On a small job the choice between bagged concrete and a ready-mix lorry comes down to three things: volume, access and labour. Material price is only part of it.
How much a bag actually makes
A 25 kg bag of pre-mixed concrete makes roughly 0.011–0.012 m³, so you need around 87 bags for one cubic metre. A 20 kg bag makes about 0.0095 m³, around 105 bags per cubic metre. That is a lot of lifting, mixing and cleaning.
The real cost of bags
Add up the hidden costs of a bag pour:
- Labour. A two-person crew with a mixer realistically places 1 m³ an hour once you include fetching, mixing and placing.
- Consistency. Every batch varies a little in water content. On a large slab this shows as patchy colour and uneven strength.
- Cold joints. If one end starts to set before the other is finished, you get a weak plane through the slab.
- Handling and waste. Split bags, part bags left over and wasted water all add up.
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 | Ready-mix or volumetric lorry |
| Any volume with poor access | Ready-mix with a line pump or concrete barrow hire |
Part-load charges and minimums
Most UK suppliers add a part-load charge below about 4–6 m³, and volumetric (mix-on-site) lorries charge for time as well as volume. Even with the fee, a truck often beats bags once you count labour.
Site mixing from cement and aggregate
Mixing from cement and all-in ballast at 1:5 is the traditional UK approach. It is cheaper per cubic metre than bagged mix but heavier on labour. Allow about 13 bags of cement and 1.85 tonnes of ballast per cubic metre for a general-purpose mix.
Use the slab calculator with Supply as set to each option to compare them on your own job.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the Building Regulations and Building Control for your project.