2nd Grade · 2.MD.A.2

Measuring in inches and centimeters in 2nd grade

In 2nd grade math, measuring in inches and centimeters means measuring the same object twice, once with each unit. Your child then explains why the two numbers are different. A unit is the size of each step you count when you measure. This year your child learns that a smaller unit needs more steps, so the same object gets a bigger number.

What the standard says

The official Common Core wording, quoted exactly. The plain version follows.

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.

What it means, in plain words

An inch is about as wide as a quarter (the coin). A centimeter is much smaller, about as wide as a child's fingernail. Most school rulers show both units: inches along one edge and centimeters along the other.

When your child measures a cereal box with both edges of the ruler, the box gets two numbers, about 11 inches and about 28 centimeters. The box did not change. The number in centimeters is bigger because each centimeter is shorter, so more of them fit along the same box. This is the main idea of measuring in inches and centimeters in 2nd grade: the smaller the unit, the bigger the count.

The same rule works with any two units. A rug measured in feet and then in inches gets a bigger number in inches, because a foot is 12 inches. Your child does not convert between inches and centimeters in 2nd grade. The goal is to explain why one number is bigger and to check that both measurements make sense.

Your child should be able to:

Measure one object with the inch edge of a ruler and then with the centimeter edge
Explain that a smaller unit gives a bigger number for the same object
Say which of two units will give the bigger number before measuring
Check that two measurements of one object make sense together

Example

A book is 10 inches long. When it is measured with the centimeter side of the ruler, about how many centimeters long is it, and why is that number bigger?

Line up one end of the book with the 0 on the centimeter side.
The other end reaches the 25 mark, so the book is about 25 centimeters long.
Both numbers describe the same book. A centimeter is shorter than an inch, so it takes more centimeters to cover the book.
The book is 10 inches long and about 25 centimeters long. The bigger number comes from the smaller unit.

Example

A shoe is 20 centimeters long. Will it measure more than 20 inches or fewer than 20 inches?

An inch is longer than a centimeter.
A longer unit covers the shoe in fewer steps, so the number of inches must be smaller than 20.
With the inch side of the ruler, the shoe reaches about the 8 mark.
The shoe is about 8 inches long, which is fewer than 20 inches.

Where children usually get stuck

Your child measures a picture frame as 8 inches and 20 centimeters and says the frame is longer in centimeters.
The child reads the number as the length itself and forgets that each unit has its own size. A bigger number feels like a longer object.
What helps. Measure a spoon with large paper clips, then with pennies laid edge to edge, and count both. Ask why the pennies gave a bigger number when the spoon stayed the same.
Your child is asked for centimeters and writes the number from the inch side of the ruler.
Many rulers show inches on one edge and centimeters on the other, and the child reads the edge that faces up. The marks look alike, so nothing seems wrong.
What helps. Find the small "in" and "cm" labels on your ruler together. Before each measurement, ask your child to point to the label for the unit the question asks for.
Your child lines the object up with the 1 on the ruler instead of the 0, and every answer comes out 1 too big.
Counting starts at 1, so starting the ruler at 1 feels natural. A length is the number of spaces between marks, and the first space starts at 0.
What helps. Put a small sticker on the 0 mark of each edge of the ruler. Line the object up with the sticker every time.

Try four practice questions

Real 2nd grade questions from our question bank, easiest first. Answer them right here, check each one, and see a worked solution for any question.

1Warm-up
Each line below shows two measurements of the same object. Select all the lines where both measurements could be correct.
A
A rope is 4 feet long, and the same rope is 2 inches long.
B
A pencil is 7 inches long, and the same pencil is 1 foot long.
C
A bag of apples weighs 48 ounces, and the same bag weighs 3 pounds.
Show the answer and how to solve it
The two lines that could be correct are the pencil (7 inches = 1 foot) and the bag of apples (48 ounces = 3 pounds), because each one pairs the bigger number with the smaller unit.
1. 1When you measure the same thing in two units, the smaller unit takes more pieces to measure. So the bigger number always goes with the smaller unit. Inches are smaller than feet, and ounces are smaller than pounds.
2. 2Two lines follow this rule. The pencil is 7 inches and the same pencil is 1 foot — 7 is the bigger number, and inches is the smaller unit. That works. The bag of apples weighs 48 ounces and the same bag weighs 3 pounds — 48 is bigger, and ounces is smaller. That works too.
3. 3One line breaks the rule. The rope is 4 feet and the same rope is only 2 inches: the bigger number (4) is paired with feet, the bigger unit. A rope 4 feet long would be 48 inches, not 2 inches.
4. 4The two lines that could be correct are the pencil (7 inches = 1 foot) and the bag of apples (48 ounces = 3 pounds).
Common mistake. A common mistake is pairing the bigger number with the bigger unit. Remember, smaller units need more of them to measure the same thing, so the smaller unit always gets the bigger number.
The idea underneath. Smaller units take more pieces to measure the same thing. That's why the bigger number always pairs with the smaller unit when the same object is measured two ways.
2On level
Maya measured the same pencil two times. The first time she used inches and got 5 inches. The second time she used centimeters and got 13 centimeters.
Are both measurements correct? Look at both rulers, then choose the answer that best describes Maya's measurements.
A
Only the inches measurement is correct.
B
Neither measurement is correct.
C
Only the centimeters measurement is correct.
D
Both measurements are correct.
Show the answer and how to solve it
Both measurements are correct. The pencil is 5 inches and 13 centimeters. Maya's first measurement (5 inches) matches the inches ruler, and her second measurement (13 centimeters) matches the cm ruler.
1. 1Look at the inches ruler. The pencil starts at 0 and ends at 5. The pencil is 5 inches long.
2. 2Look at the centimeters ruler. The pencil starts at 0 and ends at 13. The pencil is 13 centimeters long.
3. 3Maya reported two measurements: 5 inches the first time and 13 centimeters the second time. Compare each one to its ruler: 5 inches DOES match (the inches ruler shows 5). 13 centimeters DOES match (the cm ruler shows 13).
4. 4Both of Maya's measurements match the rulers exactly. Both the inches measurement and the centimeters measurement are correct.
Common mistake. A common mistake is checking only one ruler and trusting that the other measurement must also be right. You have to look at BOTH rulers and compare each of Maya's measurements to its ruler, one at a time.
The idea underneath. When the same thing is measured in two different units, you have to check each measurement on its own ruler. One measurement being right doesn't mean the other one is right too — they are two separate facts to check.
3On level
Each sentence gives two measurements of the same object in two different units. Select all the sentences where both measurements could be correct.
A
A cane is 6 feet long, and the same cane is 72 inches long.
B
A fence is 84 feet long, and the same fence is 7 inches long.
C
A ladder is 5 feet long, and the same ladder is 60 inches long.
D
A post is 96 feet long, and the same post is 8 inches long.
Show the answer and how to solve it
Select the ladder sentence and the cane sentence. In each, the larger number (60 inches and 72 inches) is paired with the smaller unit. The fence and post sentences have the numbers swapped — they wrongly pair the larger number with feet, which is the larger unit.
1. 1When you measure the same object with two different units, the smaller unit always gives you the larger number. That is because more small units fit in the same length. Inches are smaller than feet, so the inches number should be larger than the feet number.
2. 2Look at the fence sentence. The same fence is 84 feet long AND 7 inches long. A fence that is 84 feet long would be enormous! The numbers are swapped. The correct pairing is 7 feet and 84 inches, since $7 \times 12 = 84$ . This sentence is not correct.
3. 3Look at the ladder sentence. The same ladder is 5 feet long AND 60 inches long. Since 1 foot is 12 inches, 5 feet equals 60 inches. The larger number (60) is paired with inches, the smaller unit. This sentence is correct!
4. 4Look at the post sentence. The same post is 96 feet long AND 8 inches long. A post that is 96 feet tall would be taller than a building! The numbers are swapped. The correct pairing is 8 feet and 96 inches, since $8 \times 12 = 96$ . This sentence is not correct.
5. 5Look at the cane sentence. The same cane is 6 feet long AND 72 inches long. Since 1 foot is 12 inches, 6 feet equals 72 inches. The larger number (72) is paired with inches. This sentence is correct!
6. 6The correct sentences are the ladder and the cane. Both pair the larger number with inches (the smaller unit). The fence and post sentences have the numbers swapped, so they should NOT be selected.
Common mistake. A common mistake is ignoring the size of the real object. When a sentence says 'A fence is 84 feet long, and the same fence is 7 inches long,' a student should notice that a real fence is not 84 feet long. The larger number must always go with the smaller unit, because more small units fit in the same length.
The idea underneath. When you measure the same length with two units, the smaller unit always gives the larger number — because it takes more small units to cover the same distance.
4Stretch
A hallway is 9 in one unit and 3 in another. The units are yards and feet. Which pairing is correct?
A
9 in yards, 3 in feet
B
3 in yards, 3 in feet
C
9 in yards, 9 in feet
D
3 in yards, 9 in feet
Show the answer and how to solve it
The correct pairing is 3 in yards, 9 in feet. A yard is bigger than a foot, so we need fewer yards (3) and more feet (9) to measure the same hallway.
1. 1Look at the two numbers and the two units. The numbers are 9 and 3, and the units are yards and feet.
2. 2A yard is bigger than a foot. In fact, 1 yard is the same length as 3 feet. So yards is the bigger unit and feet is the smaller unit.
3. 3When you measure with a smaller unit, you need MORE of them. When you measure with a bigger unit, you need FEWER of them. So the bigger number (9) goes with the smaller unit (feet), and the smaller number (3) goes with the bigger unit (yards).
4. 4Check: 3 yards means $3 \times 3 = 9$ feet. So the correct pairing is 3 in yards, 9 in feet — both describe the same hallway.
Common mistake. A common mistake is matching the bigger number to the bigger unit, picking '9 in yards, 3 in feet'. But a hallway that is 9 yards long would be as long as three cars parked end to end — way too long for a hallway! A bigger unit needs FEWER of them, not more.
The idea underneath. A bigger unit (like yards) means you need fewer of them to measure the same length. A smaller unit (like feet) means you need more.