Gravel Calculators
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.
A note on this estimate
Hole walls collapse and tamping changes real usage, so treat the per-post figure as a planning number. The gravel here is a drainage layer only — the rest of the hole is concrete or tamped soil.
Last updated: 8 November 2026 · Report an error
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 on our homepage for that.
Hole sizes by post and fence height
Two rules of thumb size the hole. Diameter: about three times the post's width, so there is room to tamp the gravel or concrete around it in lifts. Depth: about half the above-ground height for typical fences, as the table below shows — and in cold regions the hole must reach below the frost line regardless of what the ratio says, or frost heave will walk the post out of the ground.
| Post | Hole diameter | Fence height above ground | Hole depth |
|---|---|---|---|
| 4 × 4 in (3.5 in actual) | 10–12 in | 4 ft | 24–30 in |
| 4 × 4 in (3.5 in actual) | 10–12 in | 6 ft | 30–36 in |
| 6 × 6 in (5.5 in actual) | 14–16 in | 6 ft | 32–38 in |
| 6 × 6 in gate post | 16 in | 6 ft | 36–42 in — gates lever their posts hardest |
Worked example: 10 posts in 12-inch diameter holes, with a 6-inch gravel bed in each. Each bed is π × 6² × 6 ≈ 679 in³ = 0.39 ft³ — about 39 lb of #57 stone, the calculator’s default material. The run of ten totals 3.93 ft³ ≈ 0.15 yd³, roughly 392 lb (0.20 tons) — about 8 standard bags, which is exactly the size of job bags exist for. Filling the whole 36-inch hole instead would take six times as much; that approach belongs to the general calculator.
Material choice: angular crushed stone, tamped in lifts — it settles tight around the post and drains freely. A clean stone like #57 never locks the way crusher run does, but in a post hole the job is drainage and a firm bed, not a structural lock. Round pea gravel is the common mistake: it rolls, never packs firm, and the post works loose in its socket within a season or two. Keep the top of the fill a few inches below grade and crown soil or turf over it so surface water sheds away from the post instead of ponding in the hole.
Related tools & guides
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?
About half the above-ground height as a rule of thumb, plus the gravel bed — a 4-foot fence wants a 24–30 inch hole and a 6-foot fence 30–36 inches, and in frost country the hole must pass below the frost line whatever the ratio says. Hole diameter is typically 3 times the post width (10–12 inches for a 4×4 post).
Can I fill the whole post hole with gravel instead of concrete?
Yes — some installers backfill entirely with compacted stone. It drains freely and lets you pull or reset a post later, but it takes about six times the gravel of a 6-inch drainage layer (a 12-inch, 36-inch hole is 2.36 ft³ against 0.39 ft³), and a post set in stone alone can shift more than one set in concrete. Use the general gravel calculator on the homepage to size a full-hole fill.
How many bags of gravel do I need for 10 posts?
For 12-inch holes with a 6-inch layer, about 8 standard 0.5 ft³ bags: each post takes 0.39 ft³, so ten posts need 3.93 ft³ — roughly 392 lb (0.20 tons) of #57 stone. Bags beat bulk at this size almost every time.