An estimate is a smart guess. You can estimate a length by comparing it with lengths you already know, then check by measuring (Measuring with a Ruler).
| Unit | About as long as |
|---|---|
| 1 centimeter | the width of your little finger |
| 1 inch | the width of a quarter |
| 1 foot | a ruler, or a sheet of paper (11 inches) |
| 1 yard | a big step for a grown-up |
| 1 meter | a little more than a yard |
Why it works
If you know about how long 1 inch is, you can picture inch after inch along an object and count them. An estimate also helps you catch mistakes: if a ruler seems to say a pencil is 70 inches long, you know something went wrong.
Worked example: Pick a sensible estimate
About how tall is a classroom door: 2 centimeters, 2 meters or 20 meters?
- A door is taller than a grown-up.
- 2 centimeters is tiny, and 20 meters is taller than a house.
- About 2 meters makes sense.
Answer: about 2 meters
Worked example: Choose the unit
Would you measure a soccer field in centimeters or meters?
- A soccer field is very long.
- Meters are the bigger unit, so the number stays easy to use.
Answer: meters
Key idea
Estimate by comparing with a length you know. Use small units for small things and big units for big things.
Common mistake
Choosing the right number with the wrong unit, like “a pencil is 7 feet long.” Check: picture the unit. A foot is about as long as a ruler, and no pencil is 7 rulers long. It is about 7 inches.
Grown-up tip: Before you measure anything together, ask for a guess first, then see how close it was.