How much concrete does a fence post take?
More than most people guess. A 4×4 post in a hole three times its width and 2 feet deep takes nearly three 50-pound bags, and the number climbs fast as the hole gets deeper:
| Hole depth | Concrete, cu ft | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 24 in | 1.0 | 2.8 | 2.3 | 1.7 |
| 30 in | 1.3 | 3.4 | 2.9 | 2.2 |
| 36 in | 1.5 | 4.1 | 3.4 | 2.6 |
| 42 in | 1.8 | 4.8 | 4.0 | 3.0 |
That table is for a 4×4 post in a 10½-inch hole with no gravel. The calculator handles the other sizes.
The math, if you want to check it
A post hole is a cylinder. Its volume is 3.14 times the radius squared times the depth. Take away the space the post itself fills, and what's left is concrete.
Then divide by what one bag makes. A 50-pound bag makes about 0.375 cubic feet, a 60-pound bag 0.45 and an 80-pound bag 0.6. We round the total up to whole bags, because nobody sells half of one.
How wide and how deep
The concrete maker's own rule is a hole three times as wide as the post, and as deep as a third to a half of the post's height above the ground. For a 6-foot fence that means at least 2 feet down.
In cold places the hole also has to reach below the frost line, or the ground will heave the post up over a few winters. Your building department knows the depth for your town.
Why the gravel
Six inches of gravel in the bottom of the hole gives water somewhere to go. Without it the end of the post sits in a cup of concrete that fills up when it rains, and that's where a wood post rots first.
It also saves concrete, which is why the bag count drops when you tick the box.
Fast-setting or regular mix
Fast-setting concrete is made for this job. You pour it into the hole dry, add water and it hardens around the post in well under an hour. You pay more per pound for the convenience.
Regular mix is cheaper, and you have to mix it with water in a wheelbarrow first. On a long fence the savings add up. So does the mixing.
Either way, plumb the post and brace it before you pour, and slope the top of the concrete away from the post so rain runs off instead of pooling against the wood.
Call 811 a few business days before you dig. It's free, and they come out and mark the buried lines.
Fair questions
Does every fence post need concrete?
No. Plenty of fences are built with posts set in tamped gravel or soil, and metal T-posts on a wire fence are simply driven in. Concrete gives the firmest hold, and it's the norm for gate posts and corners, which carry the most load.
How long before I can hang the fence?
Fast-setting mix holds a post upright within the hour, but give it several hours before you put real weight on it. The bag gives the exact time.
How many bags does my whole fence need?
Enter your post count above. If you don't know it yet, the fence picket calculator counts posts, rails and pickets from the length of the fence.
Sources
- Quikrete, Setting Posts in Concrete: hole width, hole depth and how much concrete a post takes.
- Call 811: the national call-before-you-dig service, for when and how to have buried utilities marked.
