You can measure the same thing with different units: inches or centimeters, feet or yards. The length doesn’t change, but the number does. You need Measuring with a Ruler for this.
Why it works
A centimeter is shorter than an inch, so it takes more centimeters to cover the straw. A big unit covers more length each time, so you need fewer of them. Small unit, big number. Big unit, small number.
| Bigger unit | is the same as |
|---|---|
| 1 foot | 12 inches |
| 1 yard | 3 feet |
| 1 meter | 100 centimeters |
Worked example: Feet to inches
A board is 2 feet long. How many inches is that?
- Each foot is 12 inches.
- $12 + 12 = 24$.
- 2 feet and 24 inches are the same length.
Answer: 24 inches
Worked example: Which is longer?
Which is longer: 30 inches or 2 feet?
- Change to the same unit: 2 feet is 24 inches.
- 30 inches is more than 24 inches.
Answer: 30 inches
Key idea
The bigger the unit, the fewer you need. To compare two lengths, change them to the same unit first.
Common mistake
Thinking 24 inches is longer than 2 feet because 24 is a bigger number. They are exactly the same length! Check by changing to the same unit: 2 feet = 24 inches.
Grown-up tip: Measure a table with a ruler in inches and again with a yardstick, and compare the two numbers.