In Grade 1 you laid cubes end to end. A ruler has those units printed on already.
Step 1: Let's Learn
Read it, or press Listen and follow the words.
The units on this ruler are centimetres.
Centimetre is a unit of length. Your fingernail is about one centimetre wide.
Line up the end with 0
Put one end of the object at 0. Then the number at the other end is the length.
Why zero matters
If the object starts at 1, the far end reads one too many. The number is the distance from 0.
What a ruler really is
A ruler is a row of equal units printed in a straight line so you never have to lay out the units yourself. Before rulers, people measured by placing copies of a unit end to end. The ruler just does that once and keeps it, which is why it must be used carefully — the units only line up if you start in the right place.
Start at zero, not at the edge
The most common measuring mistake is lining the object up with the end of the ruler instead of the 0 mark. Many rulers have a small blank strip before the 0. Find the 0, put the end of the object exactly there, and read where the other end lands.
When the end lands between marks
Real objects rarely end exactly on a line. If a pencil ends between 9 and 10, say it is about 9 centimetres or nearly 10 centimetres, whichever mark it sits closer to. Saying "about" is not a failure — every measurement in the world is approximate.
Step 2: Try It Yourself
Slide the object off zero and see what goes wrong.
centimetres
05101520
It starts at 0 and ends at 8, so it is 8 centimetres long.
Step 3: Watch an Example
One step at a time.
Watch Ana Measure a Pencil
Ana has a pencil and a centimetre ruler.
- Step 1
She lines the blunt end up with the 0, not with the edge of the ruler.
Step 4: Your Turn
Practice makes it stick.
The Wrong Start
Problem 1 of 2
Leo starts a 6 cm crayon at the 2 mark. What number does the far end land on?
on the ruler
Measuring a Leaf
Problem 2 of 2
A leaf starts at 0 and ends at 11 on the ruler. How long is the leaf?
cm
Read the Ruler
1 of 3
An object goes from 0 to 7. How long is it?
2 of 3
An object goes from 3 to 9. How long is it?
3 of 3
An object goes from 2 to 10. How long is it?
Try Measuring Badly
Move the start away from 0 and watch the note explain the mistake.
centimetres
05101520
The object does not start at 0. Reading the far end gives 8, but it is only 5 centimetres long.
Step 5: Quick Check
Show what you know.
Question 1 of 2
Where should one end of the object go?
Question 2 of 2
An object goes from 0 to 14 cm. How long is it?
What You Learned
- A ruler has the units printed on it.
- Line one end up with 0.
- The length is the distance from 0, not the tick you happen to see.