R3 · Lesson 3: Multiply Decimals

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Why this matters for Grade 6: Grade 6 requires fluency multiplying multi-digit decimals (6.NS.3). You need this every time you scale a recipe, calculate sales tax, or find the total cost of several items at the same price.

Learn It

Count Decimal Places

The key rule for multiplying decimals:

Ignore the decimal points and multiply the numbers as whole numbers.
Count the total number of decimal places in both factors combined.
Place the decimal point in the product so it has that many decimal places.

Example 1: 0.5 × 0.5
Whole-number multiply: 5 × 5 = 25.
Decimal places: 0.5 has 1, and 0.5 has 1 → total = 2 places.
Place the decimal: 25 → 0.25

Example 2: 1.2 × 3
Whole-number multiply: 12 × 3 = 36.
Decimal places: 1.2 has 1, and 3 has 0 → total = 1 place.
Place the decimal: 36 → 3.6

Example 3: 2.5 × 4
Whole-number multiply: 25 × 4 = 100.
Decimal places: 2.5 has 1, and 4 has 0 → total = 1 place.
Place the decimal: 100 → 10.0 = 10

Try It · Guided

Work These Together

1. One notebook costs $1.50. What is the cost of 4 notebooks?

Hint

1.50 × 4. Whole-number: 150 × 4 = 600. 1.50 has 2 decimal places, 4 has 0 → 2 total. 600 → 6.00 = 6.00.

2. Calculate: 0.5 × 0.5

Hint

5 × 5 = 25. Total decimal places = 1 + 1 = 2. Place: 0.25.

Practice · On Your Own

You Try

3. Calculate: 1.2 × 3

Hint

12 × 3 = 36. 1.2 has 1 decimal place, 3 has 0 → 1 total. 36 → 3.6.

4. Calculate: 2.5 × 4

Hint

25 × 4 = 100. 2.5 has 1 decimal place, 4 has 0 → 1 total. 100 → 10.0 = 10.

5. A pack of pencils costs $1.25. A teacher buys 6 packs. What is the total cost?

Hint

1.25 × 6. Whole-number: 125 × 6 = 750. 1.25 has 2 decimal places → 7.50.

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Exit Ticket

Show What You Know

6. How many decimal places does the product of 0.4 × 0.3 have?

Hint

0.4 has 1 decimal place and 0.3 has 1 decimal place. Total = 2. So 4 × 3 = 12 → 0.12.

7. Calculate: 0.4 × 0.3

Hint

4 × 3 = 12. Two decimal places total → 0.12.

← Lesson 2: Add & Subtract Next: Divide Decimals →