Concrete for fence posts calculator

Enter the post size, the hole and how deep it goes. We take the post out of the hole volume and count the bags of fast-setting or regular mix per post and for every post.

Each post takes 3.8 bags of 50 lb mix (1.42 cu ft). For 16 posts, buy 67 bags and 11 bags of gravel for the hole bottoms.

The concrete fills a 10.5 in round hole from the top of the gravel to 3 in below grade, 33 in in all, less the space the post takes up. The order includes 10% extra because a dug hole is never a clean cylinder.

Bags per post and for 16 posts, by mix
Mix	Yield per bag	Per post	To buy
Fast-setting, 50 lb bags	0.375 cu ft	3.8 bags	67 bags
Concrete mix, 80 lb bags	0.6 cu ft	2.4 bags	42 bags
Concrete mix, 60 lb bags	0.45 cu ft	3.2 bags	56 bags

The whole fence: fence calculator

Posts, rails, pickets, concrete and hardware from the length and height.

Planning figures for setting wood fence posts. Bag yields vary a little by brand, so check the bag you buy. Call 811 before you dig.

How the concrete for fence posts calculator works

The concrete fills a round hole from the top of the gravel to 3 in below grade. The calculator works out that cylinder, takes away the square of the post that stands in it, and divides by the yield of one bag. For the whole fence it adds 10% and rounds up to whole bags.

Bags per post by hole size

Post and hole	Concrete	50 lb fast-setting	80 lb mix
4x4, 8 in hole, 24 in deep	0.46 cu ft	1.2 bags	0.8 bags
4x4, 10 1/2 in hole, 24 in deep	0.90 cu ft	2.4 bags	1.5 bags
4x4, 10 1/2 in hole, 36 in deep	1.42 cu ft	3.8 bags	2.4 bags
4x4, 12 in hole, 36 in deep	1.93 cu ft	5.1 bags	3.2 bags
6x6, 16 1/2 in hole, 36 in deep	3.51 cu ft	9.3 bags	5.8 bags
6x6, 16 1/2 in hole, 48 in deep	4.78 cu ft	12.7 bags	8.0 bags

Depth is the post in the ground, not counting the 6 in of gravel under it. Each row is one post, before the 10% extra.

Pouring the concrete

Fast-setting

Brace the post plumb on two sides, pour the dry mix into the hole to 3 to 4 in below grade, and soak it with the water the bag calls for. Do not stir it. Hang rails after the time on the bag.

Regular mix

Mix in a wheelbarrow or mixer to a stiff consistency, fill around the braced post, and slope the top away from the post so water runs off. Leave it to cure before the rails and pickets go on.

Next steps

To find how deep each post goes for your fence height and frost line, use the fence post depth calculator. For the whole fence, use the fence calculator. For a slab instead of post holes, the concrete slab calculator counts cubic yards and bags.

Where the numbers come from

The hole width of 3 times the post, the 6 in of gravel and filling to 3 in below grade are from Quikrete, Setting Posts in Concrete. Bag yields are Quikrete's: about 0.375 cu ft for a 50 lb bag of Fast-Setting Concrete Mix, 0.6 cu ft for an 80 lb bag of Concrete Mix and 0.45 cu ft for a 60 lb bag. The 10% extra is editorial. The set time is Quikrete's product description; check the bag.

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Questions

Anything else? Write to us or see every calculator.

Questions people ask

For a 4x4 post set 36 in deep in a 10.5 in hole, about 3.8 bags of 50 lb fast-setting mix, 2.4 bags of 80 lb mix or 3.2 bags of 60 lb mix. Set 24 in deep, 2.4 bags of 50 lb fast-setting. The hole width matters as much as the depth, so enter yours above.

A 50 lb bag of Quikrete Fast-Setting makes about 0.375 cu ft. A 4x4 in a 12 in hole 24 in deep, filled to 3 in below grade, takes 1.23 cu ft, so 3.3 bags. Buy whole bags for the job, not per post: leftover dry mix goes in the next hole.

For ten 4x4 posts 36 in deep in 10.5 in holes, 42 bags of 50 lb fast-setting with 10% extra, plus 7 half-cubic-foot bags of gravel for the hole bottoms.

Fast-setting goes in dry and is soaked with water in the hole, so there is no mixing, and it sets in about 30 minutes by the maker's description. Regular concrete mix is mixed first and takes longer to set, and an 80 lb bag fills more hole than a 50 lb bag. Both hold a wood fence post; check the time on the bag before you hang rails.