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Concrete Bags Calculator

Find how many concrete bags.

Result

46

How it is calculated

The number of bags is the volume to fill divided by the volume one bag makes: bags = ⌈volume / yield⌉. Volume is in cubic metres (length × width × depth in metres) and yield is the mixed concrete from one bag, also in cubic metres. If the yield is left empty or is zero or less, the engine uses 0.011 m³ per bag. The result is rounded up to a whole bag, because you cannot buy part of one, so 45.45 becomes 46. The tool adds no allowance for waste, spillage, uneven ground or over-digging, so many people add 5 to 10% to the volume. A bag's yield differs by weight and brand, so use the figure printed on the bag. Convert centimetres to metres first, as 20 cm is 0.2 m.

Worked example

Suppose a footing needs 0.5 m³ of concrete and a bag makes 0.011 m³ (the default yield). Divide: 0.5 / 0.011 = 45.45. Since part of a bag cannot be bought, the tool rounds up and shows 46 bags. Adding a 10% reserve to the volume (0.55 m³) would give 0.55 / 0.011 = 50 bags, which is safer if the ground is uneven.

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Common mistakes

  • Entering the volume in cubic centimetres or litres. The field expects cubic metres; 500 litres is 0.5 m³.
  • Using the bag's weight as its yield. The yield is the volume of mixed concrete in m³, usually printed on the bag.
  • Forgetting the depth conversion: 10 cm deep is 0.1 m, not 10.
  • Buying exactly the calculated number. With no waste allowance, any spillage or uneven base leaves you a bag short.

About this calculator

This calculator works out how many bags of ready-mix concrete a project needs by dividing the total volume to fill (length × width × depth) by the yield of a single bag, which depends on its weight.

DIYers and contractors use it before a trip to the hardware store for fence post footings, small slabs, or patio repairs, so they buy the right number of bags instead of guessing and making a second trip mid-pour.

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Comments (1)

  • Editorial team

    Turn the job into volume first: length times width times depth in metres, so a 2 m by 3 m slab that is 10 cm thick is 0.6 m3 and, at the default yield, 55 bags. The yield varies between bag sizes and brands, so replace the default with the figure printed on the bag you will actually buy. Which bag size and yield are you using for your job?

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