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Allmat Group

Fence post concrete calculator

Ten fence posts in 300mm holes 600mm deep, with 100mm of gravel under each 90mm post, take 0.34 m³: 4 bags of Rapid Set per hole, 40 x 20kg bags in all.

In US sizes ten 4x4 posts in 10in holes 2ft deep are 0.31 yd³ (8.4 ft³), about 20 x 80lb bags.

1. What are you doing?

2. Your job

Tap to type this size300mmTap to type this size600mmTap to type this size100mm gravelTap to type this size90mm post

Tap the hole width, depth, post or gravel on the picture to change it.

3. Sizes

Hole shape

Post shape

Product

Allmat concrete calculator

40 x 20kg Rapid Set Concrete 20kg

What we'd use

  • Rapid Set Concrete 20kg bag

    Rapid Set Concrete 20kg: 40 bags

    4 per hole × 10 holes.

You need 0.34 m³ of concrete

0.31 m³ plus extra for spillage and uneven ground (10% on the fence or gate posts).

Details: other bag sizes, water, weight, getting it home

About 40 x 20kg or 40 x 25kg bags of concrete, our estimate from bag weight.

  • Rapid Set Concrete 20kg (40 × 20 kg): Water: about 80 L. Weight: 800kg. No mixing: water in the hole, then the bag.

Bags or a truck?

No-mix Rapid Set goes straight into each hole, so bags are the way to go for posts.

Getting it home

Total weight: 800kg. About 1 ute load at 1,000kg, our estimate.

The load alone is over 750kg, the limit for an unbraked trailer, and the trailer's own weight counts too.

Click and collect or delivered across Adelaide.

How we worked it out
  • Fence or gate posts: 0.31 m³ plus 10% = 0.34 m³, which is 40 x 20kg bags.

Cement Australia: about 110 x 20kg bags make 1 m³ (their FAQ, calculator and the PROSTRENGTH Rapid Set data sheet). Rapid Set Concrete publishes no per-m³ figure, so it uses the same 110.

Trade Mortar: about 20 house bricks or 10 masonry blocks per 20kg bag at a 10mm joint (Cement Australia).

Estimates only. Check sizes and quantities against your plans, and with your engineer or builder, before you order.

Pour-day check

Type in the weather for the day you plan to pour. Temperature never changes how much you need; it changes the timing.

  • Rapid Set Concrete 20kg: 20°C is within the maker's range (10°C and up).
  • Rapid Set Concrete 20kg hardens in about 15 minutes at this temperature (guidance)
  • Keep it damp for 7 days (Cement Australia).
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How many bags of concrete per fence post?

Rounded up for each hole, because each hole is filled on its own, with 10% extra for uneven sides.

Cement Australia's bag says about 2 bags per post hole, which suits a smaller hole. A 300mm hole 600mm deep holds more, so check your own hole size.

Bags of Rapid Set Concrete (20kg) per fence post: round hole, 90mm square post, 100mm of gravel under the post, 10% extra, rounded up per hole
Hole width450mm deep600mm deep750mm deep900mm deep
200mm1223
250mm2344
300mm3457
350mm4679
Our estimate. Cement Australia publishes about 110 x 20kg bags per cubic metre for its concrete mixes but no figure for Rapid Set, so the same yield is applied here.

Bags per post in inches

For holes measured in inches, with a 4x4 post (3.5in actual) and 4in of gravel.

Bags of Rapid Set Concrete (20kg) per post: round hole, 4x4 post (3.5in actual), 4in of gravel, 10% extra, rounded up per hole
Hole width18in deep24in deep30in deep36in deep
8in2223
10in2345
12in3467
Our estimate. Cement Australia publishes about 110 x 20kg bags per cubic metre for its concrete mixes but no figure for Rapid Set, so the same yield is applied here.

How deep should fence posts be?

A common rule of thumb is to bury about a third of the post's length and to dig the hole about three times the post's width. Cement Australia's fence post guide adds 10 cm of gravel in the bottom for drainage, which is why the calculator takes the gravel out of the concrete.

The post itself also takes up room in the hole, so its size comes off the volume too. Many online calculators leave the post in, which over-orders.

Setting posts with Rapid Set

Rapid Set needs no mixing. Pre-soak the hole, pour about 2 litres of water per bag into it, then pour the bag in around the post. Cement Australia says it hardens within about 15 minutes and the supports can come off after an hour.

Rapid Set is for posts that carry no load. Tick "These posts carry weight" for a pergola, deck or carport and the calculator switches to Concrete Mix.

Questions

For a 300mm hole 600mm deep with 100mm of gravel and a 90mm post, 4 x 20kg bags. The table shows other hole sizes.

Cement Australia's guide places 10 cm of gravel in the hole for drainage before the concrete goes in.

Rapid Set is for non-structural posts. Pergola, deck and carport posts carry load, so use Concrete Mix or PROSTRENGTH Extra Strength.

About an hour with Rapid Set and 30 minutes with PROSTRENGTH Rapid Set, longer in cold weather. Always check the post is firm first.

Order it through Allmat from the result. We pick and pack the order, then it is click and collect or delivered across Adelaide.

Goes with this job

Disclaimer

This calculator gives estimates from the sizes you enter, using manufacturers' published figures and standard geometry. It is not engineering or design advice. Footing, slab and beam sizes must come from your plans or engineer. Allow for site conditions, spillage and uneven ground. Check quantities with your supplier before ordering. Allmat accepts no responsibility for decisions made using these estimates.

Sources

Last checked 5 October 2026.

Cite this page

Allmat, "Fence post concrete calculator", https://allmat.com.au/concrete-calculator/fence-posts, checked 5 October 2026.

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