Sometimes you wonder whose pencil is longer, yours or a friend's. Lay the two pencils side by side and just look, and they often seem about the same, which can be confusing. This is where a ruler tells you exactly.
A ruler has marks drawn at even intervals, with a number written at each one. Each of these numbers is a unit of length called a "centimeter (cm)." One mark-to-mark space is 1 cm.
There's one rule you always have to follow when measuring length with a ruler: line up the left end of the object exactly with the 0 mark on the ruler.
Why does it have to line up with 0? The marks on a ruler count up in order starting from 0: 1 cm, 2 cm, 3 cm, and so on. That's why lining up the object's starting point with 0 makes the number at the object's ending point exactly equal to its length.
What happens if you start measuring from 1 instead of 0? Reading the number at the end point directly would give you a length shorter than the object really is. That's why it's important to build the habit of always starting from 0.
Sometimes a ruler that's been used for a long time has a worn-out end, making the 0 mark hard to see. In that case, you can start from any mark you can see clearly, then subtract the starting mark's number from the ending mark's number. But when you're first learning, starting from 0 is the way that's least likely to cause confusion.
Measuring length comes up all over everyday life — measuring your body when buying clothes, measuring the width of a spot in your room for furniture, and using a ruler to cut colored paper in art class.
On our activity page, you can use a slider to change an object's length and see for yourself how the marks on the ruler are read, and try a quiz where you guess the length just by looking at the marks. Practice a few times and build your confidence reading a ruler.