Size of older GW Microwave

If you want to calculate the volume of an object and have the answer in cubic feet.

Volume=Length X Width X Height
With the dimensions you give
Volume=12.5in X 37in X 20in
Volume=9250 cubic inches

To convert 9250 cubic inches into cubic feet you need to understand that there are 1728 cubic inches in 1 cubic foot.
L X W X H = 12"X12"X12"=1728 cu in

You then multiply 9250cu in and (1/1728cu ft) Really you are dividing 9250 by 1728.
your answer is 5.35 cubic feet.

12 inches in 1 foot
144 square inches in 1 square foot
1728 cubic inches in 1 cubic foot

to convert you put the unit you want to keep on top of the dividend.
to conver to to feet ( randomly selecting 100 inches)
Example converting 100 inches to feet

100inches X ____1ft____ = (100/12) = 8.3 feet
12inches

or cubic inches to cubic feet
4000 cubic inches to cubic feet

4000cu in X ____1cu ft____ = ( 2.33 cu ft)
1728 cu in

if you notice that the units on top and bottom of the dividend cancel each other out leaving the numerator (top number) as your unit.
it also works in reverse

2.33 cu ft X ___1728 cu in____ = (4000 cu in)
1cu ft