Post Hole Gravel Calculator

How Does This Calculator Work?

This calculator takes your post hole dimensions and gives you an accurate estimate of how much gravel you need inside each hole. It works by calculating the volume of a cylinder, which is the shape of most post holes, then multiplies by the hole count. A circular patio or fire-pit pad is a different job: that gravel covers the whole round surface, and it belongs on the circular gravel calculator.

Formula used:
Volume = π × (radius)² × depth

You can also toggle the unit of measurement and optionally enter your own gravel density if you’re using a custom material.

Why Add Gravel to Post Holes?

Gravel serves a lot more than just filler at the bottom of a hole. It plays a crucial role in both supporting the structure and protecting it long-term.

• Structural stability – Gravel helps anchor the post and distribute load evenly.
• Drainage improvement – It prevents water from pooling at the base of your post.
• Prevents rot for wooden posts – By letting moisture drain away, it increases the lifespan of timber posts.
• Reduces frost heave – In colder climates, gravel reduces ground movement caused by freezing and thawing.

How Much Gravel Per Post Hole?

Most projects follow a basic rule of thumb: Add about 6 inches of gravel at the base of every post hole. For deeper or heavier posts (like for decks), you might want to use 8–12 inches instead.

A typical post hole is:

• 6 to 12 inches in diameter
• 2 to 3 feet deep

Let’s break down an example manually:

Suppose you're digging a 10-inch diameter hole and you want to add 6 inches (0.5 feet) of gravel:

• Convert diameter to radius:
  10 ÷ 2 = 5 inches = 0.417 feet
• Use the formula:
  Volume = π × (0.417)² × 0.5 ≈ 0.27 cubic feet per hole

If you have 10 holes, that’s roughly 2.7 cubic feet of gravel total.

Post Hole Projects That Use Gravel

This calculator comes in handy for a variety of projects that rely on post holes, including:

• Fencing – both residential backyard fences and agricultural perimeter fencing
• Mailboxes – especially for community or heavy-duty mail stations
• Sign posts – road signs, business signage, or private property markers
• Pergolas and shade structures – posts need to be stable and moisture-protected
• Deck footings – often used in combination with concrete for stronger support

Gravel Types Suitable for Post Holes

Some commonly used gravel types in post holes include:

• Crushed stone – Sharp edges allow it to compact firmly; ideal for structural support.
• Pea gravel – Smooth and round; not the best for load-bearing but okay for drainage layers.
• ¾" minus gravel – Contains a mix of gravel and fine dust that compacts well; great for post holes.
• Class 5 or CA6 aggregate – Common in road base and post settings; ideal if you're using compactors.

For structural projects, avoid decorative gravel as it's not meant to hold weight or resist shifting.

Can You Skip Gravel and Use Concrete Only?

It’s not recommended. Gravel ensures proper drainage and helps prevent rot or frost damage, something concrete alone can’t do.

Should You Use Gravel for Wood or Metal Posts?

Yes, both benefit. Wood needs drainage, while metal needs stable footing to avoid tilt or sink.

Can I Reuse Gravel from an Old Hole?

If it’s clean and free of soil or organic matter, yes. But compacted or contaminated gravel is better replaced.

Recommended Tools and Supplies

Here’s what you’ll typically need when working with post hole gravel:

• Post hole digger or auger – For digging clean, round holes efficiently
• Gravel bags or bulk delivery – Depending on project size
• Tamper or compacting tool – To press the gravel down firmly

Optional:

• Post level – Helps ensure upright alignment
• Concrete mix – Often poured on top of gravel for added strength

Gravel per hole at 6 inches deep

The manual example above is one 10 inch hole: about 0.27 cubic feet. The same cylinder, counted by the tool at 1,600 kg/m³, is the table below. Set the diameter unit to inches. Depth stays in inches. The 6 inch row is the usual drainage layer, not a hole filled entirely with gravel.

Worked example: a 12-hole fence line

Choose inches for the diameter, type 10, leave the depth at 6 inches, and set the hole count to 12. The order line reads 0.12 cubic yards, 0.09 cubic meters, 0.16 short tons, 0.15 tonnes, and 7 bags of 0.5 cubic feet. That is 3.27 cubic feet, which is 12 × 0.27. Ten of the same holes are the 2.7 cubic feet already estimated by hand above (the tool shows 2.73).

Filling the whole 2 foot hole with gravel, instead of a 6 inch drain layer, is four times this volume. Type that deeper number if that is the detail. Concrete around the post is a different material and a different page.

Post hole gravel questions

How much gravel do I need per post hole?

Six inches in a 10 inch hole is 0.27 cubic feet. An 8 inch hole at the same depth is 0.17 cubic feet, and a 12 inch hole is 0.39 cubic feet. Those are the gravel plugs at the bottom, which is the depth most fence jobs actually place.

A deck footing that calls for 8 to 12 inches of gravel uses the same diameter and a larger depth. Do not type the full hole depth unless the whole hole is gravel.

How many bags of gravel for 12 post holes?

Twelve holes at 10 inches by 6 inches are 3.27 cubic feet, which is 7 bags of 0.5 cubic feet. Ten holes are 6 bags. One hole is a single bag, with most of the bag left over.

By weight the 12-hole line is 0.16 short tons, about 327 pounds, or 7 bags of 50 pounds if the stone is near 1.35 tons per yard. Buy an extra bag if you are setting the posts in a full gravel backfill.

How much does the gravel under a fence weigh?

At 1,600 kg/m³ the 12-hole example is 0.16 short tons and 0.15 tonnes. A cubic yard of the same stone is 1.35 short tons, but a fence rarely needs a full yard unless the holes are deep and numerous.

A 100-hole agricultural line at 10 by 6 inches is about 1.01 cubic yards and 1.36 short tons. That is the point where a bulk delivery beats bags.

Should I put gravel under a 4x4 fence post?

Yes, as the drain at the bottom, usually 6 inches in a hole a few inches wider than the post. A 4x4 often sits in an 8 or 10 inch auger hole. The 8 inch row is 0.17 cubic feet of gravel per post at 6 inches deep.

Angular crushed stone locks around the wood better than round pea gravel. Pea gravel drains, and it also lets the post shift.

Can I fill the post hole with concrete and skip the gravel?

The drainage layer is what keeps the end of a wood post out of a cup of water. Concrete on top of 6 inches of gravel is the usual detail. Concrete alone, poured to the bottom of a tight hole, holds moisture against the wood.

If the spec really is concrete only, calculate that volume on the concrete calculator and do not pretend it is gravel.

How much gravel for a mailbox or deck footing?

A single mailbox hole at 10 by 6 inches is the 0.27 cubic foot row, one bag. A deck footing that wants 12 inches of gravel in a 12 inch hole is 0.79 cubic feet, about 2 bags of 0.5 cubic feet, before any concrete above it.

Count every footing. Four deck posts at that larger plug are 3.14 cubic feet, still a bag job, not a truck.