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How to Calculate Percentages

Updated June 2025 · 5 min read

Percentages show up everywhere, from sale prices and tax rates to test scores and statistics. Once you understand the three core percentage calculations, you can handle almost any percentage problem you encounter in daily life.

The Three Core Percentage Calculations

  • What is X% of Y? Find a percentage of a number (e.g., what is 20% of 80?).
  • X is what percent of Y? Find what percentage one number is of another.
  • Percentage change. How much did something increase or decrease in percent terms?

Calculation 1: What Is X% of Y?

This is the most common percentage calculation. You want to find a specific portion of a number.

Formula:

Result = (Percentage / 100) x Number

Examples:

  • What is 20% of 80? = (20 / 100) x 80 = 0.20 x 80 = 16
  • What is 15% tip on a $45 meal? = 0.15 x 45 = $6.75
  • What is 7% sales tax on $120? = 0.07 x 120 = $8.40

Calculation 2: X Is What Percent of Y?

Here you already have two numbers and want to express their relationship as a percentage. Divide the part by the whole, then multiply by 100.

Formula:

Percentage = (Part / Whole) x 100

Examples:

  • 30 is what percent of 150? = (30 / 150) x 100 = 20%
  • You scored 47/50 on a quiz. What is your percentage? = (47 / 50) x 100 = 94%
  • A store sold 320 of 400 items. What percent sold? = (320 / 400) x 100 = 80%

Calculation 3: Percentage Change

Use this when something has increased or decreased and you want to express the change as a percentage of the original value.

Formula:

Percentage Change = ((New Value - Old Value) / Old Value) x 100

Examples:

  • Price went from $50 to $65. Change = ((65 - 50) / 50) x 100 = +30% (increase)
  • Subscribers dropped from 1,200 to 900. Change = ((900 - 1200) / 1200) x 100 = -25% (decrease)
  • Your grade went from 72 to 90. Change = ((90 - 72) / 72) x 100 = +25%

Finding the Original Value

Sometimes you know the result and the percentage but need to find the original number. Divide by the decimal form of the percentage.

Formula:

Original = Result / (Percentage / 100)

Example:

A price after a 20% discount is $64. What was the original price?
After a 20% discount, you paid 80% of the original.
Original = 64 / 0.80 = $80

Quick Reference: Common Percentages

PercentageDecimalFractionExample (of 200)
10%0.101/1020
15%0.153/2030
20%0.201/540
25%0.251/450
33%0.331/366
50%0.501/2100
75%0.753/4150

Percentage Tips and Tricks

  • 10% trick: To find 10% of any number, just move the decimal point one place to the left. 10% of 340 = 34. Then double it for 20%, halve it for 5%, etc.
  • Reverse the order: X% of Y always equals Y% of X. So 8% of 50 equals 50% of 8, which is just 4. Use whichever version is easier to calculate.
  • Increase vs. markup: A 25% increase on $100 gives $125. But to mark something up so the profit is 25% of the selling price (not cost), divide by 0.75 instead: $100 / 0.75 = $133.33.

Skip the math

Use our Percentage Calculator to instantly solve any of the three core percentage problems, including percentage change and finding the original value.

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