Bagged concrete guide
Turn form volume into bags using the product’s stated wet yield.
Back to calculator ↑Measure and prepare
Measure form length, width and actual pour depth in metres. Small depth errors can add substantial volume; check the form after excavation.
Measure inside the prepared forms rather than their outside dimensions. Check depth at several points because sub-base levels may vary. For zones of different thickness, calculate each volume separately and add them before estimating bags. Replace the example wet yield with the exact product label value.
How the estimate works
Cubic metres equal length × width × depth. Multiply by 1,000 for litres, add allowance, divide by the stated mixed yield per bag and round up.
The example is 4 × 3 × 0.1 m = 1.2 m³, or 1,200 L. Ten percent allowance gives 1,320 L. With a stated mixed yield of 17 L per bag, 1,320 ÷ 17 = 77.65, rounded to 78 bags. Do not divide wet volume directly by dry bag kilograms; these are different units.
Check the order
Use the exact product’s wet mixed yield; dry bag mass cannot replace it. For a large pour, compare bag mixing with ready-mix delivery.
Check whether the bagged mix is suitable for the intended application and thickness. Check labour, water, equipment and mixing capacity before planning a continuous pour of many bags. Have the installer plan placing and curing.
Limits of the estimate
Structural work needs a professional design; this is a bag-count estimate.
This is a material volume estimate, not a safe slab thickness for a given load. It does not design reinforcement, joints, settlement control or required strength. The 10% extra is editable for expected site loss. Load-bearing slabs and footings require a professionally checked design. If forms have pipe voids, recesses or thickened edges, calculate those volumes separately and add or subtract them before rounding bags.
Common questions
Does every bag yield 17 L?
No. It is one product example; replace it with your bag’s stated yield.
Can I enter centimetres?
The field uses metres; 10 cm is 0.10 m.
What if the bag count is large?
Compare bag mixing with ready-mix and plan a continuous pour.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.