40 lb Concrete Bags for Fence Posts

Work out how many bags of concrete each fence post needs.

Your result

Enter your numbers and press the button. Your result appears here.

A 40 lb bag of Quikrete Concrete Mix yields about 0.30 cubic feet.

How the bags are counted

Concrete per hole = (π × (hole width ÷ 2)² − post area) × (depth − gravel). Bags per post = concrete per hole ÷ bag yield, rounded up.

Example: a 4×4 post in a 10.5 in hole 30 in deep, with 6 in of gravel, takes 1.14 ft³ of concrete: 4 bags per post, 40 for 10 posts.

Concrete needed at a glance

Worked out with the same settings as the calculator above, changing only the hole depth.

Hole depth	Bags per post	Total bags	Concrete
24 in	3	30	8.5 ft³
30 in	4	40	11.4 ft³
36 in	5	50	14.2 ft³
42 in	6	60	17.0 ft³
48 in	7	70	19.9 ft³

Bag yields

Bag	Yield	Bags per cubic yard
40 lb concrete mix	0.30 ft³	90
50 lb concrete or fast-setting mix	0.375 ft³	72
60 lb concrete mix	0.45 ft³	60
80 lb concrete mix	0.60 ft³	45

Yields from Quikrete product data. Pour fast-setting mix dry into the hole and add water; it sets in 20 to 40 minutes.

Common questions

How many bags of concrete per fence post?

For a 4×4 post in a hole about 10 to 12 in wide and 2 to 3 ft deep, plan on 2 to 4 bags of 50 lb fast-setting mix. Bigger holes and 6×6 posts take more.

Should I put gravel in the bottom of the hole?

A few inches of gravel under the post helps water drain away from the end grain. Leave the top of the concrete just above the ground and slope it away from the post.

Fast-setting or regular mix?

Fast-setting mix is poured dry into the hole and soaked with water, and it sets in 20 to 40 minutes. Regular mix is cheaper per cubic foot but must be mixed first.

Sources

Quikrete, Concrete Mix data sheet (bag yields): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf
Quikrete, project calculator (post depth example): https://www.quikrete.com/calculator/