How many yards of concrete for 20x20x4in slab?

Answer 1

20 inches is 20/36 yard = 5/9 = 0.0.555555555555555recurring)yard

The other 20 inches also equals 0.5555555555555555555yard

The 4 inches = 4/36 yard = 1/9 = 0.1111111111111111111yard. Multiplying yards by yards by yards will give us "cubic yards"; that's what the concrete makers and builders mean by "yard".

Calculating 5/9 x 5/9 x 1/9 we have 25/729 = 0.3429 cu. yd (to 5 decimal places), and that is just a bit more than "a third of a yard".