Fence post concrete calculator

Ten 4x4 posts in 12 in holes, 38 in deep with 6 in of gravel underneath, take 28 bags of 80 lb concrete (1.634 cu ft per hole), or 37 bags of 60 lb. Enter your own post, hole and count.

Post

Hole diameter

Hole depth

Gravel in the bottom

Left for soil at the top

Number of posts

28 80 lb bags

37 60 lb bags

44 50 lb bags

Concrete in each hole	1.634 cu ft (28 in deep)
Concrete for all posts	16.34 cu ft (0.61 cu yd)
40 lb bags	55 in all (5.4 per post)
50 lb bags	44 in all (4.4 per post)
60 lb bags	37 in all (3.6 per post)
80 lb bags	28 in all (2.7 per post)
90 lb bags	25 in all (2.4 per post)
Gravel for the bottoms	3.93 cu ft (8 bags of 0.5 cu ft)

How it was worked out

Concrete depth: 38 in − 6 in gravel − 4 in soil = 28 in

(π × (12 in ÷ 2)² − post 12.25 sq in) × 28 in = 1.634 cu ft per post

QUIKRETE's rule for this post is a hole about 10.5 in across (three times the post's width).

Sizing the hole

QUIKRETE's guide to setting posts gives three rules:

Make the hole about three times as wide as the post. For a 4x4, which measures 3½ in, that is about 10½ in, so a 10 or 12 in auger.
Dig one third of the post's overall length, plus 6 in for a layer of gravel in the bottom to drain water away from the end of the post.
Fill with concrete to 3 to 4 in below the top and finish with soil.

A post that holds up a structure, such as a deck, must sit at least 6 in below your area's frost line. The calculator takes the gravel and the top layer of soil out of the hole, and takes out the space the post fills, so it counts only the concrete.

Hole depth by QUIKRETE's rule (one third of the post, plus 6 in of gravel)
Overall post length	Hole depth	Post in the ground	Post above ground
6 ft	30 in	24 in	48 in
7 ft	34 in	28 in	56 in
8 ft	38 in	32 in	64 in
9 ft	42 in	36 in	72 in
10 ft	46 in	40 in	80 in
12 ft	54 in	48 in	96 in

Worked example

Concrete depth: 38 in − 6 in gravel − 4 in soil = 28 in

(π × (12 in ÷ 2)² − post 12.25 sq in) × 28 in = 1.634 cu ft per post

10 posts × 1.634 cu ft = 16.34 cu ft ÷ 0.60 cu ft per 80 lb bag = 27.2, so 28 bags

Round up the total for the whole fence, not each post: part bags left from one hole go into the next.

Bags per 4x4 post

Concrete around one 4x4 post (concrete depth excludes the gravel and the soil on top)
Hole	Concrete depth	Cu ft	80 lb bags	60 lb bags	50 lb bags
8 in	24 in	0.53	0.9	1.2	1.4
8 in	30 in	0.66	1.1	1.5	1.8
8 in	36 in	0.79	1.3	1.8	2.1
10 in	24 in	0.92	1.5	2	2.5
10 in	30 in	1.15	1.9	2.6	3.1
10 in	36 in	1.38	2.3	3.1	3.7
12 in	24 in	1.4	2.3	3.1	3.7
12 in	30 in	1.75	2.9	3.9	4.7
12 in	36 in	2.1	3.5	4.7	5.6

QUIKRETE's 60 lb Fast Setting Concrete Mix yields 0.45 cu ft, the same as a 60 lb bag of its regular Concrete Mix, so the 60 lb column works for either. It is poured dry into the hole and then soaked with water, so there is nothing to mix.

Sources

Setting Posts (project sheet), The QUIKRETE Companies, current sheet. Hole 3 times the post diameter; depth one third of the post length plus 6 in of gravel; concrete to 3 to 4 in below the top; 6 in below the frost line for structural posts.
QUIKRETE Concrete Mix (No. 1101) data sheet, The QUIKRETE Companies, revised October 2022. Bag yields: 40 lb 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60, 90 lb 0.675; about 140 lb per cubic foot.
QUIKRETE Fast Setting Concrete Mix (No. 1004) data sheet, The QUIKRETE Companies, current sheet. A 60 lb bag yields about 0.45 cu ft.