Measuring in inches and in centimeters
In 2nd grade, children learn to measure one object in both inches and centimeters and to explain why the smaller unit gives the larger number.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 2nd grade practice. Tap the speaker to hear one read aloud, or the light bulb for a hint, and talk about the answer together.
Question 1 of 3
Maria measured her book using small blocks and counted 10 blocks. If she measures the same book using bigger blocks, what will be true about the number of bigger blocks she counts?
- She will count fewer bigger blocks.
- She will count more bigger blocks.
- She will count the same number of bigger blocks.
Why this matters
This is where your child discovers that a measurement needs both a number and a unit. Seeing that smaller units give bigger numbers is surprisingly important later: in 3rd grade the same idea helps with fractions, where more pieces in a whole means smaller pieces, and in 4th and 5th grade they convert between units, such as knowing that 3 feet is 36 inches.
Where kids can get stuck
- Thinking the bigger number means longer, so a book measuring 25 centimeters seems longer than the same book at 10 inches.
- Expecting a bigger unit to give a bigger number, so predicting more feet than inches for the same object.
- Writing a measurement without a unit, such as 25, so no one can tell if it means inches or centimeters.
How you can help
- Measure the kitchen table with a ruler in inches, then in centimeters, and ask your child why the second number is bigger even though the table stayed the same.
- Measure a hallway in your child's footsteps and then in your own, and talk about why the child needed more steps. Connect it to small units and big units.
- Measure a toy in both paper clips and spoons, and before measuring with the second, ask your child to predict whether the number will be bigger or smaller.
If it’s helpful, you can watch the kid-friendly video below, or sign up for free, adaptive practice in Bobby's Math Club.
Kid-friendly explanation
Questions parents ask
Why is my child measuring the same thing twice?
Measuring the same object with two units shows your child that the number depends on the size of the unit. A smaller unit, like a centimeter, fits more times than a larger unit, like an inch. This builds understanding of what measurements mean and why the unit must always be named.
Why does a smaller unit give a bigger number?
A smaller unit takes up less space, so you need more of them to cover the same length. If a pencil is about 6 inches long, it is about 15 centimeters long, because centimeters are shorter. Using footsteps of different sizes is a good way to help your child see this.
Does my child need to convert between inches and centimeters?
No. In 2nd grade, children describe how two measurements relate to the size of the units, such as noticing that more centimeters than inches fit along an object. Converting units within one system, such as feet to inches, comes in 4th grade. Converting between inches and centimeters is not expected in elementary school.
Make learning math fun
Bobby's Math Club turns 2nd grade math practice into something fun to do together.
- Unlimited free practice
- A few minutes a day, at just the right level
- Level-ups, badges, and a cheer for every win
Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.
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Join the clubNote for teachers: This skill is covered by Common Core math standard 2.MD.A.2: “Measure the length of an object twice, using length units of different lengths for the two measurements; describe how the two measurements relate to the size of the unit chosen.” Learn how Bobby's Math Club can help your students grow. (Common Core 2.MD.2)