Concrete for post holes

How this estimate works

A round hole has a cylinder volume: π × (diameter ÷ 2)² × fill depth. When the post is already in the hole, it occupies part of that space. For a square post, subtract post width² × fill depth. Multiply the difference by the number of identical holes, add your waste allowance, then divide by the bag yield.

Enter the actual measured width of the post, not just its nominal lumber name. Enter concrete fill depth after any gravel base or unfilled top section has been accounted for.

Worked example: four 12-inch holes

For four holes, each 12 inches across and filled 24 inches deep around a 3.5-inch square post: one hole’s cylinder holds about 1.571 ft³. The embedded post occupies about 0.170 ft³. Net concrete is about 1.401 ft³ per hole, or 5.603 ft³ total. With 10% extra, order about 6.163 ft³ (0.228 yd³). At 0.60 ft³ per 80 lb bag, round up to 11 bags.

Before you buy

Confirm the actual hole diameter and fill depth at the site. Soil conditions, frost depth, structural loads and post-setting method affect the design; this is a quantity estimate only. Check the yield on the chosen bag: fast-setting and specialty mixes may differ from the standard concrete-mix values used here.

For different-sized holes, calculate each group separately or use the multi-pour calculator and add the results.

Bag-yield source

Default yields follow the QUIKRETE Concrete Mix data sheet: about 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb. Always defer to the specific product label.