How Many Bags of Concrete per Fence Post?

How many bags of concrete per fence post? For a typical 4×4 fence post in a 10-inch hole about 30 inches deep, plan on about three 50-pound bags of fast-setting mix or two 80-pound bags of regular concrete mix. Wider or deeper holes, and bigger posts, take more. The table below shows common sizes, and the method underneath lets you figure any hole.

Concrete calculator (post holes) →

Start with the hole size

Quikrete's post-setting instructions say to dig the hole about three times the width of the post, to a depth of one-third to one-half of the post's above-ground height, plus 6 inches for gravel. Put 6 inches of gravel in the bottom, set and brace the post, and fill with concrete to 3 to 4 inches below ground level (Quikrete: setting posts). In cold climates, local codes or the fence maker may require going below the frost line, which can mean deeper holes.

Bags per post for common holes

The table subtracts the post itself, the 6-inch gravel layer and the top 3 inches left for soil. Yields come from Quikrete's data sheets: about 0.375 cubic feet per 50-pound bag of Fast-Setting Concrete Mix and about 0.60 cubic feet per 80-pound bag of Concrete Mix (Fast-Setting data sheet, Concrete Mix data sheet).

Post	Hole	Concrete per hole	50 lb fast-setting	80 lb concrete mix
4×4	10 in wide × 24 in deep	~0.58 ft³	2	1
4×4	10 in × 30 in	~0.81 ft³	3	2
4×4	12 in × 30 in	~1.23 ft³	4	3
4×4	12 in × 36 in	~1.58 ft³	5	3
6×6	16 in × 36 in	~2.67 ft³	8	5
6×6	18 in × 42 in	~4.28 ft³	12	8

Post sizes use actual dimensions (a 4×4 is about 3½ inches square, a 6×6 about 5½). Bags are rounded up per hole. If you skip the gravel or fill to the top, you will need more.

Figure your own holes

Concrete depth = hole depth − gravel layer − space left at the top.
Hole volume = 3.1416 × (hole diameter ÷ 2)² × concrete depth, in cubic inches.
Subtract the post: post width × post width × concrete depth.
Convert: divide by 1,728 for cubic feet, then divide by the bag yield and round up.

Or let the concrete calculator do it: tap Post holes, enter the diameter, depth and count. It counts the full hole, which builds in a little extra.

Fast-setting or regular mix?

Fast-setting mix is designed to be poured dry into the hole and soaked with water; Quikrete says it sets in 20 to 40 minutes. Regular concrete mix is mixed in a tub or mixer first and is often cheaper per cubic foot, which adds up on a long fence. For big gate or pergola posts, many people mix regular concrete.

Whole-fence totals

Multiply bags per hole by the number of posts. Fence posts are commonly spaced up to about 8 feet apart, so a 100-foot run has roughly 13 to 14 posts plus gate posts. At three bags each, that is around 40 bags. Check the fence kit's spacing and add a couple of spare bags.

Questions people ask

How many 50 lb bags of concrete do I need per fence post?

For a 4x4 post in a 10-inch hole about 30 inches deep with gravel at the bottom, about three 50-pound bags of fast-setting mix. Bigger holes take more.

How many 80 lb bags of concrete per post?

About two 80-pound bags for a 4x4 in a 10 by 30 inch hole, or about three for a 12 by 30 inch hole, based on Quikrete's 0.60 cubic foot yield.

How big should a fence post hole be?

Quikrete recommends about three times the post width and one-third to one-half of the above-ground post height deep, plus 6 inches for gravel. Local frost depth can require more.

Should concrete go to the top of the post hole?

Quikrete's instructions fill to 3 to 4 inches below ground level, then backfill with soil or sod after the concrete sets.

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