Size the hole to the post
Oversized holes burn bags without much extra holding power. Many manufacturers aim near three times the post width — about a 10″ hole for a 4×4.
DIY fence materials estimate
How many bags of concrete per fence post?
Conclusion: A common 4×4 in a 10″ × 30″ hole, no gravel, 50 lb fast-set + 10% cushion ≈ 4 bags per post (~1.15 cu ft). Adjust the form for your hole.
Post-hole bag estimator
Calculate bags per hole
Your estimate
— bags per post
Enter hole and post details to see the estimate.
Bags per post—rounded up
Total bags—for all posts
Concrete volume—cu ft per post (before cushion)
Assumptions appear here after a valid calculation.
The estimate, explained
How post-hole concrete bags are calculated
Bagged concrete for fence posts is almost always a volume problem: the cylindrical hole, minus the buried post, minus any gravel base, converted to the yield printed on the bag. Partial bags still cost a full bag, so counts round up.
The short version
hole area = π × (diameter ÷ 2)²
concrete depth = hole depth − gravel base
cu ft = (hole area − post area) × concrete depth ÷ 1,728
bags = ceil( cu ft × (1 + waste%) ÷ bag yield )
1,728 cubic inches = 1 cubic foot. Typical printed yields: 50 lb fast-set ≈ 0.375 cu ft; 60 lb ≈ 0.45; 80 lb ≈ 0.60. Confirm your bag label.
Worked example (default on this page)
A 10″ hole that is 30″ deep has a cross-section of about 78.5 sq in. A dressed 4×4 is 3.5″ × 3.5″ (12.25 sq in), so concrete fills about 66.3 sq in of each inch of depth. Over 30″ that is about 1,988 cu in ≈ 1.15 cu ft. With a 10% cushion and 0.375 cu ft per 50 lb bag, that is about 3.37 bags — buy 4 bags per post.
Quick reference
Bags per post — 4×4, no gravel, 10% cushion, 50 lb fast-set
| Hole | Depth | Cu ft (approx.) | Bags to buy |
|---|---|---|---|
| 8″ | 24″ | ~0.53 | 2 |
| 8″ | 30″ | ~0.66 | 2 |
| 10″ | 24″ | ~0.92 | 3 |
| 10″ | 30″ | ~1.15 | 4 |
| 10″ | 36″ | ~1.38 | 5 |
| 12″ | 30″ | ~1.74 | 6 |
| 12″ | 36″ | ~2.09 | 7 |
Rounded up from the same formula as the live calculator. Switch bag size or add gravel in the form for your exact job.
Same 10″ × 30″ hole by bag weight
| Bag | Yield used | Bags / post (10% cushion) |
|---|---|---|
| 40 lb | 0.30 cu ft | 5 |
| 50 lb fast-set | 0.375 cu ft | 4 |
| 60 lb | 0.45 cu ft | 3 |
| 80 lb | 0.60 cu ft | 3 |
Fewer trips to the store
Practical tips before you mix
Read the bag yield
Fast-set and standard mixes list slightly different cubic-foot yields. If your pallet is 80 lb bags, switch the dropdown so you do not overbuy 50 lb counts.
Check frost and set method
Depth, gravel, and whether you dry-pack or slurry-mix must follow local frost lines and the instructions on the bag. This page only estimates quantity.
Common questions
Fence post concrete FAQs
How many bags of concrete do I need per fence post?
It depends on hole diameter, depth, post size, gravel, and bag weight. The default example on this page — a 4×4 in a 10″ × 30″ hole with 50 lb fast-set and a 10% cushion — is about 4 bags per post.
Do I subtract the post from the hole volume?
Yes. The buried post displaces concrete. This calculator subtracts the actual post cross-section before converting to cubic feet and bags.
How deep should a fence post hole be?
A common rule of thumb is about one-third of the above-ground length, then adjust for local frost depth and wind exposure. Enter the depth you will dig; this tool does not prescribe frost depth.
Should I put gravel under the post?
Many installers add a few inches of gravel for drainage. If you do, enter that depth so the concrete column is shortened and bags are not over-counted.
What bag size should I buy?
Fast-setting mixes are often 50 lb (~0.375 cu ft). Standard mixes commonly come in 60 lb and 80 lb bags. Always confirm the printed yield and round up.
Is this structural or frost-line advice?
No. It is a materials quantity estimate only — not a substitute for local code, manufacturer instructions, or a structural engineer.
Planning estimate only. Results depend on the hole you dig, the mix you buy, and how you place it. Not engineering, code, or frost-line advice.
Sources & method notes
- Volume math: cylinder hole area minus post cross-section, times concrete depth, ÷ 1,728 → cubic feet.
- Bag yields used as typical U.S. printed values (confirm on your bag): 40 lb ≈ 0.30 cu ft; 50 lb fast-set ≈ 0.375; 60 lb ≈ 0.45; 80 lb ≈ 0.60. Manufacturer data sheets (e.g. Quikrete / Sakrete post-setting tables) publish similar hole→bag examples.
- Hole sizing guidance is a common industry rule of thumb (~3× post width); local frost depth and wind loads are site-specific.
Last updated 2026-10-16 (Asia/Shanghai). Independent tool — not affiliated with Quikrete, Sakrete, or any retailer.