Key takeaways
- Subtraction finds what's left: Difference = a − b.
- Line up digits by place value before subtracting.
- If the top digit is smaller than the bottom one, borrow 1 from the next column.
- b − a is not the same as a − b unless the two numbers are equal.
In this guide
What is subtraction?
Subtraction finds the difference between two numbers. If you have 10 candies and eat 4, subtraction tells you how many remain: 6.
Why does subtraction matter?
Subtraction shows up anywhere something gets used up or taken away: change from a purchase, battery left on your phone, or days remaining until an exam. It also powers comparisons, like how much more expensive one phone is than another.
A negative answer in subtraction isn't a mistake — it just means the second number was bigger than the first.
The formula
| Symbol | What it means | Example |
|---|---|---|
| Difference | What's left after subtracting | 32 |
| a | The starting amount | 50 |
| b | The amount taken away | 18 |
How to subtract step by step
- 1Write the larger starting amount as a and the amount being removed as b, lined up by place value.
- 2Subtract the ones column first. If the top digit is smaller than the bottom digit, borrow 1 from the column to its left.
- 3Subtract the tens column (adjusting for any borrow), then continue through the remaining columns.
- 4Read off the final difference.
Now try your own numbers in the calculator below to see every borrowing step.
Try it yourself
Pre-filled with the example — change any value.
Common mistakes to avoid
- Subtracting in the wrong order. a − b is not the same as b − a unless both numbers are equal.
- Forgetting to lower the borrowed-from digit. When you borrow 1, the column you took it from drops by 1.
- Panicking at a negative answer. If b is bigger than a, the difference is negative — that's correct, not an error.
Tips and tricks
- Check your work by adding the difference back to b — you should get a again.
- Estimate first by rounding both numbers to the nearest ten to sanity-check your final answer.
- Subtraction is the reverse of addition, so if you're unsure, add your answer back to double-check.
Where you'll use it in real life
- Shopping: working out how much change you should get back from a purchase.
- Travel: finding out how many days remain until a trip or event.
- Budgeting: subtracting expenses from your income to see what's left.
- Comparisons: using percentage change to see how much a price rose or fell.
Quick summary
To subtract one number from another, line the digits up by place value and subtract column by column, borrowing from the next column whenever the top digit is smaller. A negative result simply means the number taken away was bigger. Use the calculator above to see every borrowing step for your own numbers.
Worked example
You had $50 in your wallet and spent $18 on lunch and a coffee. How much money do you have left?
- 01Difference = a - b
- 02Difference = 50 - 18
- 03Difference = 32
These numbers are pre-filled in the calculator above.
Frequently asked questions
What does it mean if subtraction gives a negative number?
It means the second number was bigger than the first. For example 4 − 10 = −6, which just shows you're 6 short of the amount you started with.
How is subtraction different from addition?
Addition combines amounts into a bigger total, while subtraction removes an amount from a total to find what's left. They undo each other.
How do I subtract when I need to borrow?
If the top digit in a column is smaller than the bottom digit, borrow 1 from the column to its left (which lowers that digit by 1 and adds 10 to the current column), then subtract as normal.
How do I check that a subtraction answer is correct?
Add the difference back to the number you subtracted (b). If you get the starting number (a) back, your subtraction was correct.
