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Australia

Concrete Post Hole Calculator

Enter the hole and the post, in millimetres and metres, and the number of posts. The calculator takes the post out of the hole and gives the 20kg bags for the whole fence or pergola.

What are you pouring?
Fence or deck posts set in concrete
ground Hole diameter or width Hole depth Post size (100 mm = 4×4)

Tip: metres and millimetres, or feet and inches if your plans use them, like 10' 6": we convert them.

Use your own prices (optional)

You need

– m³

–

Poured, it weighs about – kg.

Premixed bags

Bag Bags To carry Est. cost

Mixing it yourself

Cement Australia, General Purpose Cement product data sheet (PDS C01, 2016): mix by volume cement : sand : aggregate and 20kg bags per m³ – high strength 1 : 1.5 : 3 (17 bags), paths and driveways 1 : 2 : 3 (16), foundations and footings 1 : 3 : 3 (13). The sheet gives no water: we use half the cement weight (water/cement 0.5). Kilos of sand and aggregate from 2,400 kg per m³ of fresh concrete, split by the volume parts.

Ready-mix truck

–

How we price

Link to this calculation Download estimate (PDF)

Your materials at a glance

How to calculate concrete

  1. Measure the hole. Diameter (auger) or width (dug by hand) and depth.
  2. Measure the post. 100 × 100 mm for most timber posts, 50 × 50 mm for many steel fence posts.
  3. Subtract the post. Hole volume − post volume = concrete for one hole.
  4. Multiply by the posts. Then add 5–10% and divide by 0.0094 m³ for 20kg bags.

Worked example: a pergola with 8 posts, 400 mm holes 600 mm deep

  1. One hole: π × 0.2² × 0.6 − 0.1 × 0.1 × 0.6 = 0.0694 m³.
  2. Eight holes plus 5%: 0.583 m³.
  3. 20kg bags: 63, about 1,260 kg.

Concrete per post hole

Round hole (diameter × depth) Post m³ 20kg bags
250 × 500 mm 50 × 50 mm 0.023 3
300 × 600 mm 50 × 50 mm 0.041 5
300 × 600 mm 100 × 100 mm 0.036 4
350 × 700 mm 100 × 100 mm 0.060 7
400 × 600 mm 100 × 100 mm 0.069 8
450 × 900 mm 125 × 125 mm 0.129 14

One hole, square post, no waste included. 20kg bags at about 0.0094 m³ each.

Concrete formulas by shape

All measurements in metres; divide millimetres by 1,000 first. Results are in cubic metres.

Post hole
(hole area − post area) × depth; round hole π × (diameter ÷ 2)², square hole width²
Column / pier
π × (diameter ÷ 2)² × height

1 m³ = 1,000 litres.

Frequently asked questions

How deep should a fence post hole be?

A common rule of thumb is about a third of the post's height above ground, often 600 mm for a 1.8 m fence, with a hole about three times the post's width. Wind, soil and your fence supplier's instructions come first.

Should I use rapid-set concrete for posts?

Quick set mixes are made for posts and set in minutes; the bag count is the same, because Australian Builders gives its quick set mix the same 10 litres a bag (we count 9.4 for both).

How much concrete for a pergola post?

A 400 mm hole 600 mm deep around a 100 × 100 mm post takes about 0.07 m³: 8 bags of 20kg.

About the prices

Costs are estimated ranges with source and date, never a quote. Enter your local prices under "Use your own prices", or read how we estimate prices.