How Much Concrete per Fence Post? (4x4 & 6x6)

The answer depends on three numbers: hole diameter, hole depth and post size. This calculator works out the cylinder volume, subtracts the 6 in gravel drainage layer and the post itself, and converts what's left into bags.

Formula

Concrete (cu in) = π × (hole diameter ÷ 2)² × concrete depth − post width² × concrete depth. Concrete depth is the hole depth minus the gravel layer. Divide by 1,728 for cubic feet. Standard Concrete Mix yields 0.60 cu ft per 80 lb bag, and QUIKRETE Fast-Setting Concrete Mix yields about 0.375 cu ft per 50 lb bag.

Rules of thumb for post holes

Diameter: about 3× the post width, so 10–12 in for a 4x4 and 12–16 in for a 6x6.
Depth: roughly ⅓ of the post's height above ground, plus 6 in of gravel, and always below your local frost line. A 6 ft fence typically needs holes 30–36 in deep.
Crown the top of the concrete so water runs away from the post.

4x4 post: concrete per hole (6 in gravel base)

Hole dia.	Hole depth	Cu ft	80 lb bags	50 lb fast-set bags
8 in	24 in	0.396	0.66	1.06
8 in	30 in	0.528	0.88	1.41
8 in	36 in	0.66	1.1	1.76
10 in	24 in	0.691	1.15	1.84
10 in	30 in	0.921	1.53	2.46
10 in	36 in	1.151	1.92	3.07
12 in	24 in	1.05	1.75	2.8
12 in	30 in	1.401	2.33	3.74
12 in	36 in	1.751	2.92	4.67

6x6 post: concrete per hole (6 in gravel base)

Hole dia.	Hole depth	Cu ft	80 lb bags	50 lb fast-set bags
12 in	24 in	0.863	1.44	2.3
12 in	30 in	1.151	1.92	3.07
12 in	36 in	1.438	2.4	3.84
14 in	24 in	1.288	2.15	3.44
14 in	30 in	1.718	2.86	4.58
14 in	36 in	2.147	3.58	5.73
16 in	24 in	1.779	2.97	4.74
16 in	30 in	2.372	3.95	6.33
16 in	36 in	2.965	4.94	7.91

Bag counts in the tables are fractional so you can total several posts. Round the total up. Yields: QUIKRETE Concrete Mix (No. 1101) data sheet; QUIKRETE Fast-Setting Concrete Mix product data (50 lb ≈ 0.375 cu ft).

Frequently asked questions

How many bags of concrete per 4x4 post?

For a 10 in hole 30 in deep with 6 in of gravel, about 0.92 cu ft: roughly 1.5 × 80 lb bags or 2.5 × 50 lb fast-setting bags. Most people buy 2 × 80 lb or 3 × 50 lb fast-set per post.

How many bags of concrete for a 6x6 post?

A 12 in hole 36 in deep with 6 in of gravel takes about 1.9 cu ft, a little over 3 × 80 lb bags.

Should I use fast-setting concrete for fence posts?

It's convenient: you pour it dry into a water-filled hole and it sets in 20–40 minutes. Bag for bag it yields less volume, so you need more bags.

Do I need gravel under a fence post?

A 4–6 in layer of gravel at the bottom of the hole lets water drain away from the end grain, which helps wood posts last longer.

Last reviewed October 2026 by Jim Frith. These are planning estimates; confirm quantities with your supplier and follow local codes.