Lesson 5.3 · Measurement
Estimating and comparing lengths
Sometimes you don't have a ruler. You can still make a smart guess, called an estimate. And when you do measure two things, you can find how much longer one is.
Making an estimate
To estimate, compare the object to something you already know.
| Benchmark | About how long |
|---|---|
| width of your fingertip | 1 centimeter |
| a paper clip | 1 inch |
| a school ruler | 1 foot |
| a big step | 1 yard |
| a meter stick | 1 meter |
Definition
Estimate
An estimate is a careful guess about a measurement. It does not have to be exact, but it should be close.
Worked example: Estimate a book
A book is about as long as 10 paper clips. About how long is the book?
Each paper clip is about 1 inch. So the book is about 10 inches long.
Comparing lengths
To find how much longer one thing is, subtract the shorter length from the longer length. Both lengths must use the same unit.
How much longer?
Longer length − shorter length = how much longer
Worked example: Two ribbons
A red ribbon is 8 inches long. A blue ribbon is 5 inches long. How much longer is the red ribbon?
. The red ribbon is 3 inches longer.
Worked example: Check an estimate
Kim guessed her shoe was 20 cm long. She measured it. It was 23 cm. How far off was her guess?
. Her estimate was off by 3 cm. That is a close guess!
Common mistake
Don't compare numbers with different units. A 3-foot desk is longer than a 20-inch box, even though 3 is less than 20. A foot is 12 inches, so 3 feet is 36 inches.
Practice
About how long is a paper clip?
What is the best estimate for how tall a door is?
A green crayon is 9 cm long. A yellow crayon is 5 cm long. How many centimeters longer is the green crayon?
Type a number.
A pencil is about as long as 6 paper clips. About how many inches long is the pencil?
Type a number.
A fence board is 40 inches long. A stick is 25 inches long. How many inches longer is the board?
Type a number.
Omar guessed a table was 60 inches long. It really was 55 inches. How many inches off was his guess?
Type a number.
A rope is 2 yards long. A string is 5 feet long. Which is longer?