How many bags of concrete do I need?
Last reviewed 1 July 2026
Short answer
Divide the volume by the bag's yield and round up. A 25 kg bag yields about 0.0118 m³, so a cubic metre needs about 85 bags. An 80 lb bag yields 0.60 ft³, so a cubic yard needs about 45 bags. Always add a waste allowance before rounding.
Work it out for your project with the concrete slab calculator →
Bag yields
| Bag | Yield | Bags per m³ | Bags per yd³ |
|---|---|---|---|
| 20 kg | 0.0094 m³ (9.4 L) | 107 | 82 |
| 25 kg | 0.0118 m³ (11.8 L) | 85 | 65 |
| 40 lb (18.1 kg) | 0.30 ft³ | 118 | 90 |
| 50 lb (22.7 kg) | 0.375 ft³ | 94 | 72 |
| 60 lb (27.2 kg) | 0.45 ft³ | 79 | 60 |
| 80 lb (36.3 kg) | 0.60 ft³ | 59 | 45 |
US figures are the published manufacturer yields. Metric bag yields are derived from the same volumetric rate of about 0.00047 m³ per kg of dry mix.
Why the yield is less than you would expect
A 25 kg bag of dry mix does not make 25 kg of concrete divided by 2,400 kg/m³, which would be 0.0104 m³. It makes slightly more, because you add water — roughly 2 to 2.5 litres per 25 kg — and that water becomes part of the volume.
It is also why the yield varies between manufacturers: the aggregate grading and the water demand differ. If you are ordering more than a couple of dozen bags, check the yield printed on the bag rather than relying on a table.
Bag counts for common jobs
| Job | Volume | 20 kg bags | 25 kg bags | 80 lb bags |
|---|---|---|---|---|
| Fence post hole (300 × 600 mm) | 0.042 m³ | 5 | 4 | 3 |
| Shed base 2.4 × 1.8 m at 100 mm | 0.43 m³ | 51 | 41 | 29 |
| Patio slab 3 × 2.4 m at 100 mm | 0.72 m³ | 85 | 68 | 47 |
| Garage floor 6 × 3 m at 150 mm | 2.70 m³ | 317 | 252 | 175 |
The garage floor line is exactly why ready-mix exists — 252 bags is over six tonnes of bags to handle.
Practical notes
- Buy whole pallets if you can. Merchants often price a pallet of 40–56 bags well below the individual bag price.
- Keep them dry. A bag that has taken up moisture will have lumps of set cement in it and will never mix properly.
- Mix consistently. Varying the water between batches varies the strength and the colour. Measure the water rather than judging it.
- Do not stretch the mix. Adding sand or ballast to a bagged mix to make it go further reduces the cement content and the strength.
Note: Round up per pour, not per project
Safety notice: Material planning only