On a small job the choice between bagged concrete and a premix truck comes down to three things: volume, access and labour. Material price is only part of the answer. A pour that looks cheaper in bags can easily cost more once you count the hours, the mixer hire and the risk of a poor finish.
How much a bag actually makes
A 20 kg bag of concrete mix makes about 0.0095 m³, so you need around 105 bags for one cubic metre. That is roughly 2.1 tonnes of material to lift, tip, mix and place for every cubic metre.
The real cost of bags
Add up the hidden costs of a bag pour:
- Labour. A two-person crew with a mixer realistically places about 1 m³ an hour once you include fetching bags, loading, mixing, barrowing and placing. A small slab can easily take a full day for two people.
- Consistency. Every batch varies a little in water content. On a large slab this shows as patchy colour, uneven strength and cracking.
- Cold joints. If one end of the slab starts to set before the other is finished, you get a weak plane right through it.
- Handling and waste. Split bags, part bags left over, wet bags and wasted water all add up.
- Equipment. Mixer hire, plus transport for a pallet or two of bags. A pallet often weighs over a tonne, beyond what most utes or box trailers can carry.
Rules of thumb
| Volume | Usually best |
|---|---|
| Under 0.25 m³ (post holes, small pads) | Bags |
| 0.25 to 1 m³ | Bags with a mixer, or a small-load supplier |
| Over 1 m³ | Premix truck |
| Any volume with poor access | Ready-mix with a line pump, or powered barrow hire |
Short-load fees and minimums
Most Australian plants add a short-load (small-load) fee below about 3–4 m³, and some charge extra for Saturday, early or late pours. Mini-mix and mix-on-site trucks suit small jobs and jobs where the volume is uncertain. Even with the fee, a truck often beats bags once you count labour, because a crew can place a small load in under an hour and spend the rest of the day finishing.
A quick comparison
Take a 2 m³ shed slab. In bags that's about 210 bags of 20 kg, plus mixer hire, and two people for most of a day. By truck it's 2 m³ plus a short-load fee, and the pour takes about 30 minutes. Unless your labour is free, the truck usually wins. Use the slab calculator to get your volume, then compare your own bag price and labour rate against a phone quote from a local plant.
Site mixing from cement and aggregate
Mixing from GP cement and a concrete blend (sand and 10–20 mm aggregate, sold by the cubic metre at landscape yards) is cheaper per cubic metre than bagged mix but heavier on labour. Allow about 16 bags of 20 kg cement and 1.15 m³ of blend per cubic metre for a general-purpose mix. Measure by the bucket, not the shovel, and keep the water consistent between batches.
When bags are the right answer
- Fence posts, sign posts and mailbox posts, one hole at a time.
- Small repairs, steps and kerbs.
- Places a truck can't reach and a pump can't justify.
- Jobs spread over several days where a single big pour isn't possible.
Use the slab calculator with Supply as set to each option to compare quantities on your own job, and the fence post concrete calculator for post holes.
Questions to ask the ready-mix plant
- What's the minimum load, and what's the short-load fee below it?
- How long is the free unloading time, and what do they charge per minute after it?
- Can the truck reach the pour, and how far will the chute go? Do they offer a line pump or a conveyor?
- What mix do they recommend for the job, and is it available at short notice?
- Do they charge extra for Saturdays, early starts or retarders, accelerators or fibres?
Getting the pour day right
Whichever way you go, the pour itself goes better with a plan. Have the forms checked, the reinforcement on chairs and the sub-base damp (not wet) before the concrete arrives. Have enough hands: at least three people for a truck pour of a few cubic metres, one discharging, one spreading and one screeding. Keep a few bags on site as a backup in case the estimate comes up a little short. A wheelbarrow of bagged mix can finish an edge without calling for another load.
For bag pours, set up a production line before you start: bags stacked next to the mixer, water measured into buckets, and a clear barrow route to the forms. Pour from the far end back toward the mixer, so you never have to cross fresh concrete.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.