How Much Concrete Per Fence Post? The Bag Count That Actually Holds
For a typical fence post, plan on 1 to 2 bags of 80-lb concrete: a 10-inch hole dug 2.5 feet deep with a 4×4 post in it takes about 1.4 bags. Multiply by your post count, add 10% so the last hole doesn’t run dry, and that’s the order. Here’s where those numbers come from and when they change.
Bags per post, by hole size
80-lb bags, with the post’s own volume subtracted (a real 4×4 measures 3.5 by 3.5 inches).
| Hole diameter | Depth | Post | Bags per post |
|---|---|---|---|
| 8 in | 2 ft | 4×4 | 0.8 |
| 10 in | 2.5 ft | 4×4 | 1.4 |
| 10 in | 3 ft | 4×4 | 1.7 |
| 12 in | 3 ft | 4×4 | 2.6 |
| 12 in | 3 ft | 6×6 | 2.4 |
| 16 in | 3.5 ft | 6×6 | 4.9 |
Notice the jump from 10 to 12 inches: a hole’s volume grows with the square of its diameter, so two extra inches of width adds more concrete than a foot of depth. Size holes to the job, not to the biggest auger on the rental wall.
Post hole calculator: exact bags for your fence line →
The sizing rules that keep fences straight
- Depth: a third of the post’s above-ground height, 2 feet minimum. A 6-foot privacy fence wants posts 2.5 to 3 feet down.
- Diameter: three times the post width. A 4×4 gets a 10-to-12-inch hole; a 6×6 gets 16 to 18.
- Frost: in freeze country, get below the frost line or winter will lift and rack the fence. Your building department publishes the local depth.
- Gates and corners: upsize those holes one step. They carry all the swing and tension loads.
A 100-foot fence, worked
Posts at 8-foot spacing means 13 posts. At 1.4 bags each that’s 18.2 bags, call it 20 with the safety margin. At $7.50 a bag you’re at $150 in concrete for the run, plus gravel for the hole bottoms (a 6-inch drainage bed per post is cheap insurance against rot). Run your exact count through the calculator, which handles the post deduction and waste in one pass.
Wet mix or dry pour?
The bag makers’ instructions say mix with water, and for anything structural that’s the answer: mixed concrete cures stronger and bonds the post evenly. The dry-pour shortcut (dump the bag in, add water on top) sets up fast and works acceptably for a mailbox or a temporary post, but it cures unevenly and weaker, and on a windy fence line that difference shows up in a few years. Mix it.
Setting posts that stay put
- Put 6 inches of gravel under the post before concrete, so water drains instead of rotting the end grain.
- Crown the concrete at the surface, sloped away from the post, so rain sheds off rather than pooling at the collar.
- Brace posts plumb both directions and leave them braced until the concrete firms, at least a few hours, ideally overnight.
- Wait 24 to 48 hours before hanging rails and panels; the leverage of a full panel can shift a green footing.
Post hole depth chart by fence height
Depth follows the fence, not the shovel’s enthusiasm. The third-of-height rule with a 2-foot floor, in table form:
| Fence height | Hole depth | Hole diameter (4×4 post) | 80-lb bags per post |
|---|---|---|---|
| 4 ft picket | 2 ft | 10 in | 1 |
| 5 ft privacy | 2.5 ft | 10 in | 1.4 |
| 6 ft privacy | 2.5 – 3 ft | 10 – 12 in | 1.5 – 2.6 |
| 8 ft privacy or wind exposure | 3 – 3.5 ft | 12 in | 2.6 – 3 |
In frost country the frost line overrides this table whenever it’s deeper. And gates get one size up on the hole regardless of fence height, because hinges never stop pulling.
Concrete or gravel for fence posts?
Concrete wins on strength and is the default for privacy fences, gates, and anything catching wind. Its weakness is water: a post set in a concrete collar that traps rain will rot at the ground line in 8 to 15 years. Gravel-set posts drain beautifully and are easier to replace, and they’re common on farm and ranch fence, but they give up stiffness on tall solid fences. The best practice borrows from both: 6 inches of gravel at the bottom of the hole for drainage, concrete above it for strength, crowned at the top so water sheds away from the wood.
Fast or standard setting concrete?
Fast-setting mix firms in 20 to 40 minutes and lets you pour it dry into the hole and add water on top per the bag’s own instructions, which makes it the fence-line favorite. It costs a dollar or two more per bag. Standard mix is cheaper per pound and slightly stronger fully cured, but every post needs bracing while it sets. For a long run, most crews pay the fast-set premium just to keep moving; the math flips only on jobs where you’re mixing in a tub anyway.
Quick answers
How many bags of concrete for a 4×4 fence post? One to two 80-lb bags for a typical 10-inch hole at 2 to 2.5 feet. Deeper 12-inch holes for 6-foot privacy fences run 2 to 3 bags.
How many bags for a 6×6 post? A 6×6 wants a 16-to-18-inch hole, which takes 4 to 5 bags at 3 feet deep. The bigger hole, not the bigger post, drives the count.
Can I set a fence post without concrete? Yes, with compacted gravel or foam. Gravel works well for line posts in firm soil; keep concrete for corners, gates, and tall panels.
How long before I can attach the fence? Fast-set: 4 hours for light loads. Standard mix: wait overnight, ideally 24 to 48 hours before panels go on. Rushing this step is how straight fences turn wavy.