How Many Bags

How many bags of concrete each fence post takes

A 4x4 post in a 10 inch hole 30 inches deep needs 1.15 cubic feet of concrete once the post's own volume is taken out. That is 3.07 bags of 50 lb fast-setting mix at the 0.375 cubic foot yield printed on the bag. So four bags are opened, or two 80 lb bags. Sakrete's own chart for the same hole says 3 bags, which matches rounding to the nearest quarter bag at its 0.38 cubic foot yield.

The concrete calculator's post-hole shape: any post, hole and depth, with the post subtracted

By the How Many Bags team. Bag yields, hole-sizing rules and the one maker chart come from the Quikrete and Sakrete data sheets and project pages, and post depths from conservation-service fence specifications and two university extension pages. Every bag count was checked against those documents. Last updated on . How these pages are made.

In brief
  • Concrete per post = hole volume minus post volume; bags = that volume ÷ 0.375 cu ft (50 lb fast-setting), 0.45 (60 lb) or 0.60 (80 lb), rounded up.
  • Both makers say to dig the hole three times the post width; Quikrete fills to 3 to 4 inches below grade over 6 inches of gravel.
  • Depth rules differ: half the above-ground height (Quikrete sheets), a third to a half plus 6 in (Quikrete project page), a third of the post length (Sakrete). A 6 ft fence lands at 2 to 3 ft.
  • Forty 4x4 posts in 10 in holes 30 in deep take 136 bags of 50 lb mix with 10 percent extra.

The arithmetic the makers use

Concrete fills the hole around the post, not the whole hole. The volume to buy is the cylinder of the hole minus the post inside it. The bag count is that volume divided by the yield printed on the bag. The yield is 0.375 cubic feet for a 50 lb bag of Quikrete fast-setting mix (Sakrete prints 0.38). It is 0.45 for a 60 lb bag and 0.60 for an 80 lb bag of standard mix. Quikrete's own online calculator does exactly this subtraction, and Sakrete's chart for a 10 inch hole matches hole-minus-post to the quarter bag. A nominal 4x4 post is 3 1/2 inches square and a 6x6 is 5 1/2, the dressed sizes in the lumber standard. So a 4x4 in a 30 inch hole displaces 0.21 cubic feet, half a bag. That is why counts that ignore the post come out one bag high.

Section through a fence post hole: gravel at the bottom, the post on the gravel, concrete to just below grade Post hole, side view, not to scale 4x4 post 10 in across 30 in deep 6 in gravel concrete stops 3 to 4 in below grade grade
A 4x4 in a 10 inch hole 30 inches deep, set the way Quikrete's sheet describes: 6 inches of gravel, the post standing on it, concrete stopped 3 to 4 inches below grade. Filled to the brim, the hole holds 1.15 cubic feet of concrete around the post, the calculator's figure; the maker's shorter fill is why its own example needs fewer bags.

Bags per post, by post, hole and depth

Counts are rounded up to whole bags per hole, with the hole filled to the top. The 50 lb column is fast-setting mix; 60 and 80 lb are standard mix.

Post Hole Depth Concrete 50 lb bags 60 lb bags 80 lb bags
4x4 8 in 24 in 0.53 cu ft 2 2 1
4x4 10 in 24 in 0.92 cu ft 3 3 2
4x4 10 in 30 in 1.15 cu ft 4 3 2
4x4 10 in 36 in 1.38 cu ft 4 4 3
4x4 12 in 30 in 1.75 cu ft 5 4 3
4x4 12 in 36 in 2.10 cu ft 6 5 4
6x6 12 in 30 in 1.44 cu ft 4 4 3
6x6 12 in 36 in 1.73 cu ft 5 4 3
6x6 14 in 36 in 2.58 cu ft 7 6 5
6x6 16 in 36 in 3.56 cu ft 10 8 6
2 3/8 in pipe 8 in 24 in 0.64 cu ft 2 2 2
2 3/8 in pipe 10 in 30 in 1.29 cu ft 4 3 3

The 4x4 in a 10 inch hole at 30 inches is 1.15 cubic feet, 3.07 bags of 50 lb. The table rounds it to 4 because a hole cannot take a fraction of a bag. Sakrete's chart prints 3 for the same hole, consistent with rounding to the nearest quarter bag at its 0.38 cubic foot yield (3.03). Its 18 inch row prints 2 where that arithmetic gives 1.82. Across a fence, whole-fence rounding is the honest count. Forty of those posts take 46 cubic feet, or 50.6 with 10 percent extra. That is 136 bags of 50 lb, not 160. Bags of concrete in a yard has the per-yard counts for each bag size.

Why Quikrete's example says two bags

Quikrete's data sheet says two 50 lb bags set a 3 inch square post in a 9 inch hole 30 inches deep. Filled to the brim that hole is 0.95 cubic feet, 2.5 bags. The sheet's own method stops the dry mix 3 to 4 inches below grade so the top can be backfilled with soil. At 26.5 inches of fill the volume is 2.2 bags, which rounds to the two it prints. Sakrete's 10 inch hole chart, 1.25 bags at 12 inches, 2 at 18, 2.5 at 24 and 3 at 30, follows the same arithmetic. The table here fills to the top, so it runs a little high and never short.

The depth rules, side by side

Both makers size the hole at three times the post width. That is 10 1/2 inches for a 4x4, about 16 for a 6x6, which is wider than most people dig. On depth they disagree with each other and with themselves. Quikrete's data sheets say half the above-ground height, or a third of the post's full length, which is the same rule. A 6 ft fence gets a 9 ft post set 3 ft deep. Its project page says a third to a half of the above-ground length plus 6 inches, which is 2 1/2 to 3 1/2 ft. Sakrete's sheet says a third of the post's length, 2 ft 8 in for an 8 ft post. Its web page says a third of the above-ground height, 2 ft for a 6 ft fence.

The conservation-service fence specifications written for livestock fences give fixed depths instead. For line posts it is 24 inches in ordinary soil, 18 in rock and 30 in sand. For corner, gate and end posts it is 36 to 42 inches. The North Carolina version notes that a corner assembly set 2 1/2 ft deep has about half the strength of one set at 3 1/2. Texas's version buries every post at least 3 ft. Penn State's horse-fence guidance is 36 inches for line posts and about 48 for corners and gates. Below the frost line is the floor everywhere it freezes, and the deeper hole is the one to buy bags for. A 4x4 in a 10 inch hole goes from 3 bags at 24 inches to 4 at 36.

Gravel, fill height and the top of the hole

Quikrete's method puts 6 inches of gravel in the bottom of the hole for drainage and fills concrete to 3 to 4 inches below grade. Sakrete calls for 4 to 6 inches of compacted gravel. The gravel reduces the concrete depth without changing the hole depth. So a 36 inch hole with 6 inches of gravel and the fill stopped 3 inches down holds 27 inches of concrete. The Missouri conservation specification wants the concrete to extend at least 6 inches beyond the post on every side. For a 4x4, that is a 15 1/2 inch hole. The Texas specification allows the top 12 inches to be soil. The calculator's post-hole shape takes the hole diameter, the concrete depth and the post size separately. So the gravel and the unfilled top come straight off the order.

Fast-setting or standard mix

Fast-setting mix is poured dry into the hole and watered, about a gallon per 50 lb bag by the Quikrete sheet. It sets in 20 to 40 minutes and takes load after 4 hours. Standard mix is mixed wet and needs bracing while it cures. For a run of posts the choice is time against bags. A 40-post fence in 50 lb fast-setting bags is 136 bags. The same fence in 80 lb standard mix is 85 bags and a mixer.

Questions about concrete for fence posts

How many bags of concrete for a 4x4 post?

In a 10 inch hole filled to the top: 3 bags of 50 lb fast-setting mix at 24 inches deep, 4 at 30 inches and 4 at 36 inches. In 80 lb bags it is 2, 2 and 3. Sakrete's chart for a 10 inch hole reads 2.5 bags at 24 inches and 3 at 30, the same volume rounded to the quarter bag.

How many bags of concrete for a 6x6 post?

In a 12 inch hole 36 inches deep, 1.73 cubic feet: 5 bags of 50 lb, 4 of 60 lb or 3 of 80 lb. Widen the hole to 16 inches, the three-times rule for a 5 1/2 inch post, and it is 3.56 cubic feet. That is 10 bags of 50 lb or 6 of 80 lb.

How deep should a fence post be set?

The makers' rules put a 6 ft fence post 2 to 3 ft in the ground. Quikrete's data sheets say half the above-ground height, which is the same as a third of the post's full length. Its project page says a third to a half of the above-ground length plus 6 inches. Sakrete's sheet says a third of the post's length, and its web page a third of the above-ground height, 2 ft for a 6 ft fence. Conservation-service fence specifications set 24 inches for line posts in ordinary soil and 36 to 42 inches for corner, gate and end posts. Penn State's horse-fence guidance is 36 inches for line posts and about 48 for corners. Frost depth in your area and your fence code can require more.

Do I need to subtract the post from the hole volume?

Yes, and most online counts do not. A 3 1/2 inch square post takes 0.21 cubic feet out of a 30 inch hole, over half a 50 lb bag per post. Across a 40-post fence that is about 22 bags. The calculator's post-hole shape subtracts it, and Quikrete's own calculator subtracts the post the same way.

How much concrete for a chain-link post?

The chain-link makers' institute shows line-post holes 8 inches wide at the top widening to 10 at the bottom. For terminal-post holes it shows 10 widening to 12. A 2 3/8 inch pipe in an 8 inch hole 24 inches deep takes 0.64 cubic feet, which is 2 bags of 50 lb. In a 10 inch hole 30 inches deep it takes 1.29 cubic feet, which is 4 bags of 50 lb or 3 of 80 lb.

Calculators and guides for this job

13 source documents
  1. Sakrete (Oldcastle APG), Fast Setting Concrete Mix data sheet: 0.38 cu ft per 50 lb bag; post-setting chart for a 10 in hole at 12, 18, 24 and 30 in; hole 3 times the post width and 1/3 of the post length deep; 4 to 6 in of gravel. Read 2 November 2026.
  2. The Quikrete Companies, Fast-Setting Concrete Mix 1004-50 data sheet, revised November 2025: 0.375 cu ft per 50 lb bag; hole 3 times the post diameter; two bags set a 3 in post in a 9 in hole 30 in deep. Read 2 November 2026.
  3. The Quikrete Companies, Fast-Setting Concrete Mix 1004-60 data sheet: 0.45 cu ft per 60 lb bag; depth one-half of the above-ground post height. Read 2 November 2026.
  4. The Quikrete Companies, Setting Posts in Concrete, project page: hole three times the post width, 1/3 to 1/2 of the above-ground length plus 6 in deep, 6 in of gravel, fill to 3 to 4 in below grade. Read 2 November 2026.
  5. The Quikrete Companies, Concrete Mix 1101 data sheet: 0.60 cu ft per 80 lb bag. Read 2 November 2026.
  6. USDA Natural Resources Conservation Service, North Carolina, Conservation practice specification 382, Fence, February 2008: hand-set holes at least 6 in larger than the post; line posts 24 in deep (18 rocky, 30 sandy); pull posts 36 to 42 in; a corner assembly set 2 1/2 ft deep has about half the strength of one set 3 1/2 ft. Read 2 November 2026.
  7. USDA Natural Resources Conservation Service, Missouri, Practice specification 382, Fence, September 2008: corner and gate posts 30 to 36 in deep; concrete at least 6 in beyond the post. Read 2 November 2026.
  8. USDA Natural Resources Conservation Service, Texas, Practice specification 382, Fence, March 2021: posts buried no less than 3 ft; deeper in sandy or wet soil; the top 12 in of the hole may be backfilled with soil. Read 2 November 2026.
  9. Penn State Extension, Fence Planning for Horses: line posts 36 in deep, corner and gate posts often 48 in. Read 2 November 2026.
  10. Chain Link Fence Manufacturers Institute, Step-by-Step Method for Installing Your Own Chain Link Fence, March 2018: line-post holes 8 to 10 in, terminal-post holes 10 to 12 in. Read 2 November 2026.
  11. NIST and the American Lumber Standard Committee, Voluntary Product Standard PS 20-25, Table 3: a nominal 4 in post is 3 1/2 in dressed, a nominal 6 in post 5 1/2 in. Read 2 November 2026.
  12. The Quikrete Companies, Quantity calculator, post holes: bags = (hole area minus post area) × depth ÷ 0.375 cu ft, rounded up. Read 2 November 2026.
  13. Sakrete (Oldcastle APG), Pro tips: how to set posts and poles (hole 3 times the post width; depth about 1/3 of the post's height above ground). Read 2 November 2026.