Sorry, this is perhaps unfair; some radical simplifications have rid us of the cubit, the chain and the barleycorn, but we are still stuck with the thumb and the foot for measurement. And of course the yard, the mile and many more besides ...
The English word inch comes from Latin uncia meaning "one twelfth part" (in this case, one twelfth of a foot). Many other countries in Europe ignored the Romans and simply use the word "thumb".
You can imagine carpenters and stonemasons of old measureing out lengths by walking toe to heel, or their thumb (from knucle to nail). Hence the expression "rule of thumb" ..
Small wonder that the buildings of the era had what my realtor calls "rustic charm" - code for no right angles or level floors, walls or ceilings. In my opinion this rustic charm is typified in residential builds in California ... "craftsman style".
Anyway, for those with a post 20th century education, and a need for accurate mesurements, level floors and windows etc. the constant multiplication of ridiculous fractions (3 * 2"11/16) is tedious beyond belief.
So why hurt your brain? Ideally, mail order a real tape measure, or if you really have to - do the math in decimal and then convert to fractions using the tool below. This will help you talk to the assistant at your local big-box DIY store without them assuming you are Martian, French or something ...