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Level 2Grade 2skill: doubles· 6 min read

Doubles: Adding a Number to Itself

In short: To double a number, add it to itself: double 6 is 6 + 6 = 12. For bigger numbers, double the tens and the ones separately, then add — and remember every real double is an even number.

How to double a number

To double a number, you add it to itself. That's the whole idea: one number, written down twice, added together. Double 4 means 4 + 4. Double 30 means 30 + 30, which is 60. There is only ever one number to remember, which is why doubles are the fastest addition facts to lock in.

For small numbers you can just count on: double 3 is 3 and then 3 more, which lands on 6. For bigger numbers, break them into tens and ones, double each part, and add the two pieces back together. Double 26 becomes double 20 (that's 40) plus double 6 (that's 12), and 40 + 12 makes 52.

  1. Say the number, then say it again — that is what you are adding.
  2. If it is a single digit, count on from the number to get its double.
  3. If it has tens and ones, double the tens, double the ones, then add the two answers.
  4. Check: every double must be an even number (ends in 0, 2, 4, 6, or 8).

Why doubles are always even Why it works

1 to 5
Double each single digit: 1 to 2, 2 to 4, 3 to 6, 4 to 8, 5 to 10.

Double 4 is 8

6 to 9
These tip past ten: 6 to 12, 7 to 14, 8 to 16, 9 to 18.

Double 8 is 16

Whole tens
Double the tens digit and keep the zero on the end.

Double 40 is 80

Teens
Double the ten and double the ones, then add: double 15 is 20 + 10.

Double 15 is 20 + 10 = 30

Here is the one thing worth carrying with you: a double is always even. You added two equal groups, so the total can always be split into two equal groups again. If you ever double a number and land on an odd answer like 15 or 21, you already know something slipped — real doubles never end in 1, 3, 5, 7, or 9.

Worked examples

Double 7 = 14
  1. Write it as 7 + 7.
  2. Count on from 7: 8, 9, 10, 11, 12, 13, 14.
  3. That is 7 more than 7.
Double 20 = 40
  1. Write it as 20 + 20.
  2. Double the tens digit: 2 doubled is 4.
  3. Keep the zero, so 4 becomes 40.
Double 15 = 30
  1. Split into tens and ones: 10 and 5.
  2. Double the ten: 10 + 10 = 20.
  3. Double the ones: 5 + 5 = 10.
  4. Add the pieces: 20 + 10 = 30.
Double 26 = 52
  1. Split into 20 and 6.
  2. Double the tens: 20 + 20 = 40.
  3. Double the ones: 6 + 6 = 12.
  4. Add: 40 + 12 = 52 (the extra ten from 12 carries in).
Double 45 = 90
  1. Split into 40 and 5.
  2. Double the tens: 40 + 40 = 80.
  3. Double the ones: 5 + 5 = 10.
  4. Add: 80 + 10 = 90.

Common mistakes

Adding the wrong number instead of doubling

✗ 8

Doubling 6 means 6 + 6, but it is easy to slip into adding 2 (6 + 2) because "double" gets muddled with "add a couple." That gives 8 instead of 12.

Fix: Say the number out loud twice before you add: "six plus six." You are always adding the same number, never a small extra like 1 or 2.

wrong_operation

Landing one short when counting on

✗ 15

For double 8 you count on eight steps past 8. The slip is saying "8" as your first count word instead of "9." Put the two counts side by side: the correct one goes 9, 10, 11, 12, 13, 14, 15, 16 and stops on 16, but the miscount goes 8, 9, 10, 11, 12, 13, 14, 15 and stops on 15 — one short.

Fix: Use the even-number check. 15 is odd, and no real double is odd, so you know to recount. Double 8 is 16.

off_by_one

Forgetting the place value on tens

✗ 8

To double 40 you double the 4 to get 8 — but that 8 stands for 8 tens, not 8 ones. Writing plain 8 drops the number by a factor of ten.

Fix: When you double a tens digit, put the zero back straight away: 4 doubled is 8, so 40 doubled is 80.

place_value_slip

Losing the carried ten

✗ 42

For double 26 you get 40 from the tens and 12 from the ones. If you write only the 2 from that 12 and forget the extra ten hiding inside it, you land on 42 instead of 52.

Fix: When doubling the ones makes a number bigger than 9, that overflow is a whole ten. Carry it over: 40 + 12 is 52, not 42.

missed_carry

Practice doubles

Try a mix of single-digit, whole-ten, and two-digit doubles. Watch for the tricky ones where doubling the ones pushes you past ten and you have to carry.

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Frequently asked questions

What does it mean to double a number?

Doubling a number means adding it to itself. Double 5 is 5 + 5 = 10, and double 12 is 12 + 12 = 24. There is only one number to work with — you just use it twice.

Is doubling the same as multiplying by 2?

Yes. Double 7 (which is 7 + 7 = 14) gives exactly the same answer as 7 x 2 = 14. Doubling is the friendly, add-it-to-itself way of thinking about multiplying by 2.

Are all doubles even numbers?

Always. Because you are adding two equal groups, the total can be split back into two equal groups, which is what makes a number even. If you ever double a number and get an odd answer, you know a mistake crept in.

How do I double a two-digit number like 34?

Split it into tens and ones. Double the tens (30 + 30 = 60), double the ones (4 + 4 = 8), then add the two pieces: 60 + 8 = 68. Watch for cases like double 26, where doubling the ones makes 12 and you carry an extra ten.

Why are doubles worth memorizing?

Doubles are the quickest facts to recall, and they unlock nearby facts too. Once you know double 7 is 14, you can work out 7 + 8 as "double 7 plus 1" — 15 — without starting over.