Percentage Calculator — X% of Y, What % of, Reverse %, % Change

Instant results. Private by design—everything runs locally in your browser.

Calculations

Find X% of Y

Tips, discounts, VAT, markups—enter a percent and a base.

%
?
Result: —

What percent is A of B?

Great for test scores, statistics, ratios, tax rates.

?
Result: —

Reverse a Percentage (Find the Original Number)

If A is B% of the original, what was the original?

% of what?
Result: —

Percent Increase/Decrease

Percent change = (New − Old) ÷ Old × 100.

Result: —

Quick Reference

  • X% of Y → (X / 100) × Y
  • What % is A of B → (A / B) × 100
  • Reverse % (“A is B% of what?”) → A ÷ (B / 100)
  • % Change → ((New − Old) / Old) × 100

Tip: Press Ctrl/Cmd + K to focus site search.

Percentage Examples

  • 20% of 150 = 30
  • What percent is 30 of 120? → 25%
  • 25 is 10% of what? → 250
  • From 80 to 100 is a 25% increase

5 Fun Facts about Percentages

Percent literally means “per 100”

The % symbol came from the Italian per cento (for 100). Writing 12.5% is just “12.5 per hundred.”

Origins

Stacked percents multiply

10% of 10% is 1% because 0.10 × 0.10 = 0.01. Chaining percentages multiplies them together.

Compound idea

Order of discounts matters

Two discounts don’t add: 20% off then 10% off is a 28% total cut, not 30%, because you’re compounding reductions.

Shopping math

Gains and losses aren’t symmetric

A 50% drop needs a 100% rise to get back. Percent change is directional; up and down aren’t mirror images.

Asymmetry

Percent vs percentage point

Moving a rate from 5% to 6% is a 1 percentage point shift, but it’s a 20% relative increase. Words matter.

Clear wording

Frequently Asked Questions

How do I calculate percent increase or decrease?

Use: (New − Old) ÷ Old × 100. Positive means increase; negative means decrease.

What’s the difference between “X% of Y” and “What percent is A of B”?

“X% of Y” multiplies Y by a percentage. “What percent is A of B” divides A by B and converts to a percentage.

Can I reverse a discount with this tool?

Yes. Use “A is B% of what?” to find the original price before a discount.

Are my inputs private?

Absolutely. Everything runs locally in your browser—no uploads.

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