Converting units in 4th and 5th grade: multiply or divide?

By 15Q20M Editorial · Last updated October 28, 2026

With a fact like 1 gallon = 4 quarts, multiply to change a bigger unit into a smaller one, and divide to change a smaller unit into a bigger one. A smaller unit takes more pieces to cover the same amount, so the number grows: 3 gallons = 12 quarts. Going the other way, 24 quarts is 6 gallons.

Why does my child get unit conversions backward?

Your child reads that Carlos has 2,500 g of clay and Priya has 3 kg, and picks Carlos "because 2,500 is a larger number than 3." That is a real wrong choice on Day 49, Question 14 of our Grade 5 workbook. Another choice on the same problem says 2,500 g = 25 kg. The right answer is Priya: 3 kg is 3,000 g.

Both wrong choices skip the same step: getting the two amounts into one unit, with the right number of grams in a kilogram. With measurements, the unit decides what the number means. A child who first asks "which unit is bigger?" can predict whether the number should grow or shrink, and that prediction points to multiply or divide.

When do you multiply and when do you divide to convert units?

Look at the size of the units before picking an operation. Each row below is a real workbook problem. The factor in parentheses is the fact your child needs; our Grade 5 book prints it on 69 of its 78 conversion problems.

Our workbook asks for the reason as well as the rule. Day 51, Question 15 asks what to do when changing a smaller unit to a larger one. The right choice is to divide, "because it takes fewer large units to represent the same amount." One wrong choice says to multiply "because larger units contain more small units," which states a true fact and then draws the wrong conclusion from it.

What is the difference between 4th and 5th grade unit conversions?

Fourth grade works mostly in one direction. Fifth grade broadens the work to both directions, often with decimals. The fourth grade standards put conversion under the heading "conversion of measurements from a larger unit to a smaller unit," so the Grade 4 problems here all multiply: 4 m = 400 cm, 3 hours = 180 minutes, 5 pounds = 80 ounces (Grade 4, Days 53 and 54). The Illustrative Mathematics family materials for Grade 4 describe the same step: students solve problems that convert a larger unit to a smaller one.

In 5th grade, students also convert from smaller units to larger ones. Our Grade 5 workbook includes 60 inches = 5 feet (Day 50, Q8) and 250 cm = 2.5 m (Day 51, Q8), where the second answer has a decimal. Schools also teach the step in different ways. One Eureka Math parent tip sheet shows 579 mL turned into liters by multiplying 579 by 0.001 L. That gives the same 0.579 as dividing by 1,000. If your child's class writes it that way, keep their method and use the bigger-or-smaller check below to confirm the answer.

How can a child tell whether to multiply or divide?

Ask four questions, in this order, before any arithmetic. Here they are on Day 51, Question 9: a kitten weighs 72 oz. How many pounds is that?

1. Which unit is bigger, an ounce or a pound? "A pound."
2. We are going from ounces to pounds. Will the new number be bigger or smaller than 72? "Smaller. It takes fewer pounds than ounces."
3. How many of the small unit fit in one big unit? "16 ounces in a pound."
4. Do the math and compare with your guess. 72 ÷ 16 = 4.5. Is 4.5 smaller than 72? "Yes." So the kitten weighs 4.5 lb.

Step 2 is the one that catches mistakes. If your child predicts a smaller number and gets 1,152 (72 × 16), the mismatch shows up before you say a word. Keep the questions short and let your child answer out loud: "Which one is bigger?" and "More of them or fewer?" are usually enough.

What mistakes do kids make converting units?

Each row is a wrong answer choice from a real workbook problem. The middle column is the likely cause, since a choice on a page shows the slip but not the thinking behind it.

If you check only one thing first, check the top row. A child who compares the numbers alone will get some problems right by luck, whenever the bigger number also happens to be the bigger amount, so a few correct answers can hide the habit.

Is it normal to mix up multiplying and dividing?

If your child reverses the operation, ask the bigger-or-smaller question and let them fix it. If they can, try another conversion in the opposite direction. A wrong answer your child can correct this way is part of learning the step.

When is it worth more practice?

Look closer if one of these keeps happening:

• Your child cannot say which of two units is bigger, such as an ounce or a pound. That is the 4th grade foundation, and the conversion step depends on it.
• Your child still chooses an answer by comparing the numbers alone, as in the 2,500 g vs 3 kg problem.
• Two-unit word problems fall apart, like 2 km and 800 m in all (Grade 5, Day 49, Q11: 2,800 m). These ask for a conversion and then another operation.
• Your child does not know the customary factors (12 inches in a foot, 16 ounces in a pound). Our Grade 5 workbook prints the factor on most conversion problems, though classroom tests vary. Write the factors from tonight's page on an index card so your child can focus on choosing multiply or divide.

What can you try with your child tonight?

1. Pick two units from the kitchen, such as a cup and a quart, and ask which is bigger. Then ask how many cups it takes to fill the quart.
2. Make a two-column table together: feet on the left, inches on the right, from 1 to 5. Ask your child to read off 3 feet and then 48 inches.
3. Do the kitten problem with the four questions out loud, then try 3,500 cm to meters the same way.
4. Give one problem with the wrong operation already done (2.5 gallons = 0.625 quarts) and ask your child to explain why it cannot be right.

Why are decimals so hard in 5th grade?

What 5th grade Common Core math covers, standard by standard

Frequently asked questions