Converting feet to inches and kilograms to grams
In 4th grade, children learn how units of measure compare in size and how to convert a larger unit into a smaller one in the same system, like 4 feet to 48 inches.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 4th 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
Amara buys a 5-pound bag of oranges. How many ounces does the bag weigh?
- 60 ounces
- 21 ounces
- 80 ounces
Why this matters
Converting units is something your child will do all their life, like knowing that a 2-hour movie is 120 minutes or that a 3-foot shelf is 36 inches, and it helps them judge whether a measurement makes sense. Next year they convert in both directions, including inches back into feet, and use conversions in longer problems. In later science classes, moving smoothly between grams, liters, and meters becomes an everyday need.
Where kids can get stuck
- Dividing instead of multiplying when changing to a smaller unit, so 4 feet comes out as fewer than 4 inches.
- Assuming every unit works in tens, so 1 hour becomes 100 minutes or 1 pound becomes 10 ounces.
- Trying to convert feet straight into centimeters, when this year's work stays inside one system at a time.
How you can help
- Measure family members with a tape measure. If someone is 5 feet tall, ask your child to work out how many inches that is, then measure again in inches to check the answer of 60.
- Read food labels together. If a bag of flour weighs 2 pounds, ask how many ounces that is, and check whether the label also lists 32 ounces.
- Talk about time in the car. If the drive takes 2 hours, ask your child how many minutes that is, and how they knew to multiply by 60.
Want a little more? Try Bobby's Math Club for free practice that adapts to your child.
Questions parents ask
Why is my child learning to convert units this way?
Teachers often use two-column tables, such as 1 foot to 12 inches, 2 feet to 24 inches, because the table shows the pattern behind every conversion. It connects converting to multiplication your child already knows. Knowing roughly how big each unit is also helps your child check whether an answer makes sense, such as noticing that a door is not 7 inches tall.
Which unit conversions should my child know in 4th grade?
Common ones include: 1 foot is 12 inches, 1 yard is 3 feet, 1 pound is 16 ounces, 1 hour is 60 minutes, 1 minute is 60 seconds, 1 meter is 100 centimeters, 1 kilometer is 1,000 meters, 1 kilogram is 1,000 grams and 1 liter is 1,000 milliliters. Classes vary in which ones they practice most, so your child's teacher may focus on a few at a time.
Why does the number get bigger when converting to a smaller unit?
It takes more of a small unit to measure the same thing. A board that is 4 feet long is also 48 inches long, because inches are much shorter than feet. The board has not changed; only the size of the unit has. Laying a ruler end to end along a longer object can help your child see this.
Make learning math fun
Bobby's Math Club turns 4th grade math practice into something fun to do together.
- Unlimited free practice
- A few minutes a day, at just the right level
- You see their progress and what to work on next
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 4.MD.A.1: “Know relative sizes of measurement units within one system of units including km, m, cm; kg, g; lb, oz.; l, ml; hr, min, sec. Within a single system of measurement, express measurements in a larger unit in terms of a smaller unit. Record measurement equivalents in a two-column table. For example, know that 1 ft is 12 times as long as 1 in. Express the length of a 4 ft snake as 48 in. Generate a conversion table for feet and inches listing the number pairs (1, 12), (2, 24), (3, 36), ....” Learn how Bobby's Math Club can help your students grow. (Common Core 4.MD.1)