on bag count to choose structural post depth.
- Confirm local fence, deck, frost, and wind requirements.
FAQ
How many bags for one fence post hole?
Many common fence post holes need 1 to 3 bags, but the count depends on hole diameter, depth, post size, and bag yield.
How many 80 lb bags for a 10 inch post hole?
A 10 inch diameter by 30 inch deep hole around a 4x4 post often rounds to about 2 bags of 80 lb mix.
Should I subtract the post from the hole volume?
Yes for a better estimate. The buried post displaces concrete, especially in smaller holes.
Do deck posts use the same bag count?
Not necessarily. Deck footings can have different diameter, depth, bell shape, inspection, and load requirements.
Can I use this for structural posts?
Use it for material planning only. Structural post sizing and footing depth should follow plans, code, and qualified local guidance.
Next step
Run the exact hole size in the Post Hole Concrete Calculator, then review the contractor scope in the Concrete Quote Reviewer. Contractors can write the post-hole 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.