Post holes / concrete bags
Fence Post Concrete Calculator
One dressed 4x4 post in a 12 in wide, 36 in deep hole needs 2.1 cu ft of concrete, or 6 bags of 50 lb fast-setting mix. We subtract the post's actual 3.5 in square footprint.
Sources checked
Your estimate
6 50 lb bags
2.1 cu ft per post × 1 = 2.1 cu ft (0.078 cu yd) before 0% extra.
- 60 lb bags4.67 before rounding5 bags
- 80 lb bags3.5 before rounding4 bags
0.375 cu ft per fast-setting 50 lb bag. Makers suggest a hole about 3 times the post width (10.5 in here).
How to count concrete for a post hole
- Treat the hole as a cylinder: π × (12 / 2)² × 36 / 1728 = 2.36 cu ft. The same cylinder is counted for round piers and form tubes by the Sonotube concrete calculator.
- Subtract the 3.5 × 3.5 in post through 36 in of depth: 2.1 cu ft around one post. Turn this off for hollow posts filled with concrete or when the post is not yet in the hole.
- For the default, divide 2.1 by 0.375 cu ft per 50 lb bag and round up to 6 bags. Multiply the unrounded volume by the number of posts before rounding bags. For what other bags yield see how many cubic feet in a bag of concrete; for how deep the hole itself goes see how deep to set fence posts.
- Dressed 4x4 post face
- 3.5 inNIST PS 20-25, American Softwood Lumber Standard (Jan 2025)
- Suggested hole diameter
- 3 × post widthQUIKRETE Concrete Mix 1101 data sheet (Oct 2022); Sakrete 5000 Plus Concrete Mix data sheet (2023)
- 50 lb fast-setting bag yield
- 0.375 cu ftQUIKRETE Fast-Setting Concrete Mix 1004 data sheet (Nov 2025)
- 80 lb ordinary bag yield
- 0.6 cu ftQUIKRETE Concrete Mix 1101 data sheet (Oct 2022)
- Water for dry-poured 50 lb bag
- 1 galQUIKRETE Fast-Setting Concrete Mix 1004 data sheet (Nov 2025)
The concrete depth excludes gravel. If the hole is deeper than the maker’s fill-lift limit, follow the product’s mixing and lift instructions.
Bags for a 12 in by 36 in hole
| Post | Concrete | 50 lb fast-setting | 80 lb ordinary | Maker check |
|---|---|---|---|---|
| Dressed 4x4 | 2.1 cu ft | 6 | 4 | 6 bags of 50 lb for nominal 4 in; QUIKRETE Concrete Calculator (run 13 Oct 2026); Sakrete concrete calculators (run 13 Oct 2026) |
Bags per post hole by hole size and depth
| Hole diameter | 24 in deep | 30 in deep | 36 in deep | 48 in deep |
|---|---|---|---|---|
| 10 in | 3 / 2 | 4 / 2 | 4 / 3 | 5 / 4 |
| 12 in | 4 / 3 | 5 / 3 | 6 / 4 | 8 / 5 |
| 16 in | 7 / 5 | 9 / 6 | 11 / 7 | 14 / 9 |
Where sources disagree
QUIKRETE gives two counts for the same 3 in post and 9 in by 30 in hole
- 2 bags on its Fast-Setting data sheetQUIKRETE Fast-Setting Concrete Mix 1004 data sheet (Nov 2025)
- 3 bags in its online calculatorQUIKRETE Concrete Calculator (run 13 Oct 2026)
Our form uses cylindrical hole volume minus the square post, then the bag’s printed yield. The makers may round or allow for loss differently; add extra explicitly if your site needs it.
Fence post concrete questions
How many bags of concrete per fence post?
For one dressed 4x4 in a 12 in hole 36 in deep, 6 bags of 50 lb fast-setting concrete by geometric volume and the bag’s 0.375 cu ft yield, 300 lb of mix. QUIKRETE and Sakrete online calculators each gave 6 bags for a nominal 4 in post in that hole.
How many bags of concrete for 14 fence posts?
14 posts in 12 in holes 36 in deep take 29.41 cu ft, 79 bags of 50 lb fast-setting concrete or 50 bags of 80 lb, rounded once for the whole fence. The number of posts for your layout comes from the fence calculator.
How many bags of concrete for a 6x6 or vinyl fence post?
A dressed 6x6 in a 17 in hole, three post widths wide, takes 11 bags of 50 lb for 36 in of concrete. Bufftech digs 12 in for its 5x5 vinyl post: 5 bags at the same depth.
How many bags of concrete for a mailbox post?
A mailbox post is a 4x4 in a hole, so the same arithmetic applies: in a 10 in hole 24 in deep it takes 0.92 cu ft, 3 bags of 50 lb fast-setting concrete or 2 of 80 lb. Enter your own hole to change it.
Do you subtract the fence post from the concrete?
The default subtracts the 3.5 in square dressed post. In a 10 in by 36 in hole that gives 1.38 cu ft; DIYCalcs reported 1.38 cu ft for a nominal 4x4. Turn subtraction off if you need the full cylindrical volume.
How wide should the hole be?
QUIKRETE and Sakrete print about 3 times the post width. For a 3.5 in dressed 4x4, that is about 10.5 in. Fence system instructions may specify a different diameter.
Does the concrete fill the whole hole?
No: QUIKRETE puts 6 in of gravel under the post, so a hole dug 6 in deeper than the setting depth holds concrete only above it. The box asks for the concrete depth; the depth of the setting itself is on how deep to set fence posts.
Can I use an 80 lb bag instead?
Yes. An 80 lb bag of ordinary concrete mix yields 0.6 cu ft, versus 0.375 cu ft for the selected 50 lb fast-setting bag. The result lists both counts.
A planning aid, not an engineering calculation: your local code and building inspector have the final word. Terms