How this calculator works
It treats each hole as a cylinder — π × radius² × depth — to find the volume in cubic feet, multiplies by your hole count, and divides by the 0.375 cubic-foot yield of a standard 50 lb bag of concrete mix.
Worked example — 10 holes, 10 in diameter, 36 in deep:
π × (5 ÷ 12)² × 3 = 0.55 cu ft per hole × 10 = 5.5 cu ft → 15 bags.
π × (5 ÷ 12)² × 3 = 0.55 cu ft per hole × 10 = 5.5 cu ft → 15 bags.
How deep should a post hole be?
A common rule is to bury at least one-third of the post length. For a 6 ft fence using 8 ft posts, that means going 2.5–3 ft deep — deeper in freeze-thaw climates to get below the frost line.
Frequently asked questions
How many bags of concrete per fence post?
About 1–3 bags (50 lb each) depending on hole size — typically 2 bags for a standard 10-inch diameter, 30-inch deep hole.
What diameter should a post hole be?
About 3 times the post width — so a 4-inch post needs roughly a 12-inch hole.