What is a cubic foot?

To work out a cubic foot you need to have all 3 dimensions. E.G. Length, Width and Height. Multiply the dims together and divide by 1728
1 cubic foot = a box 12 inches long by 12 inches wide by 12 inches heigh.
So if you measure a box that is 18 inches long by 18 inches wide and 24 inches heigh you would multiply 18x18x24 = 7776 then divide by 1728 = 4.5 cubic feet.
Why is it that the magic number 1728 makes this work?
12x12x12 (1 cubic foot dims) multiplied together = 1728
Hows that for some adult math?
Cheers The Moving Dr
PS more volume conversions are available here