10 Mental Maths Tricks to Calculate Faster in Your Head

TL;DR

Why Mental Maths Still Matters

You have a calculator on your phone. So does everyone else.

But reaching for it every time a number comes up slows you down. At a market stall. In a meeting. At the checkout with a queue behind you.

Mental maths is not about being clever. It is about having a few shortcuts that work fast enough to be useful in real life. Once these become automatic, you stop second guessing yourself around numbers.

These 10 tricks are practical. Each one covers a situation you will actually encounter.

Trick 1: Find 10% Of Any Number

This is the foundation for almost every other percentage shortcut. Move the decimal point one place to the left.

Trick 2: Find 5% Of Any Number

Find 10% first, then halve it.

Trick 3: Find 15% Of Any Number

Find 10%, then find 5% (halve the 10%), then add them together.

Trick 4: Find 20% Of Any Number

Find 10% and double it.

Trick 5: Multiply Any Number By 5

Instead of multiplying by 5 directly, multiply by 10 first and then halve the result.

Trick 6: Multiply Any Two Digit Number By 11

Add the two digits together. Place the result in the middle of the original number.

Trick 7: Square any number ending in 5

Multiply the tens digit by the next number up, then attach 25 at the end.

Trick 8: Divide Any Number By 5

Multiply by 2, then divide by 10 (move the decimal one place left).

Trick 9: Check If A Number Is Divisible By 3

Add all the digits together. If the total is divisible by 3, the original number is too.

Trick 10: Double And Halve For Tricky Multiplications

When one number is awkward to multiply, double one factor and halve the other until you get a simpler pair.

How These Tricks Work Together In Real Life

None of these trick’s lives in isolation. The real value is in combining them.

Imagine you are at a shop. A jacket is $220. There is a 15% off sale plus a 10% loyalty discount.

You want to know the real price before you get to the counter.

1. 15% of $220: 10% = $22, 5% = $11, so 15% = $33. Price drops to $187.
2. Now 10% off $187: move decimal = $18.70. Price drops to $168.30.
3. Total saving: $220 minus $168.30 = $51.70.
4. Actual combined discount: $51.70 / $220 = 23.5%, not 25%.

That took about 15 seconds. No phone needed.

For more on how stacked discounts work, see: how to calculate a discount.

Which Tricks To Learn First

You do not need all ten immediately. Most people encounter about four or five situations regularly.

How To Make These Stick

Reading a trick once does not make it automatic. Retrieval does.

The fastest way to learn these is to use them on real numbers in daily life. Pick one trick per week and apply it whenever the opportunity comes up naturally.

• Week 1: practice Trick 1 (10%) on every price tag you see
• Week 2: add Trick 4 (20%) and use it to estimate discounts
• Week 3: combine 1 and 2 (Tricks 2 and 3) to calculate 5% and 15%
• By week 4: you will have the core percentage shortcuts automatic

After a month of casual practice, most of these become instant. The numbers stop feeling like work.

If you want the full formula reference behind these shortcuts, see: basic maths formulas every adult should know.

Common Mistakes When Using Mental Maths

✕ Trying to be exact every time. Mental maths is for fast estimates, not final figures on anything important.

✕ Skipping the estimate step entirely and reaching straight for a calculator. Estimation builds number sense. A calculator does not.

✕ Using the wrong base for percentage calculations. Always apply the percentage to the current price, not the original, when discounts are stacked.

✕ Trying to learn all 10 tricks at once. Pick two and make them automatic before adding more.

✓ Estimating first, then verifying with a calculator for anything above $50 or anything that goes on an invoice.

✓ Using trick combinations. 15% = 10% + 5%. 25% = 10% + 10% + 5%. Build up from what you know.

✓ Practicing on real numbers in daily life rather than made up exercises. Real practice sticks faster.

Frequently Asked Questions

Is mental maths worth learning if I always have a phone with me?

Yes, for a few reasons. First, estimation catches errors. If you type the wrong number into a calculator and the result looks roughly right, you accept it. If you have a mental estimate, you notice when the calculator gives you something unexpected. Second, mental maths is faster for simple calculations than unlocking a phone and opening an app. Third, it is a genuine confidence builder around numbers in social situations like splitting bills or negotiating prices.

Which is the most useful trick for everyday life?

Trick 1 (finding 10%) is the most versatile. Nearly every other percentage shortcut in this guide starts from it. Once 10% is automatic, 5%, 15%, 20%, and 25% all follow with one extra step each.

How long does it take to get good at mental maths?

For the core percentage tricks (1 through 4), most people feel confident within two to three weeks of regular daily use. The key is applying the tricks to real situations rather than doing practice exercises. A month of casual use while shopping and tipping is worth more than an hour of deliberate drills.

What if my mental calculation does not match the receipt?

Small differences are normal because mental maths involves rounding. If the difference is larger than you expect, recalculate exactly using a calculator. A big discrepancy is worth checking because the register can be wrong too.

Related Reading

These posts go well with what you just read:

• Basic maths formulas every adult should know — the full formula reference these shortcuts are built on
• How to calculate a discount — put tricks 1, 3 and 4 to work at the checkout
• How to calculate your monthly budget in 10 minutes — another place where quick number sense pays off every month

Numbers become less intimidating once a few shortcuts are automatic. Start with Trick 1 this week. By the time you reach Trick 10, you will not remember what it felt like to need a calculator for basic percentages.