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How to Multiply Decimals

Level 2 Decimals · Reviewed by The NumThink editorial team · Last reviewed

Short version: Ignore the points, multiply the whole numbers, then give the answer as many decimal places as the two numbers have together: 0.3 × 0.4 → 3 × 4 = 12 → 0.12.

This page is part of the NumThink Decimals course. Read why the method works, watch it run on worked examples, then drill it in the practice box below until it feels automatic.

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Why it works

0.3 is 3 tenths and 0.4 is 4 tenths. A tenth of a tenth is a hundredth (0.1 × 0.1 = 0.01), so 0.3 × 0.4 is 12 hundredths: 0.12.

Each decimal place is one division by 10. Removing the points multiplies by 10 once for every decimal place; putting the same number of places back undoes it exactly.

The steps

  1. Count the decimal places. 2.4 × 0.9: one place and one place — two in total.
  2. Multiply as whole numbers. 24 × 9 = 216.
  3. Put the places back. Two places: 2.16.

Worked examples

Easy 0.3 × 4

Show the thinking

3 × 4 = 12

One decimal place in total

So 0.3 × 4 = 1.2.

Answer: 1.2

Medium 2.4 × 0.9

Show the thinking

24 × 9 = 216

Two decimal places in total

So 2.4 × 0.9 = 2.16.

Answer: 2.16

Hard 0.25 × 1.2

Show the thinking

25 × 12 = 300

Three decimal places in total: 0.300

So 0.25 × 1.2 = 0.3.

Answer: 0.3

When to use it

  • Prices: 1.5 kg at 4.2 per kg costs 6.3.
  • Areas: a rug 2.5 m by 1.2 m covers 3 m².

When NOT to use it

  • Multiplying by 10, 100 or 1,000 — just move the point.
  • When a fraction is easier: 0.25 × 48 is a quarter of 48, which is 12.

Common mistakes

Counting the places of only one number. Answering 0.3 × 0.4 = 1.2. Fix: Count the places of BOTH numbers: one and one make two, so 0.12.
Expecting multiplication to make it bigger. Thinking 2.4 × 0.9 must be more than 2.4. Fix: Multiplying by a number less than 1 makes the answer smaller: 2.16 is less than 2.4.
Check yourself: Estimate: 2.4 × 0.9 is a little under 2.4 × 1 = 2.4, and 2.16 fits ✓.