How to Calculate Percentages Fast: Tricks and Tools

"What's 20% of 75?" If that question makes you freeze up, you're not alone. But percentages are everywhere—from sales discounts to restaurant tips—and learning a few tricks can make you look like a math wizard.

Trick 1: The 10% Method

This is the most powerful trick in the book. Finding 10% of any number is easy: just move the decimal point one place to the left.

Once you have 10%, you can easily find other percentages:

• 20%: Double the 10% (7.5 × 2 = 15)
• 5%: Halve the 10% (7.5 / 2 = 3.75)
• 15%: Add the 10% and the 5% (7.5 + 3.75 = 11.25)

Trick 2: The Reversible Rule

Did you know that X% of Y is the same as Y% of X?

Let's say you need to find 8% of 50. That sounds hard.

Flip it: Find 50% of 8. That's easy! Half of 8 is 4.

So, 8% of 50 is also 4. Mind blown?

Common Percentage Scenarios

Discounts

A store offers 30% off a $50 jacket.

• 10% of $50 is $5.
• 30% is 3 times that ($5 × 3 = $15).
• You save $15. The new price is $35.

Tips

You want to leave a 20% tip on a $42 bill.

• 10% of $42 is $4.20.
• Double it to get $8.40.

Conclusion

Percentages are a tool for comparison. Whether you're comparing prices, analyzing grade improvements, or checking battery life, they help us make sense of the world.