Concrete footing, pier and sonotube calculator

Four pad footings 450mm square and 450mm deep need 0.38 m³ with 5% extra, which is 43 x 20kg bags of Concrete Mix.

In US sizes four 18in square pads 18in deep is 0.53 yd³ (14 ft³), about 25 x 80lb bags.

1. What are you doing?

2. Your job

Tap a size on the picture to change it; the number of pads is below.

3. Sizes

Number of pads

Length (mm)

Width (mm)

Depth (mm)

Product

Extra allowed (%)

Allmat concrete calculator

What we'd use

Concrete Mix 20kg: 43 bags

Allow an equivalent brand to specification

You need 0.38 m³ of concrete

0.36 m³ plus extra for spillage and uneven ground (5% on the pad footings (square holes)).

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

About 43 x 20kg or 34 x 25kg bags of concrete, our estimate from bag weight.

Concrete Mix 20kg (43 × 20 kg): Water: about 75.3 L. Weight: 860kg. Mixer loads at 2 bags a load: 22.

Bags or a truck?

Bags work, or mix your own with Builders Cement, sand and aggregate.

Cement Australia's guide: under 0.1 m³ use bags, 0.1 to 0.5 m³ mix your own, over 0.5 m³ ready-mix. Up to about 1 m³ we still show the bags; past that, and for a house slab or a driveway beyond a small patch, it is a truck pour.

Getting it home

Total weight: 860kg. 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

Pad footings (square holes): 0.36 m³ plus 5% = 0.38 m³, which is 43 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.

Air temp (°C)

Wind (km/h)

Humidity (%)

Concrete Mix 20kg: 20°C is within the maker's range (5°C to 35°C).
Keep it damp for 7 days (Cement Australia).

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Strip, pad or pier footings

A strip footing runs under a wall: 10m of it at 450mm wide and 300mm deep is 1.35 m³. A pad sits under a single post or stump. A pier is a round hole, often formed with a cardboard tube.

Footing sizes come from your plans or engineer. This calculator turns those sizes into concrete and bags; it does not design the footing.

Concrete pier and tube volumes

A round pier is a cylinder: 3.1416 × radius × radius × height. A 300mm pier 1.2m deep holds 0.085 m³, or 0.11 cubic yards.

Footings in colder countries

In the US and Canada a footing has to sit below the local frost line, which can be over a metre down. Australian building rules have no frost-depth requirement, so check your own local rules if you are building somewhere that freezes.

Questions

Length × width × depth in metres gives cubic metres. For a round pier use 3.1416 × radius × radius × depth. Add 5% for the sides of the hole.

A 300mm round pier 1.2m deep holds about 0.085 m³, about 10 x 20kg bags of Concrete Mix with 5% extra.

Yes. Choose "Round column or pier" and enter the tube diameter and height.

Concrete Mix suits footings on small projects; PROSTRENGTH Extra Strength gains strength faster. Rapid Set is not for footings that carry load.

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

Cement Australia: Concrete Mix
Cement Australia: frequently asked questions
Allmat arithmetic (this calculator)

Last checked 5 October 2026.

Cite this page

Allmat, "Concrete footing, pier and sonotube calculator", https://allmat.com.au/concrete-calculator/footings, checked 5 October 2026.