Post hole concrete calculator

Post holes are small, but there are a lot of them. Getting the count right saves a second trip in the middle of a fence line.

How it works

Each hole is a cylinder: 3.1416 x radius² x depth, measured above the gravel. The calculator takes away the part of the post that sits in the concrete, then divides by the bag yield.

Example: a 12 in hole, 36 in deep with 6 in of gravel, holding a 4x4 (3.5 in actual) needs about 1.75 cu ft of concrete. That is 4.7 bags of 50 lb mix or 3.9 bags of 60 lb mix per post.

Hole size: Quikrete’s rule

Quikrete’s setting-posts guide says to dig the hole about 3 times the width of the post and 1/3 to 1/2 of the post’s above-ground height deep, plus 6 in for gravel. For a 6 ft fence, that is a hole at least 2 ft deep before gravel. A 4x4 post (3.5 in) gets a hole about 10 to 12 in across.

In cold climates, posts should go below the local frost line so they do not heave. Your building department can tell you that depth.

Fast-setting or regular mix?

Fast-setting mix is poured into the hole dry and then soaked; Quikrete’s data sheet says it sets in 20 to 40 minutes. Regular mix is mixed first and takes longer, but costs less per bag. Both are counted the same way here.

Sources

• Quikrete: Setting Posts in Concrete (hole 3 times the post width, depth 1/3 to 1/2 of the above-ground post, plus 6 in)
• Quikrete Fast-Setting Concrete Mix data sheet (sets in 20 to 40 minutes)
• Quikrete Concrete Mix (No. 1101) data sheet: bag yields 0.30, 0.375, 0.45 and 0.60 cu ft for 40, 50, 60 and 80 lb

Sources last checked 1 November 2026.