Post Hole Concrete Calculator (Fence Posts & Footings)

Calculate the concrete volume for fence-post holes — by hole, total across all holes, with post-profile displacement and a 10% waste buffer.

Download on the App Store Free · iPhone & iPad · Save projects, work offline

Calculator

All calculations are local — nothing leaves your browser.

Post profile (optional — deducts post volume per hole)

Enter the hole and post details to see the concrete needed.

Save this on the jobsite — open in the iOS app to keep this calculation, work offline and unlock the full toolset.
Download on the App Store

Post hole concrete formula

V = holes × ((π × r² × depth) − (post area × depth))

Work out one hole as a cylinder — pi times the radius squared times the depth — then subtract the volume the post itself occupies below ground. Multiply the result by the number of holes and add 10% for waste.

Worked example: Six 300 mm holes dug 600 mm deep, each taking a 100 mm square post, is 6 × ((π × 0.15² × 0.6) − (0.1² × 0.6)) = 0.22 m³, so order 0.24 m³ with the buffer.

Setting fence posts is the highest-volume use of bagged concrete in residential construction. The math per hole is simple — π × (radius)² × depth — but real jobs add three wrinkles that this calculator handles: many holes, the post itself takes up space, and the variability in real-world digs.

How to measure

Hole diameter: this is the diameter of the auger or post-hole digger you’re using. Common sizes are 200 mm (8"), 250 mm (10") and 300 mm (12").

Hole depth: measure from the top of the drainage gravel (or the bottom of the hole if you’re not using gravel) to the finished ground level. Typical values:

Number of holes: count the posts. For a typical run, that’s the line posts plus the corner and end posts.

Post profile (optional): if you set the post profile and dimensions, the calculator deducts the post’s volume from each hole — so you order the actual concrete needed, not the gross hole volume.

Why deduct the post’s volume?

A 100 mm round post in a 250 mm hole displaces about 16% of the hole volume. Across 20 posts, that’s enough difference to short-deliver a real bag count. The deduction matters most when:

For ready-mix or oversize holes, the deduction is small and the 10% buffer covers it either way.

Worked example

Eight standard fence posts:

Per hole (gross): π × 0.125² × 0.6 = 0.0295 m³ Post volume per hole: π × 0.05² × 0.6 = 0.0047 m³ Net per hole: 0.0247 m³ All 8 holes: 0.198 m³ With 10% buffer: 0.218 m³

That’s about 11 × 20 kg bags. Order 12 to be safe — the price difference is negligible and you don’t want to be a bag short on the last hole.

On the jobsite

The Concrete Calculator app for iOS has this calculator plus four others (rectangular slabs, circular slabs, irregular slab area, circular columns), saved projects, and full offline support — perfect for fence runs where you’re working through a long list of posts. Get it on the App Store.

Setting a fence specifically? The fence post concrete calculator covers the same maths with hole-sizing and depth rules for fencing, plus concrete and bag counts per post for common post sizes.

Related concrete calculators

Pouring a different shape? These cover the rest of the common jobs.

All five concrete calculators on your phone

Rectangular slabs, circular slabs, irregular slab area, circular columns and post holes — with saved projects, full offline use, and imperial / metric units across the board.

Download on the App Store

Free · 4.8 ★ on the App Store

Frequently asked questions

How much concrete do I need per hole?
For a typical 100 mm post in a 250 mm × 600 mm hole, you need about 0.025 m³ of concrete after the post displaces its share — roughly two 20 kg bags. Multiply by your number of holes and add the 10% buffer to get the order quantity.
How deep should a fence post hole be?
Rule of thumb is one-third the above-ground post height, with a minimum of 600 mm in standard soil. Frost line, soil type and post height all affect this. Check your local code — colder regions often require deeper footings.
Does the post displace concrete? Should I subtract its volume?
Yes — when you set a real post in the hole, it displaces some of the concrete. This calculator's optional Post Profile setting deducts the post's volume per hole so you order the right amount and don't end up with leftover concrete.
What about gravel at the bottom of the hole?
Most installs add 75–100 mm of compacted drainage gravel under the post. The concrete only fills the space above the gravel — measure depth from the top of the gravel to ground level, not to the bottom of the dig.
Why is the 10% buffer especially important for post holes?
Post holes are notoriously variable — augers wander, sides cave in, depths run over. A 10% buffer covers all of these. For a run of more than 10 posts, consider 15% — the variability adds up.