Mental Addition Tricks and Shortcuts
With the right strategies, you can add numbers in your head faster than reaching for a calculator. These tricks work because they exploit patterns in our base-10 number system.
Trick 1 – Making Hundreds (or Thousands)
Round one number up to the nearest hundred (or thousand), add, then subtract the rounding adjustment.
Why it works: Adding a round number like 100 or 1000 is instant — you just change one digit. So instead of adding an awkward number like 99, pretend it's 100 and add that. But since you added 1 too many, take that 1 back off at the end. You get the same answer, with much less effort.
Steps:
- Look at the number that's close to a round hundred or thousand (like 99, 998, or 297).
- Work out how far it is from that round number — this is your adjustment.
- Add the round number instead of the original number.
- Subtract (or add back) the adjustment to correct the total.
Trick 2 – Doubles and Near-Doubles
If the two numbers are close to each other, use the nearest double and adjust.
Why it works: Most people memorise doubles (like 7+7=14 or 25+25=50) as instant facts. If two numbers are only 1 or 2 apart, the sum is almost the same as doubling one of them — you just nudge the answer up or down by the small difference.
Steps:
- Check whether the two numbers are close together (within about 1-2 of each other).
- Pick whichever number you find easiest to double.
- Double it using a fact you already know.
- Add or subtract the small difference between the doubled number and the number you didn't use.
Trick 3 – Left-to-Right Addition
Add the largest place value first (left to right), then work through smaller values. This gives a running total and is faster than carrying mentally.
Why it works: The written "carrying" method works right to left because it's designed for pen and paper. But we say and picture numbers left to right (three hundred and forty-seven), so adding hundreds first, then tens, then ones matches how your brain already holds the number — and you always know your running total so far.
Steps:
- Split each number into its place values (hundreds, tens, ones).
- Add the biggest place value first (e.g. the hundreds) to get a starting total.
- Add the next place value (the tens) to that running total.
- Add the smallest place value (the ones) last to get the final answer.
Trick 4 – Friendly Number Pairs
Look for pairs that total 10, 100, or 1000 and add them first.
Why it works: Pairs that make 10 (1+9, 2+8, 3+7, 4+6, 5+5) are number bonds most students learn early and can spot instantly. Adding several numbers is much easier once you've turned two or three of them into a clean 10, 100, or 1000 — the rest of the sum becomes simple addition of round numbers.
Steps:
- Scan all the numbers you need to add together.
- Look for any two (or more) that combine to make exactly 10, 100, or 1000.
- Add those friendly pairs first, before touching the rest.
- Add the round totals from each pair, then add on any numbers left over.
Trick 5 – Splitting to Make Easy Numbers
Split one addend to make the other a round number.
Why it works: Adding to a round number (like 80) is much easier than adding to an awkward one (like 48). By breaking the second number into "just enough to reach a round number" plus "whatever's left over," you turn one tricky sum into two easy ones.
Steps:
- Look at the first number and work out how much you'd need to add to reach the next round ten (or hundred).
- Split the second number into that exact amount, plus whatever remains.
- Add the first part to reach the round number.
- Add the leftover part to that round number to get the final answer.
Summary Table
| Trick | Best for | Example |
|---|---|---|
| Making hundreds | Numbers close to 100, 1000 | 67+99=166 |
| Near-doubles | Numbers within 1-2 of each other | 13+14=27 |
| Left-to-right | Multi-digit mental addition | 347+235=582 |
| Friendly pairs | Several single-digit numbers | 4+7+3+6=20 |
| Splitting | One awkward number | 48+37=85 |
Key Takeaways
- Rounding one number and adjusting afterwards is one of the most powerful tricks.
- Near-doubles exploit the doubling facts you already know.
- Spot friendly pairs (that make 10, 100, etc.) when adding multiple numbers.
- Left-to-right addition mirrors how we naturally think about numbers.
Practice: Try a Trick
A random trick from the list above is tested each time. If you get it wrong (or leave it blank), we'll show exactly how that trick works.