I hate using rulers. Really, I do. The lines are tiny and hard to see. Some students rulers are in eighths, some in sixteenths, some even smaller than that. Some students still confuse inches and centimeters. Some students see "mm" on the centimeter side and think their ruler doesn't have centimeters. It can be a nightmare.
Now that I've had plenty of time to teach measurement, I'm starting to get the hang of it! So if you struggle with it, I hope this post helps you!
First things first... I find a ruler clip art and blow it up, from the one inch mark to the two inch mark. That one inch takes up the entire paper, just to give you an idea of how big I make it. If you can, print it on cardstock. Get one copy for yourself and make it on an overhead transparency.
Pass out the blown up inch and tell the students (and tell them again, and again) that this really isn't an inch. Explain that an inch is very tiny. Have them look at what an inch is. Draw an inch line segment on the overhead. Explain you wanted to blow it up so they can see all the tiny marks.
We measure up to sixteenths, so make sure your ruler is divdied that way. If a student's ruler is NOT in the sixteenths, let them borrow one of yours.
As a class, label the blown up inch. As you label yours, they should label theirs. Have them keep it in their math folder. As they practice measurement, have them take it out to use. I have them measure the line segment on their worksheet, count the number of lines on their ruler, then count the same number on their blown up inch.
This website is great for measurement worksheets for homework.