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Generated on: July 20, 2026 at 09:33 PM

Post-Hole Concrete Calculator

Inputs

Post diameter10 cm
Hole diameter30 cm
Hole depth60 cm
Number of posts6
Bag yield12.7 l

This is an estimate based on a cylindrical hole and the bag yield you enter. Actual use varies with hole shape, how much the post displaces, spillage, and whether you crown the top for drainage. Set posts below the frost line and follow local code for fence, deck, and load-bearing posts; buy a spare bag, since a slightly oversized hole is easy to under-fill.

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Construction & DIY

Post-Hole Concrete Calculator

Work out how much concrete is needed to set fence or deck posts. Each hole is a cylinder minus the post that sits in it, so the calculator gives the volume and the number of bags per post and in total.

Inputs

≥ 0.1 cm
≥ 0.1 cm
≥ 1
≥ 0.1 l

Results

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Details

6 posts need about ... bags of concrete — roughly ... bags (...) per post.

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Post-Hole Concrete

Setting a fence or deck post in concrete means filling the ring of space around the post inside a dug hole. The concrete needed is the volume of the hole minus the volume the post takes up, and this calculator turns that into the number of bags per post and in total.

The hole-minus-post volume

A round hole is a cylinder, so its volume is pi times the radius squared times the depth. The post displaces a smaller cylinder of the same depth, so the concrete that surrounds one post is the difference:

V1=π(dh2)2h−π(dp2)2h\begin{aligned} V_1 &= \pi \left(\tfrac{d_h}{2}\right)^2 h - \pi \left(\tfrac{d_p}{2}\right)^2 h \end{aligned}

Multiplying by the number of posts gives the total volume, and dividing each volume by the yield yy of one bag — and rounding up — gives the bag counts:

V=V1 pB=⌈Vy⌉\begin{aligned} V &= V_1 \, p \\ B &= \left\lceil \frac{V}{y} \right\rceil \end{aligned}

Worked example

Consider six posts 10 cm wide, set in 30 cm holes dug 60 cm deep, using 60 lb bags that yield about 12.7 L each:

V1=π(0.15)2(0.6)−π(0.05)2(0.6)=0.0424−0.0047=0.0377V=0.0377×6=0.226B=⌈22612.7⌉=⌈17.8⌉=18\begin{aligned} V_1 &= \pi (0.15)^2 (0.6) - \pi (0.05)^2 (0.6) = 0.0424 - 0.0047 = 0.0377 \\ V &= 0.0377 \times 6 = 0.226 \\ B &= \left\lceil \frac{226}{12.7} \right\rceil = \lceil 17.8 \rceil = 18 \end{aligned}

So each post takes about 38 L of concrete — roughly three bags — and the six posts need about 18 bags.

Consider six posts 4 in wide, set in 12 in holes dug 24 in deep, using 60 lb bags that yield about 0.45 cu ft each:

V1=π(0.5)2(2)−π(0.167)2(2)=1.571−0.175=1.396V=1.396×6=8.38B=⌈8.380.45⌉=⌈18.6⌉=19\begin{aligned} V_1 &= \pi (0.5)^2 (2) - \pi (0.167)^2 (2) = 1.571 - 0.175 = 1.396 \\ V &= 1.396 \times 6 = 8.38 \\ B &= \left\lceil \frac{8.38}{0.45} \right\rceil = \lceil 18.6 \rceil = 19 \end{aligned}

So each post takes about 1.4 cu ft of concrete — roughly three or four bags — and the six posts need about 19 bags.

Choosing inputs

A common rule sizes the hole at about three times the post width and sets it below the local frost line, often a third of the post’s above-ground height. The bag yield matters as much as the hole size: a 60 lb bag yields about 0.45 cu ft and an 80 lb bag about 0.6 cu ft, so entering the bag you actually plan to buy keeps the count right. Buy a spare bag, since a slightly oversized hole is easy to under-fill.

For posts set on a poured footing instead, see Concrete Volume Calculator.

Frequently Asked Questions (FAQ)

How much concrete do I need per post?

Treat the hole as a cylinder: its volume is pi times the radius squared times the depth. Subtract the volume the post itself takes up, then divide by the yield of one bag and round up. A 30 cm hole 60 cm deep around a 10 cm post holds about 38 L of concrete, which is roughly three 60 lb bags.

How deep should a post hole be?

A common guideline is to bury about one third of the post’s above-ground height, and always below the local frost line so the post is not heaved by freezing ground. A 1.8 m (6 ft) tall fence post is often set 60–90 cm (2–3 ft) deep. Gate posts and corner posts carry more load and are usually set deeper and wider.

How many bags of concrete per post?

It depends on the hole size and the bag weight. A typical fence-post hole takes two to three 60 lb bags or one to two 80 lb bags. This calculator divides the concrete volume per post by the bag yield you enter and rounds up, so you can match it to the bags you plan to buy.

Disclaimer

This is an estimate based on a cylindrical hole and the bag yield you enter. Actual use varies with hole shape, how much the post displaces, spillage, and whether you crown the top for drainage. Set posts below the frost line and follow local code for fence, deck, and load-bearing posts; buy a spare bag, since a slightly oversized hole is easy to under-fill.

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