How Much Concrete for Fence Posts, Deck Footings and Sonotubes

A 4x4 fence post in the usual 10 inch hole, 30 inches deep, takes 3 bags of 80 lb concrete mix, or 4 bags of 60 lb, and a ten-post run takes 30 bags of 80 lb. A 10 inch Sonotube pier filled 48 inches deep takes 4 bags of 80 lb; a 12 inch pier takes 6. Every count carries a 10 percent buffer and assumes the hole is filled to the top around the post.

Slab / FootingRound Column / Post

Diameter (in)

Height / Depth (ft)

Waste buffer (%)

Bag size

40 lb50 lb60 lb80 lb

Price per cubic yard ($ (optional))

Volume

1.4 cu ft

Ready-mix (incl. waste)

0.06 cu yd

80 lb bags needed

3

Estimated cost

—

Bags needed by bag size

Bag size	Bags needed
40 lb	5
50 lb	4
60 lb	4
80 lb	3

Includes a 10% waste buffer. Ready-mix concrete is typically sold in quarter- or half-yard increments. Round up when ordering.

The hole comes first, and two rules set it

A post hole is the one concrete job where most of the quantity is decided with a shovel. The volume is a cylinder, so the diameter is squared in the arithmetic: widening a hole from 8 to 12 inches more than doubles the concrete at the same depth, while deepening it by half only adds half. Two rules of thumb set the hole, and both are older than any calculator.

Width: about three times the post. A nominal 4x4 measures 3.5 inches across, so three times is 10.5 inches, which is why 10 and 12 inch augers are the common sizes for fence work. An 8 inch hole around a 4x4 leaves a couple of inches of concrete on each side. That works for a low line post in firm, well-drained soil, but it gives a tall panel in wind very little to lever against. A 6x6 at 5.5 inches wants about 16 inches on the same rule, though 12 inches is commonly used where the post carries a pergola or deck load straight down rather than a fence load sideways.

Depth: about a third of what stands above ground, and below frost. A post that will show 6 feet goes 2 feet into the ground as a minimum, and 30 inches is a comfortable figure for a 6 foot privacy fence. Where the ground freezes, the bottom of the concrete has to sit below the frost line, and that is not a rule of thumb at all. Your local building department publishes it, and across the northern states it can be 36, 42 or 48 inches, which is why the deeper holes on this page exist.

The hole count is the other half of the arithmetic, and it comes from the fence layout rather than from this page. The fence calculator works posts out from the run length and panel spacing; multiply its post count by the per-hole figures below.

4x4 fence posts, hole by hole

The figures are the full cylinder with a 10 percent buffer, rounded up to whole bags. A 50 lb bag yields 0.375 cubic feet, a 60 lb bag 0.45 and an 80 lb bag 0.6.

Hole for a 4x4	Concrete with 10% buffer	50 lb bags	60 lb bags	80 lb bags
8 in wide, 24 in deep	0.77 cu ft (0.03 cu yd)	3	2	2
10 in wide, 30 in deep	1.5 cu ft (0.06 cu yd)	4	4	3
12 in wide, 36 in deep	2.59 cu ft (0.1 cu yd)	7	6	5

The 8 inch hole at 24 inches: line posts in good soil

This is the least concrete a 4x4 can reasonably be set in: 0.7 cubic feet before the buffer, 2 bags of 80 lb after it. It suits a low picket or garden fence in good ground, and a mailbox post or lamp post is the same hole with the same count. What it is not is a privacy fence footing. A 6 foot solid panel is a sail, and 2 inches of concrete on each side of the post is not enough to hold it.

The 10 inch hole at 30 inches: the privacy fence standard

The working size for a 6 foot privacy fence on a 4x4, and the hole the widget above is set to. It holds 1.36 cubic feet, 1.5 with the buffer, and that lands at 3 bags of 80 lb. The 60 lb and 50 lb counts are both 4, but they are not equal: four 50 lb bags yield exactly the 1.5 cubic feet with no slack at all, while four 60 lb bags leave 0.3 cubic feet spare. If you buy 50s for this hole, buy one extra for the run.

The 12 inch hole at 36 inches: gates, corners and the frost line

Gate posts, corner posts and end posts take the pull of the whole run, and 36 inches is also where the frost line lands across a good part of the country. The cylinder is 2.36 cubic feet, 2.59 buffered, which is 5 bags of 80 lb. A gate post therefore costs nearly twice the concrete of a line post. On a fence with two gate posts and eight line posts in 10 inch holes, that is 10 bags for the gates and 24 for the line, 34 bags of 80 lb in all.

6x6 posts: pergolas, freestanding decks and deep frost

Hole for a 6x6	Concrete with 10% buffer	50 lb bags	60 lb bags	80 lb bags
12 in wide, 36 in deep	2.59 cu ft (0.1 cu yd)	7	6	5
16 in wide, 42 in deep	5.38 cu ft (0.2 cu yd)	15	12	9

A 6x6 in a 12 inch hole is the same cylinder as the 4x4 gate post, so the counts match; the only difference is how much of the hole the post itself occupies, which is covered further down. The 16 inch hole at 42 inches is the deep-frost version for a pergola post or a freestanding deck post: 4.89 cubic feet, 5.38 with the buffer, 9 bags of 80 lb for one post. Ten of them is 90 bags, which is 2 cubic yards, and that is past the point where a short load of ready-mix is worth pricing. The bags of concrete per yard guide has the crossover.

One caution on decks. A 6x6 buried in concrete is a fence detail. Deck framing conventions put the post on top of a pier with a metal post base, so that the wood stays out of the ground and can be replaced. That is what the Sonotube section covers, and the deck calculator counts those piers from the beam span. The boards that go on top are a separate count again, and the decking guide tables them for 10x10 to 16x20 decks at 12 and 16 ft lengths.

Ten posts at a time

Hole	50 lb bags	60 lb bags	80 lb bags
4x4, 8 in x 24 in	30	20	20
4x4, 10 in x 30 in	40	40	30
4x4, 12 in x 36 in	70	60	50
6x6, 12 in x 36 in	70	60	50
6x6, 16 in x 42 in	150	120	90

The totals multiply the rounded per-hole count by ten, because that is how a fence actually gets set: one hole at a time, each with its own partial bag. Thirty bags of 80 lb is 2,400 lb in the bed of a truck, so check the payload rating before loading a whole run in one trip.

If you mix in a drum and pour hole after hole without stopping, the partial bags carry over and you can round once at the end instead. Ten 10 inch holes are 15 cubic feet with the buffer, and 15 divided by 0.6 is 25 bags of 80 lb rather than 30. Buy the 30 anyway. A hole short by a third of a bag is a hole you dig the concrete back out of, and the difference is a few dollars.

Sonotube piers at 48 inches: 8, 10, 12 and 16 inch tubes

Forty-eight inches is a convenient reference length: enough to reach below a 36 or 42 inch frost line with a few inches standing above grade. The tube is the pier only. In cold regions a pier usually stands on a spread footing poured below frost, and that pad is a separate pour that the concrete calculator handles in slab mode.

Tube	Concrete with 10% buffer	Cubic yards	50 lb bags	60 lb bags	80 lb bags	Ten piers, 80 lb bags
8 in	1.54 cu ft	0.06	5	4	3	30
10 in	2.4 cu ft	0.09	7	6	4	40
12 in	3.46 cu ft	0.13	10	8	6	60
16 in	6.14 cu ft	0.23	17	14	11	110

The 8 inch tube is a light porch or landing pier, 1.4 cubic feet unbuffered, and it takes the same 3 bags of 80 lb as the 10 inch fence post hole despite being a different shape. The 10 inch tube is the typical deck pier. The 12 inch tube under a beam is exactly pi cubic feet before the buffer, since a 1 foot diameter cylinder 4 feet tall is pi times 0.25 times 4, and it rounds to 6 bags of 80 lb. The 16 inch tube belongs under a heavy beam or a hot tub, and at 11 bags of 80 lb per pier it is the first size where mixing by hand gets tedious: ten of them is 110 bags.

Ten 12 inch piers is 60 bags of 80 lb, or 1.3 cubic yards. That is the point to price a short load against a pallet of bags.

The post takes up room the table ignores

Every figure above fills the whole cylinder, as if the hole were empty. It is not: the post is in it. A 4x4 displaces about 0.085 cubic feet for each foot it is buried, and a 6x6 about 0.21 cubic feet per foot.

Run that on the 10 inch hole. A 4x4 buried 2.5 feet takes out 0.085 times 2.5, roughly 0.21 cubic feet, which is more than the 0.14 cubic feet the buffer added to that hole. So for a 4x4 the buffer and the displacement roughly cancel, and 3 bags of 80 lb is a safe count rather than a tight one. For a 6x6 buried 3 feet the post takes out 0.21 times 3, about 0.63 cubic feet, roughly a quarter of the 2.36 cubic foot cylinder. The concrete actually placed is then about 1.73 cubic feet, and the 5 bags of 80 lb in the table hold 3, so the full-cylinder count leaves about two bags spare on each hole; 4 bags would cover it with the buffer intact. The right side to err on is still the generous one. Nobody has ever regretted the spare half bag; plenty of people have regretted the hole that was one scoop short with the post braced and the mix setting.

The 10 percent figure itself is the same allowance the whole site uses for concrete, and material waste factors explains what it covers and what it does not.

Gravel at the bottom of the hole

Before any concrete goes in, 3 to 6 inches of gravel at the bottom of the hole gives water somewhere to go. End grain sitting in a concrete cup rots from the bottom, and a bed of gravel under the post is the usual defence. It changes the arithmetic in one of two ways: either the hole is dug deeper by the gravel depth so the concrete column keeps its full height, or the hole stays the same and the concrete column loses a few inches at the bottom. The first is the better fence.

The gravel itself is a small quantity. The floor of a 10 inch hole is about 0.55 square feet, so 4 inches of gravel is around 0.18 cubic feet per hole, under 2 cubic feet for ten holes. The gravel calculator turns that into bags or a fraction of a yard, and a clean washed stone such as pea gravel or 3/4 inch stone drains better than crusher run here.

Finish the top of the concrete to match: sloped away from the post so rain runs off instead of pooling against the wood.

Fast-setting mix, standard mix, and when a dry pour is acceptable

Standard concrete mix is mixed with water in a wheelbarrow or a drum, placed stiff, and rodded to knock out voids. It is the cheaper bag and the yield the counts on this page assume, and it sets slowly, so the post has to be braced plumb for hours.

Fast-setting mix sets in roughly 20 to 40 minutes, so a post can be plumbed, braced briefly and left while you move to the next hole, which is how a ten-post run gets set in a day. It costs more per bag. The yield per bag is typically close to standard mix, so the counts above transfer, but confirm the yield printed on the bag.

Fast-setting mix is also the only product that should be dry poured. Its manufacturers allow pouring the dry mix around a braced post and adding water on top, about a gallon per 50 lb bag, and that is acceptable for a fence post. Standard mix dry poured hydrates unevenly, with a crust at the top and dry powder lower down, and never reaches its rated strength. Footings and piers that carry a deck, a porch roof or a pergola should always be mixed, whatever the bag says, and many inspectors will not pass a dry-poured footing under a structure. If a footing is being inspected, mix it.

Working the table backwards: what one bag sets

An 80 lb bag yields 0.6 cubic feet. The floor of an 8 inch hole is about 0.35 square feet, so one bag fills roughly 1.7 feet, call it 20 inches, of an 8 inch hole. The smallest hole on this page holds 0.7 cubic feet before any buffer. No hole here is a one-bag hole.

The useful reverse figures are fractions of the unbuffered cylinder. One 80 lb bag is about 85 percent of an 8 inch mailbox hole (0.6 of its 0.7 cubic feet), a little under half of a 10 inch privacy fence hole (0.6 of 1.36), a quarter of a 12 inch gate post hole (0.6 of 2.36) and about a fifth of a 12 inch Sonotube at 48 inches (0.6 of 3.14). Counted in bags rather than volume, it is one of the 2, 3, 5 and 6 bags those holes take once the buffer and the rounding are in. A pallet is typically about 42 bags of 80 lb, though that varies by retailer, which is 14 privacy fence posts or 7 deck piers at 12 inches with no spare.

For a budget, count the holes, pick the row, multiply, and round up to whole bags at the register. If the total passes about a cubic yard, 45 bags of 80 lb, run the same volume past a ready-mix plant as well, because at that size the bags stop being the cheap option and start being the heavy one.