In Grade 1 you measured with same-size units laid end to end (Measuring with Same-Size Units). A ruler is a row of units already laid out for you.
Why it works
Each number on a ruler tells how many units there are from the 0 mark. The first inch is the space from 0 to 1, not the mark at 1. So if one end of an object is at 0, the number at the other end is its length. If it isn’t at 0, count the spaces under the object.
Worked example: Starting at 0
A crayon starts at 0 and ends at 4 on a centimeter ruler. How long is it?
- It starts at 0.
- Read the number at the other end: 4.
Answer: 4 centimeters
Worked example: Not starting at 0
A pencil lies from the 2 mark to the 7 mark on an inch ruler. How long is it?
- Count the spaces: 2 to 3, 3 to 4, 4 to 5, 5 to 6, 6 to 7.
- That is 5 spaces. Or subtract: $7 - 2 = 5$.
Answer: 5 inches
Key idea
Line up one end with 0 and read the number at the other end. If it isn’t at 0, count the spaces (the units), not the marks.
Common mistake
Lining the object up with the 1 mark instead of the 0 mark. Then every length comes out one too many. Check: count the spaces under the object with your finger.
Grown-up tip: Many rulers have a little extra edge before the 0. Show your child where 0 really is.