How Much Gravel Do I Need for Fence Posts?

A 10-inch post hole with a 6-inch gravel base needs about 0.27 cubic feet of gravel — roughly one 0.5 cu ft bag per hole when you round up. Twenty of those holes is about 5.5 cu ft; with 10% waste, buy around 12 bags.

The short count shows up after lunch. You bought concrete for every hole and “a couple” of gravel bags for drainage. Halfway down the run the last holes sit on bare dirt. Someone drives back for more stone while posts lean in temporary braces.

The gravel-for-posts formula is:

cu ft per hole = π × (hole diameter ÷ 2)² × gravel depth ÷ 1,728

Measure diameter and depth in inches. Multiply by post count. Divide by your bag yield (often about 0.5 cu ft for a 50 lb all-purpose gravel bag), then add about 10% for spills and oversize holes. Estimates only — not engineering or building-code advice.

How much gravel for one fence post hole?

Treat the base as a short cylinder. Radius is half the hole diameter. Depth is the gravel layer only — usually 4 to 6 inches of ¾-inch crushed stone for drainage under wood or metal posts. Six inches is the common jobsite default.

Example: 10 in hole, 6 in base. Radius = 5 in. Volume = π × 5² × 6 = about 471 cubic inches. Divide by 1,728 → 0.27 cu ft. At 0.5 cu ft per bag that is just over half a bag, so you still buy one bag per hole unless you are pooling a full run and pouring carefully.

A 12 in hole at the same depth is about 0.39 cu ft; an 8 in hole is about 0.17 cu ft. Bigger auger, more stone.

The formula, on a tape

Measure one representative hole after you dig, not the plan diameter on a napkin. Rocky soil and a wandering auger make holes wider than the bit stamp.

• Radius (in) = hole diameter ÷ 2
• Cu in per hole = π × radius² × gravel depth (in)
• Cu ft per hole = cu in ÷ 1,728
• Total cu ft = cu ft per hole × number of posts
• Bags ≈ ceil(total cu ft ÷ bag yield × 1.10)

Example: 14 posts, 10 in holes, 6 in base, 0.5 cu ft bags. Per hole ≈ 0.27 cu ft. Total ≈ 3.8 cu ft. With 10% waste → about 4.2 cu ft ÷ 0.5 → buy 9 bags.

In Fence Takeoff Master you enter the run once — posts, rails, pickets, gates, and gravel — and get a material list on your phone. One run and walk gates are free; Pro ($79/mo) adds extra runs, drive gates, and a printable quote.

Three mistakes that short the gravel order

Buying “a couple of bags” for the whole fence. One bag covers roughly one to two standard holes. A 100 ft run at 8 ft spacing is about 14 posts — see how many fence posts do I need — closer to nine half-cubic-foot bags than two.

Skipping the base to “save” stone. A compacted 4–6 in crushed-stone layer drains water away from the post end grain. Pooling water speeds rot and frost heave. On most residential sets, concrete still sits on that base.

Using pea gravel when you need crushed stone. Rounded pea gravel does not lock when you tamp. Angular ¾-inch crushed stone interlocks under the post. Soft sand holds water. Match the product, then run the volume math on that bag yield.

Gravel base vs gravel-only set

This chart sizes the drainage base under a concrete set — a few inches of stone, then concrete. A full gravel-only set (stone packed in lifts to grade) needs far more material. Do not use the bag counts above for gravel-only backfill; use the full hole depth or the app. Gate and corner posts usually want concrete. Compact every lift either way.

Worked example for a 100 ft run

About 14 posts (100 ft at 8 ft spacing), 10 in holes, 6 in base, 0.5 cu ft bags: per hole ≈ 0.27 cu ft, total ≈ 3.8 cu ft, with 10% waste buy about 9 bags. Concrete is a separate takeoff — see how many bags of concrete do I need — because gravel depth shortens the concrete column.

For posts, pickets, gates, gravel, and a printable list, open Fence Takeoff Master. Related: how many pickets do I need, Jobber vs fence takeoff, Fence Takeoff Master review.

Estimates only — not engineering or building-code advice. Measure the hole you dug. Use crushed stone. Order the waste.

Estimate only — not engineering or building-code advice.