2nd Grade · 2.MD.A.3
Estimating length in 2nd grade
Estimating length means making a sensible guess about how long something is without measuring it. In 2nd grade, your child learns to estimate length in inches, feet, centimeters and meters by picturing a familiar object for each unit. The goal is a number that is close, not exact.
What the standard says
The official Common Core wording, quoted exactly. The plain version follows.
2.MD.A.3 Estimate lengths using units of inches, feet, centimeters, and meters.
What it means, in plain words
An estimate is a close guess, made by thinking instead of measuring. Estimating measurements starts with a benchmark, an everyday object your child already knows the size of. Each of the four units in 2nd grade has a benchmark that is easy to picture.
A centimeter is about as wide as a child's fingernail. An inch is about as wide as a quarter. A foot is about the length of a sheet of printer paper. A meter is about one big step for an adult. To estimate, your child pictures the benchmark laid along the object again and again and counts how many times it fits.
Estimating length is useful before and after measuring. It helps your child choose a sensible unit. It also catches mistakes: if a ruler seems to say a book is 90 centimeters long, a quick estimate shows that something went wrong. In 2nd grade, an estimate is good when it is close to the real length, even if it does not match.
Your child should be able to:
- Name a benchmark object for a centimeter, an inch, a foot and a meter
- Estimate the length of a familiar object in a given unit
- Tell whether an estimate such as 50 feet for a bathtub is reasonable
- Choose centimeters or inches for small things and feet or meters for big things
Example
About how many feet long is a bathtub?
- A sheet of printer paper is about 1 foot long.
- Picture sheets of paper laid end to end along the tub. About 5 sheets fit.
- Any answer from 4 to 6 feet is a good estimate. An answer of 50 feet is not, because that is longer than a school bus.
- A bathtub is about 5 feet long.
Example
About how many centimeters long is a house key?
- A child's fingernail is about 1 centimeter wide.
- Picture fingernails lined up along the key. About 5 of them fit.
- An estimate of 4, 5 or 6 centimeters is close.
- A house key is about 5 centimeters long.
Where children usually get stuck
Your child guesses that a door is 10 centimeters tall.
The child picked a number without picturing the unit. Without a benchmark, the number and the unit are just words.
What helps. Hold your child's fingernail against the door and say that its width is about 1 centimeter. Ask how many of those it would take to reach the top of the door.
After measuring, your child erases the estimate and writes the real length in its place.
The child thinks an estimate that does not match is a wrong answer. In school, an estimate only has to be close to count as a good one.
What helps. Take turns estimating a few things around the house, then measure them together. The closer guess wins the round.
Asked for meters, your child says a bed is about 6 meters long, the same number as its length in feet.
Feet and meters both measure big things, so the two units get mixed up. A meter is a little more than 3 feet, so the same bed is a much smaller number of meters.
What helps. Take one big step together and call it a meter. Then step along the bed: it is about 2 big steps long, so about 2 meters.
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.
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- 1Warm-upAbout how long is the fork in inches?
A7 inchesB60 inchesC20 inchesD2 inchesShow the answer and how to solve it
The best estimate is 7 inches. A fork is shorter than a 12-inch ruler but much longer than a 1-inch paperclip, so 7 inches is the only choice that fits.
- 1Picture one inch. An inch is about the width of an adult thumb, or about as long as a small paperclip.
- 2Now compare the fork to things you know. A school ruler is 12 inches long. A fork is a little shorter than a ruler — much longer than a paperclip.
- 3A fork is about 7 inches long.
Common mistake. A common mistake is choosing 60 inches. That would be taller than most kids — much, much longer than a fork. Remember, an inch is about a thumb's width.
The idea underneath. Estimating in inches starts with picturing one inch — about a thumb's width — and a 12-inch ruler. Then ask: is the object a fraction of a ruler, about a ruler, or much shorter? A fork is a little shorter than one ruler, so the answer is in the single digits.
- 2On levelAbout how many feet long is this door?
A3 feetB60 feetC7 feetD20 feetShow the answer and how to solve it
The door is about 7 feet long. About 7 rulers would fit along its length.
- 1Look at the picture of the door.
- 2About 1 foot is the length of a ruler. A door is tall enough for a grown-up to walk through, so it is much longer than 1 ruler.
- 3A door is about 7 feet long. That is about the same as 7 rulers laid end to end.
Common mistake. A common mistake is mixing up feet with inches. Inches are much smaller — there are 12 inches in 1 foot. A door is about 7 feet long, not 60 feet (60 feet is closer to the length of a semi-truck or 5 cars in a row).
The idea underneath. To estimate a length in feet, picture something familiar that is about 1 foot long, like a ruler. Then think how many of those would fit along the object.
- 3On levelFour students estimated the length of the banana in inches.
Select all the estimates that are reasonable.AAva: 11 inchesBDiego: 7 inchesCJamal: 1 inchDMaya: 9 inchesShow the answer and how to solve it
Maya's estimate of 9 inches and Diego's estimate of 7 inches are both reasonable for a banana. Jamal's 1 inch is much too small (paper-clip size), and Ava's 11 inches is just outside the reasonable range for a banana (about 8 inches).
- 1Look at the banana. A typical banana is about 8 inches long. Use that as your reference. A reasonable estimate should be close to 8 inches — within an inch or two.
- 2Maya's estimate is 9 inches. That is only 1 inch more than 8, so it is reasonable.
- 3Jamal's estimate is 1 inch. A 1-inch object is about the size of a paper clip, much smaller than a banana. This is not reasonable.
- 4Ava's estimate is 11 inches. That is 3 inches more than 8 — just outside the reasonable range for a banana. This is not reasonable.
- 5Diego's estimate is 7 inches. That is only 1 inch less than 8, so it is reasonable.
- 6The reasonable estimates are Maya's 9 inches and Diego's 7 inches.
Common mistake. A common mistake is picking only one reasonable estimate when two are reasonable, or thinking that 11 inches is close enough to 8 because they feel similar. Always compare each estimate to a familiar reference — a banana is about 8 inches, a paper clip is about 1 inch, and a ruler is 12 inches.
The idea underneath. Length estimates work best when you compare them to a known reference. A banana is about 8 inches, a paper clip is about 1 inch, and a ruler is 12 inches. Reasonable estimates fall close to the typical length — within about an inch or two.
- 4StretchWhich object is about 8 feet long?A
B
C
D
Show the answer and how to solve it
The camping tent is about 8 feet long, which matches the target. A park bench is shorter (about 6 feet), and a kayak (about 10 feet) and a trampoline (about 12 feet across) are both longer.
- 1Look at each picture and think about how many feet long each object is. It can help to compare to your own height — most second graders are about 4 feet tall.
- 2Use what you know about real objects: a camping tent is about 8 feet long, a park bench is about 6 feet long, a kayak is about 10 feet long, and a trampoline is about 12 feet across.
- 3The camping tent matches the target of 8 feet. The park bench is too short, and the kayak and trampoline are too long.
Common mistake. A common mistake is picking the kayak (about 10 feet) instead of the camping tent — both are long outdoor objects, but the kayak is 2 feet longer than the 8-foot target.
The idea underneath. Building length anchors helps you estimate. Your own height is about 4 feet, a couch is about 7 feet (a little less than two of you laid end to end), and a kayak or trampoline is 10 to 12 feet — much longer.