Fence Post Gravel Calculator
Fence posts don't need yards of stone — they need a drainage layer in the bottom of each hole. Enter your post count, hole diameter, and the gravel layer depth; results come in bags first, because that's how this job actually gets bought.
How the per-post math works
Each gravel layer is a small cylinder: π × (hole radius)² × layer depth. A 12-inch hole with a 6-inch layer holds π × 6² × 6 ≈ 679 in³ ≈ 0.39 ft³ — not quite one 0.5 ft³ bag per post. Multiply by your post count for the total; a 24-post fence needs about 9.4 ft³ ≈ 19 bags (buy 20).
This is the smallest job on the site — which is why it's computed in cubic feet and bags, not yards and tons. If your number comes out in yards, you're filling entire holes with gravel, which is a different (and fine) approach: use the general gravel calculator for that.
Fence post FAQ
How much gravel do I need per fence post?
A typical 12-inch-wide, 6-inch-deep gravel base in the bottom of a post hole takes about 0.39 cubic feet — not quite one 0.5 ft³ bag. Most of the hole is filled with concrete or tamped soil; the gravel layer at the bottom is for drainage under the post.
Why put gravel in a fence post hole?
A layer of gravel under the post lets water drain away from the wood end grain instead of pooling and rotting it. Six inches of tamped gravel under the post is the standard detail for wood fence posts.
How deep should fence post holes be?
One-third of the above-ground fence height as a rule of thumb — a 6-foot fence wants 2-foot-deep holes, deeper in frost country so posts sit below the frost line. Hole diameter is typically 3 times the post width (10–12 inches for a 4×4 post).