There is a saying: “I haven’t used Algebra since high school.”
That might be true for Calculus, but it is definitely false for Percentages. We use the % symbol every single day. Whether you are figuring out a discount at the store, calculating a tip at a restaurant, or checking how much your savings account grew, percentages rule the world.
Yet, in the moment, our brains often freeze. We know we need to take “20% off,” but we forget the math to actually get the number. Here is the ultimate cheat sheet for the three types of percentage problems you face in daily life.
1. The “Shopping” Formula (X% Of Y)
This is the most common scenario. You see a jacket that costs $80, and there is a sign that says 30% Off. How much do you save?
The “Move the Decimal” Trick
To turn a percentage into a number you can use, you simply move the decimal point two spots to the left.
- 30% becomes 0.30
- 50% becomes 0.50
- 5% becomes 0.05
Then, just multiply the Price by that new number.
Bad at Mental Math?
Don’t stand in the grocery aisle staring at your phone calculator. Use our tool to find the final price instantly.
﹪ Use Percentage Calculator2. The “Growth” Formula (Percentage Increase)
This is crucial for finances. Let’s say your rent was $1,500 last year, and your landlord raised it to $1,650 this year. You want to know: “What percentage increase is that?”
The formula is: (New Number – Old Number) ÷ Old Number.
- First, find the difference: 1650 – 1500 = 150.
- Divide that difference by the original: 150 ÷ 1500 = 0.10.
- Move the decimal back to the right: 10%.
You can use this to calculate salary raises, stock market returns, or weight loss progress.
3. The Tricky One: “Reverse Percentages”
This is the one that stumps everyone.
Scenario: You paid $44 for a shirt, and the receipt says that price “Includes 10% Tax.” What was the original price of the shirt before tax?
Your instinct is to calculate 10% of $44 ($4.40) and subtract it. This is wrong! Because the tax was added to the smaller original number, not the big final number.
The correct formula requires division:
The original price was $40. If you had just subtracted 10%, you would have gotten $39.60, which is incorrect. This math is notoriously hard to do in your head, which is why our calculator has a specific “Reverse” mode.
Cheat Sheet: Tipping at Dinner
When the bill comes, don’t panic. Here is the easiest way to estimate tips without a calculator:
| Goal | The Trick | Example ($40 Bill) |
|---|---|---|
| 10% Tip | Just move decimal one spot left. | $4.00 |
| 20% Tip | Find 10%, then double it. | $4.00 x 2 = $8.00 |
| 15% Tip | Find 10%, then add half of that. | $4.00 + $2.00 = $6.00 |
Conclusion: Tools Make It Easier
Percentages are everywhere. From Interest Rates on Loans (APRs) to sale stickers at the mall, understanding them saves you money.
For simple tips, use the 10% trick. For complex reverse equations or weird numbers (like 17.5% off), skip the headache and click the button below.