If the slab is 4 inches thick the formula to find cubic yards is:
Lenght x Width divided by 81
For a 6 in thick slab:
Lengh x Width divided by 54
Another formula is to convert all measurements to inches and then:
Lengh x Width divided by 46,656 (the number of cubic inches in a cubic yard)
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how many yards of concrete will i need to do a 20 x20 x4 slab
45kg
3.95 cubic yards of cement are needed for a slab that size.
1.19 Cubic Yards
About 2.11 Cubic Yards.