Get a ruler in your hands . Measure things until you start to understand how a ruler works . Measure some stuff and figure out where the center is . Say you measure a book and it ' s 78 thick . You look at your ruler and see that every eighth is divided into two sixteenths , so obviously half of 78 is going to be 716 . If you write that out you have 12 x 78 716 . And you notice that 12 is divided into 24 and then into 48 and so on , so you can convert anything to anything by multiplying all the numbers on top and then all the numbers on bottom . Other rulers are divided into 10 and 100 parts . But an inch is still an inch , so anything on one ruler can be translated to the other ruler . A half inch on one ruler is 510 or 50100 on the other . An eighth inch is just 12 . 5 marks when you have 100 marks per inch . A metric ruler divides an inch into 25 . 4 parts , so a half inch would be 12 . 7 of those parts . Pretty simple , isn ' t it ? Practice this a bit and people will think you went to wizard school . And , because somebody occasionally asks Yes , people are awed when I work fractions in my head .