Calculate from the stated yield
Use the mixed yield printed on the product label, not bag weight divided by a generic concrete density. The bag contains a dry mix; its weight alone does not tell you the finished volume.
Bags required = round up [volume in litres × (1 + allowance / 100) ÷ yield per bag in litres].
One cubic metre is 1000 litres. If a product states a yield of 10 litres per bag, 1 m³ needs 100 bags before allowance. If it states 12 litres, the same volume needs 84 bags before allowance. These example yields do not apply to every bag size or brand.
Worked example: a 0.6 m³ pour
- Convert to litres: 0.6 × 1000 = 600 L.
- Choose an example 10% allowance: 600 × 1.10 = 660 L.
- Read the label. For an example yield of 10 L per bag: 660 ÷ 10 = 66 bags.
- With a different product yielding 12 L per bag: 660 ÷ 12 = 55 bags.
Keep the units consistent and round the final bag count up once. Choose the allowance to suit the job. It is not a fixed code rule. Follow the product’s water, mixing, placement and curing instructions; do not add extra water to chase a larger yield.
Keep the working with your order
Find the slab or hole volume with the free web tool. Note the stated yield and bag count on the Chippy Tools concrete sheet.
What this calculation does not decide
It does not specify concrete strength, reinforcement or footing dimensions. Those come from the approved design and applicable requirements. For a large pour, discuss ready-mix quantities and delivery increments with the supplier.
Basis and product source
The sum turns a volume into a bag count using a known yield. The source for yield is the technical data sheet or bag label for the exact product you will buy. There is no fixed yield for each bag weight.