Weighing things and measuring liquids
In 3rd grade, children learn to measure and estimate weights in grams and kilograms and liquids in liters, and to solve one-step story problems with those amounts.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 3rd 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
The green tub holds 12 liters of water. The blue tub holds 7 liters. How many more liters does the green tub hold than the blue tub?
- 19 liters
- 15 liters
- 5 liters
Why this matters
Holding a kilogram in their hands gives your child a feel for weight that no chart can, and that feel helps them judge whether an answer is believable, such as noticing that a pencil cannot weigh 5 kilograms. In 4th grade they convert between units, like 2 kilograms to 2,000 grams, and later science classes measure in metric units throughout. It is practical at home as well, from reading food labels to filling a 2-liter bottle.
Where kids can get stuck
- Picking the wrong unit for an estimate, such as saying an apple weighs about 200 kilograms.
- Assuming a bigger object is always heavier, so a large pillow must outweigh a small rock.
- Reading a scale or container at the wrong line, such as viewing the liquid at an angle or skipping what each mark stands for.
How you can help
- Turn unpacking the groceries into a one-step story problem. If a bag of potatoes weighs 3 kilograms and a bag of onions weighs 2 kilograms, ask how much both weigh, or how many 2-liter bottles make 8 liters.
- Weigh fruit on the grocery store scale and ask your child to guess first whether a bag of apples is closer to 1 kilogram or 5 kilograms.
- Fill a 1-liter bottle with water in the bath or sink and ask your child how many times it takes to fill a bucket or pot.
Bobby's Math Club offers free practice that grows with your child.
Questions parents ask
Why is my child learning grams and liters instead of pounds and quarts?
Children in the United States usually learn both. Customary units like inches and pounds appear in other grades, and metric units like grams, kilograms, and liters are part of 3rd grade. Metric units are used in science and in most of the world, so early familiarity helps. Working with both also teaches that units are agreed sizes we use to measure.
How can I help my child get a feel for a gram or a kilogram?
Let your child hold things whose weight you know. A paper clip is about 1 gram. A 1-liter bottle of water weighs about 1 kilogram. Compare other objects to these, such as asking whether a shoe is more or less than the water bottle. Holding the real thing builds a sense of size that numbers alone cannot.
Will my child have to convert grams to kilograms?
Not in 3rd grade. Problems this year use one unit at a time, such as adding 3 kilograms and 4 kilograms. Converting between units, like knowing that 2 kilograms is 2,000 grams, is usually taught in 4th grade, once your child has a good sense of the units and works with larger numbers.
Make learning math fun
Bobby's Math Club turns 3rd grade math practice into something fun to do together.
- Free, unlimited practice for every child in your family
- Practice that starts where your child is and grows with them
- 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 3.MD.A.2: “Measure and estimate liquid volumes and masses of objects using standard units of grams (g), kilograms (kg), and liters (l). Add, subtract, multiply, or divide to solve one-step word problems involving masses or volumes that are given in the same units, e.g., by using drawings (such as a beaker with a measurement scale) to represent the problem.” Learn how Bobby's Math Club can help your students grow. (Common Core 3.MD.2)