How do you find what percent one number is of another?

Percent = (Part ÷ Whole) × 100. If 24 is what percent of 80? 24 ÷ 80 × 100 = 30%.

How do you calculate percentage decrease?

Use the same percent-change formula. If the new value is lower, the result is negative (a decrease). From 80 to 60 is a 25% decrease.

Percentage Calculator: % Of, Increase, Decrease & Reverse

Q: How do you calculate a percentage of a number?

Multiply the number by the percent divided by 100. Example: 15% of 80 = 80 × 0.15 = 12.

Q: How do you calculate percentage increase?

Percentage increase = ((New − Old) ÷ Old) × 100. Going from 50 to 65 is a 30% increase.

Q: What is a reverse percentage?

Reverse percentage finds the original amount before a percent was applied. If a price after 20% off is $80, original = 80 ÷ 0.80 = $100.

Q: Is the percentage calculator free?

Yes. ToolMars percentage calculator is free, private, and needs no account.

Percentages show up in tips, grades, discounts, and growth rates. A free percentage calculator answers the four questions people type into search: what is X% of Y, what percent is A of B, how much did a value change in %, and what was the original before a % was applied.

Below are the formulas, short examples, and when to use reverse percentage for sale prices. Pair this with the discount calculator when you are shopping percent-off deals.

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Formula 1: What Is X% of a Number?

Result = Number × (Percent ÷ 100)

Example: 15% of 80 = 80 × 0.15 = 12. Mental shortcut: 10% of 80 is 8; 5% is 4; add them for 15%.

Formula 2: A Is What Percent of B?

Percent = (Part ÷ Whole) × 100

Grade example

You scored 24 out of 80. Percent = 24 ÷ 80 × 100 = 30%. The calculator's “Is what %” mode does this without rearranging the fraction.

Formula 3: Percentage Increase or Decrease

% Change = ((New − Old) ÷ Old) × 100

From 50 to 65: (65 − 50) ÷ 50 × 100 = 30% increase. From 80 to 60: −25% (decrease). Always divide by the old value — using the new value as the base is a common mistake.

Salary raises and traffic growth use percent increase.
Weight loss and price cuts use percent decrease (negative change).

Solve % of, % change, and reverse percentages

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Formula 4: Reverse Percentage

If you know the final amount after a percent off (or a percent markup), reverse to the original:

After % off: Original = Final ÷ (1 − %/100)
After % increase: Original = Final ÷ (1 + %/100)

Example: an item is $80 after 20% off → 80 ÷ 0.80 = $100 original. This is the math behind “was $X, now $Y” reconstructions when the tag only shows the sale price and the percent.

Related Tools

Percentage Calculator — % of, is-what-%, change, reverse
Discount Calculator — sale price and stacked % off
Percent Error Calculator — lab percent error

Solve % of, % change, and reverse percentages

Free percentage calculator — four modes, instant results, no signup.

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Frequently Asked Questions

How do you calculate a percentage of a number?

Multiply by percent ÷ 100. 15% of 80 = 12.

How do you calculate percentage increase?

((New − Old) ÷ Old) × 100.

What is a reverse percentage?

Recovering the original before a percent was applied — e.g. $80 after 20% off → $100.

Is the percentage calculator free?

Yes on ToolMars.