How much concrete a fence post actually needs
Home & Yard Published October 5, 2026
Two bags of 80 lb mix per post is the number everyone quotes, and for one specific hole it is right. A 10 inch hole, 30 inches deep, with a 4x4 post in it, takes 1.15 cubic feet of concrete, which is 1.9 bags. Round up and you get the rule.
Change the hole and the rule stops holding, because the thing that moves the number most is the one people treat as an afterthought.
Hole width matters more than hole depth
Depth is linear. Go 20% deeper and you need 20% more concrete. Width is not, because the hole is a circle and its area follows the square of the diameter. A 12 inch hole is only two inches wider than a 10 inch one, but it holds 44% more, and once the post has taken its own share the concrete you buy goes up by half.
Here is the same post and the same depth at three common hole sizes, with the post’s own volume already taken out.
- 8 inch hole, 0.66 cubic feet, 2 bags
- 10 inch hole, 1.15 cubic feet, 2 bags
- 12 inch hole, 1.75 cubic feet, 3 bags
That is one bag of difference across a gap most people would call close enough when they are standing over an auger. Across 20 posts it is 20 bags, which is most of a pallet and a second trip.
The post takes its own share back
A 4x4 is not four inches across. Nominal lumber is planed down, so it really measures 3.5 inches, and it displaces 0.085 square feet of the hole’s cross-section. In a 10 inch hole that is about 16% of the volume, which is why a calculator that ignores the post sends you home with an extra bag per two posts.
A 6x6, really 5.5 inches, displaces two and a half times as much. If you are setting heavier posts in the same holes, your bag count per post goes down, not up.
Three times the post width is the rule worth keeping
The convention is a hole roughly three times the post’s width, so 10 to 12 inches for a 4x4 and 16 to 18 for a 6x6. It is about the concrete having enough shoulder to resist the post levering against it in wind, not about the volume.
Below three times, the collar gets thin and cracks. Above it, you are buying concrete to fill a hole you did not need to dig. If you want to see what a change costs before you commit to a hole size, the concrete calculator has a post hole mode that subtracts the post for you and reports the bags at whatever bag size you can actually carry.
Working out your own number
The arithmetic is short enough to do on the back of the bag.
Take the hole’s radius in feet and square it, multiply by pi, and that is the hole’s area. Subtract the post’s cross-section, also in square feet. Multiply what is left by the depth in feet, and you have cubic feet per hole. An 80 lb bag yields 0.6 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.3.
For the 10 inch hole above, that reads as 0.545 square feet of hole, less 0.085 for the post, times 2.5 feet of depth, giving 1.15 cubic feet. At 0.6 per bag, 1.92 bags, so two.
Add 10% before you buy. Holes are never the clean cylinders the formula assumes, a clamshell digger leaves them wider at the top, and the last few inches of a bag always end up on the ground.
Counting the whole fence, not one post
Posts are the part people remember to calculate and the smaller half of the job. A 100 foot run at 8 foot spacing is 13 posts, so around 26 bags at two per post. The fence calculator works out the post count, the spacing and the rails together, which is the number that decides whether you are making one trip or three.
If the total lands past about one cubic yard, which is 45 bags of 80 lb, it is worth pricing a ready-mix delivery against the pallet before you start mixing. Short-load fees usually push the line further out than people expect, but at 30 posts and up it is a real comparison rather than a theoretical one.
What the number cannot tell you
Concrete around a timber post is not a guarantee of anything. Water that gets between the post and the collar has nowhere to drain, and that is where rot starts, which is the argument for gravel at the base and for sloping the top of the collar away from the post.
Dense ground changes it too. In heavy clay a post can stay put with far less concrete than the rule suggests, and in sand it needs more regardless of what the arithmetic says. The bag count is the starting point you bring to the yard, not the thing that decides whether the fence is still standing in ten years.
Questions people ask
How deep should a fence post hole be?
A third of the post's above-ground height is the usual starting point, so a 6 ft fence wants about 2 ft in the ground, and the hole goes below your local frost line if the ground freezes where you are. Gate posts and end posts take more load than line posts and are often set deeper.
Can I pour the concrete into the hole dry and add water after?
Some bagged products are sold for exactly that and say so on the label. Standard ready-mix bags are not, because the water has to reach every part of the mix to hydrate the cement, and a dry pour relies on it soaking down evenly. Follow the bag, not the forum thread.
Do I need gravel at the bottom of the hole?
A few inches of compacted gravel gives water somewhere to go instead of sitting against the end grain of a timber post. It also means slightly less concrete, since the gravel takes up depth that would otherwise be filled.
How long before I can hang the rails?
Most bagged mixes are firm enough to work around in four to six hours and are safe to load after 24 hours, with full strength at 28 days. Cold weather stretches all of those, sometimes by days.