Concrete & Masonry
Fence Post Concrete Calculator
Enter the number of posts, the hole width and depth, the post size and the gravel layer. You get bags per hole and the total to buy.
Worked example
- Concrete fills 30 - 4 = 26 in of the hole.
- Hole: 3.1416 x 5^2 x 26 = 2,042 cu in; the post takes 319 cu in, leaving 1,724 cu in = 1.00 cu ft.
- 1.00 / 0.375 cu ft per 50 lb bag = 2.66 bags per hole; x 10 posts = 27 bags (rounded up).
Common sizes at a glance
| Size | 50 lb bags in total | Per hole |
|---|---|---|
| 4 x 4 post, 10 in hole, 24 in deep | 3 | 2.05 bags |
| 4 x 4 post, 10 in hole, 36 in deep | 4 | 3.27 bags |
| 6 x 6 post, 12 in hole, 36 in deep | 5 | 4.09 bags |
| 6 x 6 post, 16 in hole, 42 in deep | 11 | 10.02 bags |
| Chain link line post, 8 in hole, 24 in | 2 | 1.70 bags |
The post takes up space
A hole is a cylinder, but you are not filling all of it. The post sits in the middle and a layer of gravel sits at the bottom for drainage. Both are subtracted before we count bags.
In the worked example, a 10 in hole dug 30 in deep with 4 in of gravel leaves 26 in for concrete. A 4 x 4 post (3.5 in actual) takes 319 cu in of that, leaving about 1 cu ft per hole.
Hole width and depth
Width matters more than people expect, because volume grows with the square of the diameter. Going from a 10 in to a 12 in hole adds nearly half again as much concrete per hole.
Depth is set by the post height and by frost depth where you live. Gate posts and corner posts usually go wider and deeper than line posts. Local code has the final word on depth.
Dug holes are never exact
A post hole digger or auger rarely leaves a neat cylinder. Holes flare at the top and wander in rocky soil. Measure a few holes after digging, not before, and use the widest number you find.
Since the calculator rounds up to whole bags on the total, buying one or two extra bags covers most odd holes.
Bag size and setting method
- 50 lb bags yield about 0.375 cu ft, so the example needs 2.66 bags per hole.
- 80 lb bags yield about 0.6 cu ft and cut the bag count for larger holes.
- Fast-setting post mix is poured dry and soaked, but the volume per bag is what counts here.
- Slope the top of the concrete away from the post so water runs off.
Plan the whole fence
To know how many holes to dig, start with the fence post calculator. For the rest of the material list, the fence calculator covers rails and pickets.
Questions
How many bags of concrete per fence post?
For a 4 x 4 post in a 10 in hole 24 in deep, about 2 bags of 50 lb. At 36 in deep it is about 3.3 bags.
How wide should a post hole be?
A common rule of thumb is about three times the post width, but follow local code and the fence maker's instructions.
Do I need gravel under the post?
A few inches of gravel helps water drain away from the post end. The calculator subtracts that layer from the concrete.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.