You need 3 bags of concrete per fence post for the common case: an 80 pound bag, a 10 inch hole 3 feet deep, and a 4x4 post standing in it. Across ten posts that is 26 bags, not 30, because buying for a whole run rounds once instead of ten times.
Bags of concrete per fence post versus per project
One hole of that size holds 1.38 cubic feet of concrete, or 1.52 once the usual 10 percent allowance is on it. An 80 pound bag yields 0.6, so the true figure is a little over two and a half bags per post. Every count on this page carries that same 10 percent.
You cannot buy part of a bag, so a single post means 3. Ten posts pool the same fractions into one order and come to 26, about $169 of material. Ten separate roundings would have you carrying four spare bags home.
Estimate the whole run at once. The rounding only happens once at the end, and on a longer fence that is the difference between a tidy order and a stack of leftovers. It is also why a per-post rule of thumb overstates any job with more than a couple of holes in it.
Hole diameter is the expensive dimension
Width costs more than depth does, because volume grows with the square of the diameter while it grows only in step with depth. Across ten holes 3 feet deep:
- 8 inch holes: 15 bags, about $98.
- 10 inch holes: 26 bags, about $169.
- 12 inch holes: 39 bags, about $254.
- 14 inch holes: 55 bags, about $358.
Going from a 10 inch hole to a 12 inch one is two inches of auger and 50 percent more concrete. That is the single most common reason a fence job runs out of bags on the last day: the holes came out wider than planned and nobody re-ran the numbers.
Measure the hole you dug, not the one you planned
Augers wander, soil collapses, and a hole started at 10 inches often finishes closer to 12 at the top. If the first two holes came out wide, re-run the count before the supply run rather than discovering it on hole nine.
Concrete in one hole
cubic feet = π × ( diameter ÷ 24 )² × depth in feet − post volume
A 10in hole 3ft deep is 1.63 cu ft, less 0.26 for a 4x4 post, leaves 1.38 · plus 10% allowance, 1.52
Depth, and the frost line
Depth is usually decided for you. The footing needs to sit below the frost line for your area, or the ground will lift the post as it freezes and drop it unevenly as it thaws, and a fence that was straight in November will not be in March.
Ten holes, 10 inches wide, with a 4x4 in each:
- 2 feet: 17 bags.
- 2.5 feet: 22 bags.
- 3 feet: 26 bags.
- 3.5 feet: 30 bags.
- 4 feet: 34 bags.
The other rule people apply is burying a third of the post. A 6 foot fence on 9 foot posts wants 3 feet in the ground, which lands on the same figure from a different direction. Take whichever is deeper: frost line or a third of the post.
Most people also put a few inches of gravel in the bottom of the hole before the post goes in, so water draining down the post has somewhere to go instead of sitting against the end grain. That displaces concrete too, so subtract it from the depth you are calculating on. Four inches of gravel under a 3 foot hole means you are pouring 2 feet 8, not 3 feet, and across ten holes that is several bags you do not need to buy. The gravel calculator handles that small quantity alongside anything else you are ordering.
The post takes up room
The post is standing in the hole, so it displaces concrete that would otherwise have to be paid for. This is small per hole and not small across a run. Ten holes, 10 inches wide, 3 feet deep:
- No post at all: 30 bags, which is what a plain hole volume gives you.
- A 4x4, actually 3.5 inches: 26 bags.
- A 6x6, actually 5.5 inches: 19 bags.
A 6x6 removes more than a third of the concrete from the same hole. Note the actual dimensions rather than the nominal ones: a 4x4 measures 3.5 inches and a 6x6 measures 5.5, and using the nominal figures overstates the displacement and leaves you short.
Bag size changes the count more than anything
The same ten holes, ordered in different bags:
- 80 pound bags: 26 bags.
- 60 pound bags: 34 bags.
- 40 pound bags: 51 bags.
Those counts come off the yields the manufacturer prints. Quikrete’s concrete mix data sheet gives 0.60 cubic feet from an 80 pound bag, 0.45 from a 60 and 0.30 from a 40.
Identical concrete, and the number nearly doubles. Any bag count you find online is meaningless without the bag size attached to it, and comparing a quote for 26 against a quote for 51 tells you nothing until you know which bag each is counting.
The weight is the real constraint
Twenty-six 80 pound bags is a little over 2,000 pounds, which is a serious load for a car and a long afternoon of lifting whatever you drive. Sixty pound bags are easier to handle one at a time and there are eight more of them to move. Choose for your back and your vehicle, then let the count follow, rather than the other way around.
Dry set or mixed first
Fast-setting mix is often poured in dry and watered in the hole, which suits fence posts because there is no structural pour to get right and no wheelbarrow to clean. Standard mix wants mixing first. Either way the bag count is the same, because the yield printed on the bag is the finished volume, not the dry powder.
The three-times rule, and what it costs
A common guideline sets the hole at three times the post width. It gives a solid footing, and it is worth seeing what it does to an order before committing to it.
- 4x4 in a 10.5 inch hole, 3 feet deep, ten posts: 29 bags, about $189.
- 6x6 in a 16.5 inch hole, same depth and count: 71 bags, about $462.
Moving up a post size under that rule roughly doubles the hole width and more than doubles the concrete, before the cost of the heavier posts themselves. On a long run it is worth pricing both before deciding, and worth knowing that at 71 bags you are into the territory where ready-mix delivery starts to compete on price and certainly on effort.
The post hole concrete calculator takes the hole size, the depth, the post and the number of holes and returns the bag count for the whole run. The fence calculator covers the rails and pickets going on top, and for anything poured rather than set the concrete calculator works in yards instead of bags.
Work out your own fence run
Frequently asked
Three 80 pound bags for a 10 inch hole 3 feet deep with a 4x4 post. Across ten posts it is 26 bags rather than 30, because a whole run rounds up once instead of once per hole.
About 2.5 bags of 80 pound mix for a 10 inch hole 3 feet deep, so 3 bags if you are buying for one post. The 4x4 itself displaces concrete, which is why ten of them take 26 bags where ten empty holes would take 30.
Below the frost line for your area, or one third of the post length, whichever is deeper. For a 6 foot fence on 9 foot posts that is 3 feet, which takes 26 bags across ten 10 inch holes.
A common guideline is three times the post width, so 10.5 inches for a 4x4. Width is expensive: going from a 10 inch hole to a 12 inch one adds 50 percent more concrete, because volume grows with the square of the diameter.
Ten holes of 10 inches by 3 feet with 4x4 posts take 34 bags of 60 pound mix, against 26 of the 80 pound. Any bag count is meaningless without the bag size attached to it.