How Much Concrete for a Fence Post

Short answer

For a typical 100 mm square fence post set in a 300 mm wide hole, with 700 mm of concrete, you need about 42 litres of concrete per post, or about 47 litres with 10% waste. That is four or five 25 kg bags of post mix, depending on the yield printed on the bag. Work it out as the volume of the hole minus the volume of the post, then divide by the bag yield and round up.

How big the hole should be

The size of the hole decides almost everything, so start there. QUIKRETE's Setting Posts project sheet gives a common rule of thumb:

Diameter: about 3 times the width of the post.
Depth: one third of the post's overall length, plus 150 mm (6 inches) for a gravel base.
Gravel: tamp the bottom, then put 150 mm of gravel or crushed stone in the hole to help drainage.
Concrete level: fill with mix to about 75 to 100 mm (3 to 4 inches) below the top, then finish with soil.

The same sheet says that a post used for structural support, such as a deck, must go at least 150 mm below the frost line. For equipment such as a basketball pole, follow the maker's instructions for hole size. And for loose or sandy soil it suggests digging larger, dish-shaped holes, which means more concrete.

The formula

A round hole is a cylinder. The concrete fills the cylinder except where the post stands.

Hole volume = π × radius² × concrete depth
Post volume = post width × post depth × concrete depth (for a square post)
Concrete per post = hole volume - post volume
Add waste, usually 10%, for uneven sides and spillage.
Bags = concrete with waste ÷ bag yield, rounded up.

Concrete depth here means the depth of concrete only: the hole depth minus the gravel layer and minus the soil cap at the top.

Construction Calc

Works out material quantities and measurements for building jobs, metric first with an imperial mode.

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In Construction Calc, the Area & Volume calculator handles step 1. Pick the circle shape, enter the radius with its unit (mm, cm or m are chosen explicitly), turn on volume and enter the depth. Do the same with the rectangle shape for a square post, subtract, then divide by the yield on your bag. The app's Concrete estimator is built for rectangular slabs (length × width × depth), so for round holes the Area & Volume route is the one to use. You can save each result to a named job as you go.

Worked example in metric

A 100 × 100 mm post, 2.4 m long.

Hole depth: one third of 2.4 m is 0.8 m. Add 150 mm of gravel and the hole is 0.95 m deep.
Hole diameter: 3 × 100 mm = 300 mm, so the radius is 0.15 m.
Concrete depth: 0.8 m above the gravel, less a 100 mm soil cap = 0.7 m.

Now the volumes:

Step	Working	Result
Hole	π × 0.15² × 0.7	0.0495 m³
Post	0.1 × 0.1 × 0.7	0.0070 m³
Concrete per post	0.0495 - 0.0070	0.0425 m³ (42.5 L)
With 10% waste	42.5 × 1.1	46.7 L

1 m³ is 1,000 litres, which is why 0.0425 m³ is 42.5 litres.

From litres to bags

Bag yield depends on the product, so read the bag. As examples, QUIKRETE's data sheets list:

Product and bag	Yield per bag
Fast-Setting Concrete Mix, 25 kg	about 11.6 L
Fast-Setting Concrete Mix, 50 lb (22.6 kg)	about 10.6 L
Concrete Mix, 80 lb (36.2 kg)	about 17 L

For the example post:

With 25 kg bags yielding 11.6 L: 46.7 ÷ 11.6 = 4.03, so 5 bags if you buy for this one post alone.
With 80 lb bags yielding 17 L: 46.7 ÷ 17 = 2.75, so 3 bags.

That first result shows why rounding matters. 4.03 bags rounds up to 5, but the 0.03 is well within the waste you already added. For a single post, 4 bags is reasonable. For a full fence, work out the total first and round once.

A whole fence: total first, round once

Six posts of the size above need 6 × 46.7 = 280 L with waste. At 11.6 L per bag that is 24.2, so 25 bags. Rounding each post up to 5 bags would have bought 30. Totalling first saves money and leaves less half-used mix.

Fast-setting mix or ordinary concrete

QUIKRETE's Fast-Setting Concrete Mix is designed to set hard in about 20 to 40 minutes, and its Setting Posts sheet describes pouring the dry mix into the hole and soaking it with water, without mixing. The sheet says you need about 1 gallon (about 3.8 L) of water for each 50 lb bag, depending on soil conditions. It also notes that colder temperatures need extra curing time, and that for holes deeper than about 760 mm (2 1/2 feet) you should fill and wet the mix in stages.

Ordinary concrete mix is mixed with water first and then placed. Either way, the volume calculation above is the same. Only the yield per bag changes.

Common mistakes

Forgetting the gravel and soil cap. Using the full hole depth overstates the concrete.
Not subtracting the post. For a 100 mm post in a 300 mm hole, the post takes up about 14% of the hole.
Using diameter instead of radius. The formula uses half the width of the hole. Doubling the radius by mistake gives four times the volume.
Assuming every bag yields the same. Check the bag before you multiply.
Digging wider than planned. If the hole ends up wider than you meant, especially near the top, measure the hole you actually dug, not the one you planned.

For a slab rather than a post, use our free concrete slab calculator. For walls, see how many bricks per m2.