By Tim King · Updated 19 September 2026

Concrete Post Hole Calculator

Bags of Concrete per Fence Post from Hole Diameter and Depth

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Units

Hole diameter (e.g. 300mm)

Hole depth (e.g. 600mm or 0.6)

Number of holes

Post width, optional (e.g. 100mm)

Waste allowance (%)

Result

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How the Concrete Post Hole Calculator works

This concrete post hole calculator tells you how much concrete each fence post needs. Enter the diameter and depth of the holes, how many posts you are setting and, if you like, the width of the post so its own volume is taken out of the concrete, and the readout gives the litres or cubic feet each hole takes, the total for the job including waste, its rough weight, and how many bags to buy. Switch to imperial to type sizes as 2'6" or 12in. The table repeats the bag count for the common bag sizes so you can compare what your merchant sells.

Concrete post hole calculator: a hole is a cylinder

Why the volume matters. A fence or deck post is held upright by the concrete around its base, not by the soil alone. A hole that is too narrow or too shallow gives the post little to resist leaning, and a post that rocks in its hole often leans within a season or two.

The formula

A hole is a cylinder, so its volume is π × (diameter ÷ 2)² × depth. For a square post that is width² × depth, so the concrete per hole is the hole minus the post. The calculator then multiplies by the number of holes and adds the waste allowance. How many bags? Divide the total by the bag's yield and round up.

The first worked example is a 150 mm hole 450 mm deep. It holds 7.95 litres. A 100 mm post takes 4.5 litres of that, leaving 3.45 litres per hole.

The second worked example is the default job: six holes, 300 mm across and 600 mm deep. Each hole is 42.41 litres, the post takes 6.00 litres, and 36.41 litres is left. Six holes with 10% waste come to 240.3 litres, which is 27 bags of 20 kg, or about 577 kg of concrete.

The third is a 30 m fence at 1.8 m centres, which needs about 17 holes. With the default hole the total is about 681 litres, or 76 bags of 20 kg, weighing about 1.6 tonnes. That is a trailer load.

Hole size

The common rule of thumb is a hole about three times the width of the post, and a depth of about a third of the post's total length, with at least 600 mm in the ground for a 1.8 m fence. Gates, posts carrying load and loose or wet ground all call for deeper and wider holes. In cold climates, go below the frost line.

Subtracting the post

For a 100 mm square post in a 300 mm hole, the post occupies about 14% of the hole. That is worth entering, because over six holes it saves about four 20 kg bags. If the post width is equal to or larger than the hole diameter, the calculator ignores the post. Leave the box blank for a post that sits on a bracket above the concrete.

Waste

The default 10% covers concrete lost to an irregular hole, a bag split in the wheelbarrow, and holes that are always a little wider than marked. In loose or sandy ground where the edges fall in as you dig, use 15% to 20%. Running short mid-job is worse, because a second mix will not match the first.

Bag yields

Bags are sold by the wet volume they make, not by dry weight. The calculator uses 9 litres for a 20 kg bag, 11.25 litres for 25 kg and 18 litres for 40 kg. In imperial it uses 0.375 cubic feet for 50 lb, 0.45 for 60 lb and 0.6 for 80 lb. Fast-setting post mix is poured dry and watered in. Trust the yield printed on the bag.

Above about one cubic metre, which is roughly 25 to 30 of the default holes, delivered concrete is usually cheaper and quicker than mixing bags by hand. Ask for the total in cubic metres or yards. For a slab rather than posts, see the concrete slab calculator.

How to read the result

Concrete per hole is the volume each hole takes after the post is subtracted, before waste.
Total for the holes includes waste. This is the figure to order.
Bags is the total divided by the 20 kg or 60 lb yield, rounded up.
Approximate weight uses 2,400 kg per cubic metre, or 150 lb per cubic foot.

Tips and pitfalls

Measure depth after digging, from the ground to the bottom of the hole. Loose spoil on the floor makes the hole shallower than the line you marked.
Brace the post while the concrete sets. Check it for plumb twice, since the mix can still move as it stiffens.

The table gives the concrete per hole for common diameters and depths, with a 100 mm post subtracted and no waste added.

Hole diameter	Depth	Concrete per hole	20 kg bags per hole
150 mm	450 mm	3 litres	0.4
150 mm	600 mm	5 litres	0.5
150 mm	900 mm	7 litres	0.8
200 mm	450 mm	10 litres	1.1
200 mm	600 mm	13 litres	1.4
200 mm	750 mm	16 litres	1.8
200 mm	900 mm	19 litres	2.1
250 mm	450 mm	18 litres	2.0
250 mm	600 mm	23 litres	2.6
250 mm	900 mm	35 litres	3.9
300 mm	450 mm	27 litres	3.0
300 mm	600 mm	36 litres	4.0
300 mm	750 mm	46 litres	5.1
300 mm	900 mm	55 litres	6.1
350 mm	450 mm	39 litres	4.3
350 mm	600 mm	52 litres	5.7
350 mm	900 mm	78 litres	8.6
400 mm	450 mm	52 litres	5.8
400 mm	600 mm	69 litres	7.7
400 mm	900 mm	104 litres	11.6

FAQ

How many bags of concrete do I need per fence post?

For a typical 300mm hole 600mm deep with a 100mm post, about 36 litres, which is four 20kg bags or three 25kg bags. In imperial, a 12-inch hole 30 inches deep with a 4x4 post takes about 1.75 cubic feet, or four 60lb bags. Enter your own sizes and the readout gives the exact count.

How deep should a fence post hole be?

At least 600mm (2 feet) for a 1.8-metre fence, and generally a third of the post's overall length, so a 2.4-metre post sits 800mm deep. Deeper and wider holes are needed in loose ground, for gates and for anything exposed to strong wind, and below the frost line where the ground freezes.

Should the hole be wider than the post?

Yes, about three times the width of the post: 300mm for a 100mm post, 10 to 12 inches for a 4x4. The concrete collar around the post is what holds it upright; a hole only slightly wider than the post gives almost no resistance to leaning.

Do I subtract the post from the concrete volume?

The calculator does it for you when you enter the post width. A 100mm post in a 300mm hole displaces about 14% of the concrete, enough to save a bag over six holes. Leave it blank for posts that sit on a bracket above the concrete.

Can I use post mix such as Postcrete?

Yes. Fast-setting post mixes are poured dry and watered in, but the volume they fill per bag is similar to ordinary premix, around 9 to 11 litres for a 20kg bag depending on the brand. Use the bag count as a guide and check the yield printed on the bag.