Fence Post Calculator
Short answer: a 100 ft fence with posts every 8 ft needs 13 posts. A 12 inch hole 3 ft deep takes about 5 × 80 lb bags per hole with 20% waste. Enter your fence below.
Calculate your fence posts
How the calculation works
Posts = fence length ÷ spacing, rounded down, plus one end post. Each hole is a cylinder: volume = π × (diameter ÷ 2)² × depth. Multiply by the post count, then divide cubic feet by 27 for cubic yards.
Worked example: 100 ft of fence at 8 ft spacing = 12 + 1 = 13 posts. One 12 in hole 3 ft deep = π × (0.5 ft)² × 3 ft = 2.36 cu ft = 0.09 cubic yards. All 13 holes = 1.13 cubic yards net, 1.36 with 20% waste.
Concrete per hole (3 ft deep, with 20% waste)
| Hole diameter | Volume per hole | 80 lb bags per hole |
|---|---|---|
| 8 in | 0.04 cubic yards | 3 |
| 10 in | 0.06 cubic yards | 4 |
| 12 in | 0.09 cubic yards | 5 |
| 16 in | 0.16 cubic yards | 9 |
Volumes are net (before waste). Bag counts include the 20% waste allowance and use the 0.60 cu ft yield per 80 lb bag, rounded up.
Frequently asked questions
How many bags for one fence post hole?
A 12 inch hole 3 ft deep needs about 4 × 80 lb bags net, or 5 allowing for the 20% waste hand-dug holes usually need.
How many posts do I need for a fence?
Divide the fence length by the post spacing, round down, and add one end post. A 100 ft fence at 8 ft spacing needs 13 posts.
How deep should a fence post hole be?
Depth follows your local frost depth and fence height — holes must go below the local frost line. A 12 inch hole 36 inches deep is a common example; check your local code and fence plans.