What is in the bag
A bag of premixed concrete is cement, sand and aggregate already blended in the right proportions. You add water, mix, and place it. A 20 kg bag makes about 9.3 litres of wet concrete, which is enough for a 300 × 300 mm patch 100 mm thick, or about a fifth of a fence post hole. The figure is less than most people expect, because the dry material packs down and the water only fills the gaps rather than adding volume.
bags = volume (m³) ÷ 0.0093 so 1 m³ ≈ 108 bags of 20 kg
Yields differ slightly between brands. Some bags say 9 litres, some say 10. The calculator uses 9.3 litres as a middle figure. If your bag prints a yield, use that and adjust.
Where bags stop making sense
Bags are convenient and they are the right answer for a few post holes, a small pad for a water tank, a repair or a step. The trouble is weight and time. A cubic metre is 108 bags, which is 2.2 tonnes to lift off a trailer, tip into a mixer, and barrow into place before the first batches start to set. A concrete plant will deliver the same cubic metre mixed, tested and ready to place.
| Job | Volume | 20 kg bags | Weight | Verdict |
|---|---|---|---|---|
| 4 fence posts | 0.16 m³ | 18 | 360 kg | Bags |
| Water tank pad 2 × 2 × 0.1 | 0.40 m³ | 45 | 900 kg | Borderline |
| Shed slab 3 × 3 × 0.1 | 0.90 m³ | 102 | 2.0 t | Premix |
| Shed slab 6 × 4 × 0.1 | 2.40 m³ | 271 | 5.4 t | Premix |
Bag counts include 5 percent waste.
Mixing it properly
Most weak bagged concrete is weak because of water, not because of the bag. Add about two litres to a 20 kg bag, mix until every bit of dry material is wet, then add the rest slowly until the mix holds its shape when you cut it with the shovel. If it flows like porridge it has too much water and will finish soft and dusty. A wheelbarrow takes one bag comfortably and two if you are careful. A small electric mixer takes two or three. Mix only what you can place in about 30 minutes, and keep it out of direct sun on a hot day.
Strength
General purpose premix is usually rated around 20 MPa at 28 days when mixed as directed, which is fine for paths, post footings, small pads and repairs. Higher strength bags of 32 to 40 MPa exist for structural footings and where an engineer has specified a grade. Rapid set bags trade a little strength and a lot of working time for the convenience of setting in 15 to 30 minutes, which is why they suit posts and not slabs.
Worked example
A garden shed base of 2.4 × 1.8 m at 100 mm thick is 0.43 m³. With 5 percent waste it is 0.45 m³, or 49 bags of 20 kg. That is just under a tonne of bags, about 113 litres of water, and a long morning with a mixer. The same 0.45 m³ from a plant would be a 0.6 m³ minimum delivery. Ring both, price both, and pick the one that suits your back and your budget.
Common questions
How much concrete does a 20 kg bag make?
About 9.3 litres, or 0.0093 m³, once mixed. Check the yield printed on your bag.
How many 20 kg bags in a cubic metre?
About 108, which is over two tonnes of bags. Past half a cubic metre, premix is the better buy.
How much water per bag?
Around 2 to 2.5 litres per 20 kg bag. Add it gradually and stop when the mix holds its shape.
How strong is bagged concrete?
Most general purpose bags are about 20 MPa at 28 days when mixed as directed. High strength bags of 32 to 40 MPa are sold for footings.
Yields and water figures are typical for Australian premix bags. Follow the instructions on the bag you buy. For structural work, use the grade your engineer specifies.