The short answer is about 108 bags of 20kg, or about 86 bags of 25kg, to make one cubic metre of concrete. If that number is higher than you expected, that is the useful part of the answer.
Where the Number Comes From
One cubic metre of concrete weighs roughly 2,300 to 2,400kg. That is the figure everything else follows from, and it is worth pausing on: a cubic metre of concrete weighs about as much as a large car.
Dividing that weight by the bag size gives the bag count:
| Bag size | Bags per m3 | Bags per 0.5m3 | Bags per 0.1m3 |
|---|---|---|---|
| 10kg | about 215 | about 108 | about 22 |
| 20kg | about 108 | about 54 | about 11 |
| 25kg | about 86 | about 43 | about 9 |
| 40kg | about 54 | about 27 | about 6 |
Yield varies a little between products, so check the coverage printed on the bag and buy a few spare rather than stopping the pour to go back to the merchant. Different suppliers quote anywhere from 100 to 110 bags of 20kg per cubic metre, which is normal variation rather than a contradiction.
What That Means for a Real Job
Converting the common jobs into bags is where the number starts to bite:
- Fence post (0.03m3): about 3 bags of 20kg. Bags are clearly right.
- Small garden path, 5m x 0.6m at 75mm (0.23m3): about 25 bags. Doable, and about the practical limit.
- 3m x 2m shed base at 100mm (0.6m3): about 65 bags. Well past sensible.
- 12ft x 12ft slab at 100mm (1.34m3): about 145 bags. Not a realistic DIY mix.
- 40m2 driveway at 100mm (4m3): about 432 bags. Roughly ten tonnes of bags to carry, open and mix.
The problem is not only the count. It is that concrete you mixed two hours ago is already setting while you are still mixing the rest, which produces cold joints, weak planes between batches, and a slab that fails along them later.
Is It Cheaper to Buy Cement or Concrete?
Three different things get compared here, so it is worth separating them:
- Bagged premix is cement, sand and aggregate already combined. Most convenient, most expensive per cubic metre.
- Cement bought separately and combined with your own sand and aggregate is cheaper per cubic metre on materials, and gives you control over the proportions. Our mix ratio guide covers what those proportions should be.
- Delivered concrete is cheapest per cubic metre at volume and takes the labour and the consistency problem away entirely.
Buying cement separately genuinely is cheaper on materials. What it does not account for is mixer hire, buying sand and aggregate in quantities that rarely match what you need, disposing of the surplus, and a day of your time. Past roughly a quarter of a cubic metre those costs overtake the material saving.
The Crossover Point
As a working rule, bags make sense below about 0.25 cubic metres and delivery makes sense above it. At 2025 rates delivered C20 is roughly 90 to 120 pounds per cubic metre; a cubic metre in bags is around 108 of them, which comes to substantially more before mixer hire and labour.
The usual objection is that suppliers will not deliver small quantities, or that a part-load surcharge wipes out the saving. That is true of ready-mix, where the wagon runs partly empty. It is not true of volumetric concrete, which batches on site and meters what comes out of the chute, so there is no full-load minimum and a 0.5 cubic metre pour is a normal booking.
Work out your volume with the concrete calculator first. If it comes out above a quarter of a cubic metre, price the delivery before you buy a pallet of bags. Our guide to using bagged concrete covers doing it properly when bags are the right call.
