It is difficult to provide help when we don't know the exact trouble that you're having. I guess that you have plugged these numbers in to a calculator and the result does not make sense?
General advice: Engineering expressions/ equations usually expect the variables to be in "standard units" like meters, newtons, etc. Therefore if you take a measurement in "mm" you must convert this number to "m" before you plug it into an expression UNLESS the author explicitly states something like "variable x is the length in mm". NOTE Be careful when converting mm2 to m2, etc - since 1,000mm=1m BUT 1,000,000mm2=1m2 and 1,000,000,000mm3=1m2. Google might be able to help you out, just type "convert mm^3 to m^3" into the search box.