Percentages show up everywhere: tips, discounts, taxes, test scores, interest. The good news is that most everyday percentage problems can be done in your head once you know a few reliable tricks. Here are the ones worth memorising — and you can verify any of them with our percentage calculator.
Start with 10%, then build
The single most useful skill is finding 10%: just move the decimal point one place to the left.
- 10% of 240 = 24
- 10% of 86 = 8.6
From 10%, you can reach almost any percentage:
- 5% is half of 10%. So 5% of 240 = half of 24 = 12.
- 20% is double 10%. So 20% of 240 = 48.
- 15% is 10% + 5%. So 15% of 240 = 24 + 12 = 36.
- 1% is 10% of 10% — move the decimal two places. 1% of 240 = 2.4.
Stacking 10%, 5% and 1% lets you build numbers like 17% (10 + 5 + 1 + 1) without ever touching a calculator.
The reversal trick
Because X% of Y equals Y% of X, you can swap the numbers whenever it makes life easier. Suppose you need 4% of 25. That looks awkward — but flipped, it's 25% of 4, which is just a quarter of 4 = 1. Same answer, far less effort.
This trick turns ugly problems into friendly ones surprisingly often. Try it with 8% of 50 (= 50% of 8 = 4).
Tips and discounts in real life
Restaurant tip (18–20%): find 10%, double it for 20%, and nudge down if you want 18%. On a $45 bill, 10% is $4.50, so 20% is $9.00, and 18% is a touch under that — about $8.10.
Store discount (e.g. 30% off): find 10% (move the decimal), multiply by 3 to get the discount, then subtract from the price. A $80 jacket at 30% off: 10% is $8, so 30% is $24, leaving $56.
Percentage change: the formula that trips people up
To find a percentage increase or decrease, use:
percentage change = (new − old) ÷ old × 100
The key is dividing by the original value, not the new one. If a price rises from 50 to 65:
- Difference:
65 − 50 = 15 - Divide by the original:
15 ÷ 50 = 0.3 - Multiply by 100:
0.3 × 100 = 30%increase
A frequent error is dividing by the new value (65), which gives the wrong answer. When in doubt, our percentage calculator handles increase, decrease and "X is what percent of Y" with the steps shown.
When to use a calculator instead
Mental math is perfect for quick estimates and everyday decisions. But reach for a calculator when:
- The numbers are messy (e.g. 23.7% of 418).
- Precision matters — invoices, grades, finance.
- You're chaining several percentage steps together, where small rounding errors compound.
Practise with round numbers first
Build confidence on easy numbers — 10% of 100, 25% of 80 — until the 10%-and-build method feels automatic. Then move to real bills and price tags. Within a week you'll be calculating tips faster than you can pull out your phone.
Frequently asked questions
What is the fastest way to find 10% of a number?
Is 15% of 40 the same as 40% of 15?
CalcSolver Editorial Team
Math & Education Editors
The CalcSolver Editorial Team writes clear, accurate guides on math and calculators. Every article is reviewed for correctness and explained step by step.