Fence Post Concrete Calculator

How big to dig the hole

Quikrete and Sakrete agree: make the hole 3 times as wide as the post.

Quikrete's video and Sakrete's guide both use a 12 in hole for a 4 in post. That is the post's name size. Under the US lumber standard (NIST PS 20), a 4×4 is really 3½ in.

We use the real size, so a 4×4 gets a 10½ in hole. A 12 in hole takes about 36% more concrete.

Depth: 4 versions

• Quikrete data sheet: ⅓ of the whole post height.
• Quikrete guide: ⅓ to ½ of the height above ground, plus 6 in.
• Sakrete data sheet: ⅓ of the post length.
• Sakrete guide: about ⅓ of the height above ground (but its 8 ft fence example digs 36 in).

For gravel, Quikrete says 6 in. Sakrete says 4–6 in. The calculator uses the low end of Quikrete's guide rule: ⅓ of the height, plus 6 in of gravel. That is deeper than Sakrete's guide. The data sheets' rule digs deeper still: 36 in for a 6 ft fence.

Fill to the top, or leave room for soil?

• Sakrete fills the hole to the top and slopes it away from the post.
• Quikrete stops 3–4 in below the top. Soil or sod fills the rest.

Quikrete's data sheet gives an example: a 3 in square post, set with 2 bags of 50 lb. Its hole measures 9 in across by 30 in down. Filled to the top, that hole needs 0.95 cubic feet. The 2 bags make 0.75, only 79%.

Leave the top 3–4 in for soil, and the bags fill 88%–91% of the space. So the gap is mostly the soil on top.

Sakrete says about 2 bags per hole for a 6 ft fence. Its rule digs 24 in, gravel and all, which gives 2–2.3 bags. Our start answer is higher because we add the gravel below the 24 in.

Sakrete's bags-per-hole table

Sakrete gives the only bags-per-hole table we found. It doesn't say what post it used.

Our column takes away a 4×4 post and uses Sakrete's own 0.38 cubic feet per bag (Quikrete says 0.375). No gravel.

Our numbers land close, so its table seems to suit a 4×4.

Chain link posts

CLFMI, the chain link makers' group, has a spec guide written for commercial jobs. Its hole is 4 times the post width. It is 24 in deep, plus 3 in for each foot of fence over 4 ft. It asks for 2,500 psi concrete. It warns that frost, fence height and wind can call for bigger or deeper holes.

For a 6 ft fence:

• 2⅜ in end post: 9.5 in wide, 30 in deep, 3.1 bags of 50 lb.
• 1⅝ in line post: 6.5 in wide, 30 in deep, 1.4 bags.

Master Halco's install guide asks for less. It digs line holes 6 in wide and 18–24 in deep. End holes are 8 in wide and 30 in deep. It says about 2 bags of 60 lb per hole.

Deep holes, frost and cost

• For holes deeper than 30 in, Quikrete says to fill in stages of 30 in or less.
• No maker gives a frost depth for fence posts. For deck posts, Quikrete says the tube form goes below the frost line. See the Sonotube calculator.

Prices (Oct 2026, before tax):

• Sakrete fast-setting, 50 lb: $8.49, the middle of 5 stores ($7.99–$11.33).
• Quikrete fast-setting, 50 lb: $13.99 from only 1 store, so it is a weak price.
• Concrete mix, 80 lb: $6.49, the middle of 9 stores (6 checked by hand).

Sakrete looks cheaper than Quikrete, but these prices come from different stores. For posts that hold weight, check your local building rules or ask a builder or engineer.