About this tool
The Fence Post Concrete Calculator tells you how much concrete to buy to set fence posts firmly in the ground. For each post hole it works out the concrete volume as the hole volume minus the volume the post itself takes up. The hole is treated as a cylinder, so hole volume = π × (hole diameter ÷ 2)² × depth, and the embedded post volume is subtracted — a round post uses π × (post diameter ÷ 2)² × depth and a square post uses width² × depth, all converted from inches to feet.
A common rule of thumb is to make the hole about three times the post width and at least 24 inches deep (or one-third of the post's above-ground height, or below the frost line, whichever is deeper). Enter your own hole diameter and depth to match local code. Concrete bags are sized by their dry yield: a 40 lb bag makes about 0.30 cu ft, a 50 lb bag 0.375 cu ft, a 60 lb bag 0.45 cu ft, and an 80 lb bag 0.60 cu ft of set concrete.
The calculator reports concrete per hole, bags per hole (rounded up), total bags across every post with your waste allowance added, total cubic feet, and cubic yards (÷ 27). A 10 percent waste margin is included by default to cover spillage, over-dig, and uneven holes. Everything runs in your browser — no data leaves your device.