Answer:
Use an online converter but there is something you should know
Explanation:
There are two systems of measurement. The Imperial system (inch, mile, cups, ounces) and the Metric system (millimeter, centimeter, gram, kilogram).
Converting measurements in the imperial system by memory is very hard, but converting measurements in the metric system is more simple.
It is easier to learn by images so here is an image that explains it
Keep in mind that measurements like inches and centimeters are for distance, while gram and ounces are for weight.