Concrete in cubic metres: how to calculate a slab in m³
7 min read · Updated 2026-10-12 · BuildCalc
Outside the United States, concrete is ordered in cubic metres and comes in metric sacks, so a calculation done in feet and inches has to be converted before it means anything to a supplier. The good news is that the conversion happens once, at the start: work in metres from the beginning and every subsequent step — volume, bag count, cost — comes out in the units you will actually be quoted in.
Convert the dimensions first, not the answer
Almost every mistake in a metric concrete estimate comes from converting the final volume and getting it subtly wrong. Convert the three dimensions instead, and the volume falls out of the same multiplication you would use in feet. One metre is 3.28084 feet, so a slab measuring 10 by 12 feet with a 4 inch thickness becomes 3.05 by 3.66 metres with a 100 millimetre thickness.
Thickness is the dimension people forget to convert. Four inches is a bit over 100 millimetres, and rounding the slab thickness up from 100 to 150 millimetres changes the volume by half — which on a large slab is the difference between one delivery and two.
Cubic metres to cubic yards, and why the number is never round
A cubic metre is 1.30795 cubic yards, so a volume of one cubic metre is about 1.31 cubic yards and, in the other direction, one cubic yard is 0.7646 cubic metres. Suppliers in the United States order in fractional yards, and a result that lands on 1.31 yards will be rounded up to 1.5 or 2 in practice. Do not be surprised when the ordered figure is larger than the calculated one.
The same applies to the metric side: a result of 1.15 cubic metres is normally ordered as 1.2 or 1.5. Build the extra into your margin rather than treating it as a surprise.
Metric bag sizes and the yield figure that matters
Metric dry-mix bags come in 25 kg and 40 kg sizes. A 25 kg bag of standard concrete yields roughly 0.0146 cubic metres, and a 40 kg bag about 0.0233, though the figure is printed on every bag and varies slightly with the mix. Divide your volume by the yield per bag to get the count, then round up to whole bags — partial bags cannot be used later once mixed.
Note that 40 kg bags are noticeably heavier than the 20 kg bags sold in the United States, so check whether you can actually lift and place one before planning on hand-mixing a large volume. Above about one cubic metre, a delivered load is usually both easier and cheaper than mixing by hand.
Same job, different numbers: a worked example
A garden path 2.4 metres wide and 6 metres long, laid 100 millimetres deep, needs 2.4 × 6 × 0.1 = 1.44 cubic metres. At a 25 kg bag yielding 0.0146 cubic metres, that is 99 bags — so 100 bags. The same path in imperial would be 7.87 by 19.68 feet at 3.94 inches, giving 42.3 cubic feet or 1.57 cubic yards, which is the 1.31 figure again once rounded up.
That both answers are close, and the reason the conversion is safe to do at the end, is worth remembering — but the metric route is easier because the answer arrives directly in the unit that will be quoted back to you.
Run your own numbers
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Common questions
How many 25 kg bags make one cubic metre of concrete?
About 69, since a 25 kg bag yields roughly 0.0146 cubic metres. Check the yield figure printed on your bag, as it varies by mix.
Is one cubic metre bigger or smaller than one cubic yard?
One cubic metre is about 1.31 cubic yards, so a metric volume is the smaller quantity. A cubic yard is 0.7646 cubic metres.
Should I order more than the calculated volume?
Yes. Add five to ten per cent for uneven subgrade, formwork bulge and spillage, and remember that suppliers round fractional loads upward anyway.
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