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

It depends on the hole and post size. A 4×4 post in a 10 inch hole with 24 inches of concrete needs about 0.92 cubic feet, or about two 60 lb bags at 0.45 cubic feet per bag. A 12 inch hole at the same depth needs about three.

How deep should a fence post hole be?

QUIKRETE's guidance is one-third to one-half of the post's above-ground length, plus 6 inches for a gravel base. Where the ground freezes, local frost depth or code can require more.

Do I subtract the post when calculating concrete?

Yes for a solid wood post: the post takes up part of the hole, so subtract its volume. For a hollow vinyl post, concrete fills the post too, so count the full hole.

Should the concrete go all the way to the bottom of the hole?

In QUIKRETE's method, 6 inches of gravel goes in first and the concrete stops 3 to 4 inches below ground level, so calculate the concrete for that shorter depth.

Concrete

How to Calculate Concrete for Fence Posts

Hole volume minus post volume, multiplied by the number of posts

BuildNumerics EditorialAugust 7, 2026Updated October 18, 20264 min read

Concrete Calculator for Fence Posts — do this calculation nowOpen calculator

How much concrete per fence post? The short version

Work out the volume of the hole, subtract the part of the post that sits in the concrete, and divide by the yield of the bag you are using. A 4×4 post (3.5 in actual) in a 10 in hole with 24 in of concrete needs about 0.92 ft³, which is about 2.05 bags of a 60 lb mix that yields 0.45 ft³ per bag.

Standard hole sizing

QUIKRETE's post-setting instructions, written for its Fast-Setting Concrete Mix, call for a hole about three times the width of the post, dug to a depth of one-third to one-half of the post's above-ground length, plus 6 in for a gravel base. The gravel goes in first, and the concrete is filled to 3 to 4 in below ground level.

Local requirements can call for deeper holes, especially where the ground freezes. Check your local frost depth and any code or HOA requirements before you dig, rather than relying on a single rule of thumb.

The formula: hole volume minus post volume

A fence-post hole is typically a cylinder — π × radius² × depth. But the post itself occupies part of that cylinder and displaces concrete, so the actual concrete volume per hole is the hole's volume minus the post's own volume (also calculated as a cylinder or a rectangular prism, depending on whether the post is round or square).

Use the depth that will actually be filled with concrete. If the hole has a 6 in gravel base and the concrete stops a few inches below grade, the concrete depth is less than the hole depth. The Concrete Calculator for Fence Posts assumes concrete fills the full depth you enter, so enter the concrete depth.

Bags per post for common hole sizes

The table applies the same formula to common combinations. Bag counts use the approximate yields published for QUIKRETE Concrete Mix (No. 1101): 0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag. Other mixes, including fast-setting mixes, yield different volumes, so divide by the yield printed on your bag.

Concrete per post = (π × (hole diameter ÷ 2)² − post width²) × concrete depth. Add up all the posts before rounding up to whole bags.
Post (actual size)	Hole diameter	Concrete depth	Concrete per post	60 lb bags (0.45 ft³)	80 lb bags (0.60 ft³)
4×4 (3.5 in)	10 in	24 in	0.92 ft³	2.05	1.53
4×4 (3.5 in)	12 in	24 in	1.40 ft³	3.11	2.33
4×4 (3.5 in)	12 in	30 in	1.75 ft³	3.89	2.92
4×4 (3.5 in)	12 in	36 in	2.10 ft³	4.67	3.50
6×6 (5.5 in)	16 in	36 in	3.56 ft³	7.91	5.93

Multiply by the number of posts

Once you have the per-hole volume, multiply by the total number of posts in the fence run to get the total concrete needed. Corner and gate posts are sometimes set in larger holes than line posts — calculate those separately if their dimensions differ.

The Fence Calculator works out how many posts a fence line needs.

Hollow vinyl posts

Vinyl fence posts are hollow, so concrete fills much of the post as well as the hole around it. The Vinyl Fence Calculator therefore counts the full hole volume as concrete rather than subtracting the post.

Worked Example

A 10-inch diameter hole, 2 ft deep, with a 4×4 in (nominal) post, for 8 posts.

Hole volume: π × (5 in)² × 24 in ≈ 1,885 in³

Post volume (using actual ~3.5×3.5 in): 3.5 × 3.5 × 24 = 294 in³

Concrete per hole: 1,885 − 294 = 1,591 in³ ≈ 0.034 yd³

Total for 8 posts: 0.034 × 8 ≈ 0.27 yd³

In bags: 1,591 in³ ÷ 1,728 = 0.92 ft³ per post; × 8 = 7.37 ft³ ÷ 0.45 ft³ per 60 lb bag = 16.4 → 17 bags

Assumptions & Limitations

Hole diameter and depth follow QUIKRETE's published post-setting guidance — always confirm against local frost-depth and code requirements.
Post volume uses actual (not nominal) dimensions, since a nominal 4×4 is slightly smaller in practice.
Bag counts use QUIKRETE Concrete Mix (No. 1101) yields; check the yield on the bag you buy.

FAQ

How many bags of concrete do I need per fence post?: It depends on the hole and post size. A 4×4 post in a 10 inch hole with 24 inches of concrete needs about 0.92 cubic feet, or about two 60 lb bags at 0.45 cubic feet per bag. A 12 inch hole at the same depth needs about three.
How deep should a fence post hole be?: QUIKRETE's guidance is one-third to one-half of the post's above-ground length, plus 6 inches for a gravel base. Where the ground freezes, local frost depth or code can require more.
Do I subtract the post when calculating concrete?: Yes for a solid wood post: the post takes up part of the hole, so subtract its volume. For a hollow vinyl post, concrete fills the post too, so count the full hole.
Should the concrete go all the way to the bottom of the hole?: In QUIKRETE's method, 6 inches of gravel goes in first and the concrete stops 3 to 4 inches below ground level, so calculate the concrete for that shorter depth.

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