Drum size is not batch size
A mixer is sold by its drum volume, but it can only mix a batch of about 60 to 70% of that, because the drum is tilted and the mix needs room to tumble. A 140-liter mixer makes about 90 liters of concrete per batch; a 160-liter mixer about 100 to 110. The maker's plate or manual usually gives the mixing capacity next to the drum size.
How many batches
- Work out the volume: a 3 m × 2 m slab 10 cm thick is 3 × 2 × 0.1 = 0.6 m³, or 600 liters.
- Add about 10% for waste and uneven ground: 660 liters.
- Divide by the batch size: 660 ÷ 90 = 7.3, so 8 batches.
A batch takes a few minutes to mix and must be placed while it is workable, so for more than about a cubic meter, ready-mix delivered by truck is often easier.
What goes into a batch
A common general-purpose mix is 1 part cement, 2 parts sand and 3 parts gravel by volume, with just enough water to make it workable; extra water weakens the concrete. General-purpose concrete uses about 300 to 350 kg of cement per cubic meter, so a 90-liter batch needs roughly 27 to 32 kg, a little more than one 25 kg bag. With bagged premix, a 25 kg bag makes about 11 to 12 liters, so a 90-liter batch needs about 8 bags; check the yield printed on the bag.
The Concrete Calculator works out the volume of slabs, footings and round columns with a waste allowance, and the number of premix bags.