Concrete for Post Holes: Volume Per Hole & Bags Needed

The method

A post hole is just a cylinder with a post standing in the middle of it. The concrete you buy fills everything the post doesn’t. So:

Concrete per hole = hole volume − post volume.

Hole volume is a cylinder: π × radius² × depth.
Post volume is the part of the post sitting inside the hole — its width times its width times how deep it’s buried.
Take one off the other, then multiply by the number of holes.

That’s the whole job. The only place people slip up is forgetting the post takes up space, or using the wrong radius — the hole size is usually given as the width across (the diameter), so halve it before you square it.

Concrete per hole = hole volume − post volume (π × radius² × depth)

1 hole
2 post
3 concrete

Work it out by hand

Take a common fence post hole: 300 mm wide across, dug 600 mm deep, with a 90 mm square timber post buried the full depth.

1. Change to metres (so the answer comes out in m³ and litres cleanly).

Radius = 300 ÷ 2 = 150 mm = 0.15 m
Depth = 600 mm = 0.6 m
Post = 90 mm = 0.09 m

2. Hole volume (the cylinder).

π × 0.15² × 0.6 = 3.1416 × 0.0225 × 0.6 = 0.0424 m³ ≈ 42.4 litres

3. Post volume (the space the post takes up).

0.09 × 0.09 × 0.6 = 0.0049 m³ ≈ 4.9 litres

4. Concrete per hole.

42.4 − 4.9 = 37.5 litres per hole

5. Turn litres into bags. A 20 kg bag of pre-mix makes about 9–10 litres once mixed — the exact number is printed on the bag and changes by product, so check it. Using 10 L per bag:

37.5 ÷ 10 = 3.75 → round up to 4 × 20 kg bags per hole.

6. Multiply by the number of holes. A ten-post fence:

37.5 L × 10 = 375 litres → 375 ÷ 10 = 37.5 → round up to 40 bags for the run.

Always round up — a hole a bit wider or deeper than planned, plus some spillage, eats the difference fast. One spare bag on site beats a second trip to the shop.

Do it in 5 seconds

Sources

Cylinder volume — standard shapes: volume = π × radius² × depth. No code or rules apply; concrete amounts are just sums.
How much a bag makes (litres of wet concrete per bag) — set by the product, check the bag. The ~9–10 L per 20 kg figure used above is a working average; 25 kg and 40 kg bags make more. If you mix your own on site, the mix ratio changes it too. (Marked verify against the actual product before publish.)

Keep going

Part of the Concrete Calculations: The Complete On-Site Guide — the main page covering every concrete amount you’ll work out on site.
Next, get the conversion down: How Many Bags of Concrete Per m³? — the same litres-to-bags sums, across 20 kg, 25 kg and 40 kg bags.

Frequently asked questions

How much concrete do I need for one fence post?

For a common 300 mm wide hole dug 600 mm deep with a 90 mm post, about 37.5 litres per hole — that's the hole (a cylinder, roughly 42.4 L) minus the post that sits in it (about 4.9 L). That's roughly four 20 kg bags of pre-mix, but check how much your bag makes.

How many bags of concrete per post hole?

Divide the litres per hole by the litres one bag makes once mixed. A 20 kg pre-mix bag makes around 9–10 litres, so a 37.5 litre hole takes about four bags. Bigger 25 kg and 40 kg bags make more, so always read how much the bag makes on the label.

How deep should a fence post hole be?

A common rule of thumb is to bury about one-third of the post's height above ground, and to make the hole about three times as wide as the post. A deeper or wider hole uses more concrete, so put your real sizes into the calculator rather than guessing.

Do you fill the whole post hole with concrete?

Usually yes — up to just below ground level, then shape the top into a slope so rain runs off instead of pooling around the post. The post takes up part of the hole, so the concrete you actually buy is the hole minus the post — which is exactly how this sum works.

Does the post volume really matter?

For a slim 90 mm post in a 300 mm hole it's about 5 litres out of 42 — small on one hole, but it adds up to roughly a bag every few holes. Skip it and you'll over-order; over a long fence run those extra bags cost real money, so it's worth taking off.