Numarly

Percentage Calculator Suite

All percentage calculations in one clean tool

What is X% of Y?

Percentage Increase / Decrease

What % is X of Y?

What is it?

This calculator handles the three most common percentage problems in one tool: finding a percentage of a number, calculating percentage increase or decrease, and finding what percentage one number is of another.

Formula

  • X% of Y = (X ÷ 100) × Y
  • % Change = ((New − Old) ÷ Old) × 100
  • X is what % of Y = (X ÷ Y) × 100

Formula Explanation

All three tools rearrange the same core percentage relationship — a part divided by a whole, times 100 — just solving for a different unknown each time: the part, the percentage, or the rate of change between two values.

Example Calculation

20% of 150 is (20 ÷ 100) × 150 = 30. Going from 80 to 100 is a change of ((100 − 80) ÷ 80) × 100 = 25% increase. And 30 is (30 ÷ 150) × 100 = 20% of 150.

How to Use

  1. Choose the section matching your calculation (percentage of a value, percentage change, or reverse percentage).
  2. Enter the required values.
  3. View the result instantly as you type.

Benefits

  • Covers three common percentage problems in a single page, no need to search for separate tools.
  • Updates results instantly as you type, without a submit button.
  • Useful for shopping discounts, exam scores, growth metrics, and more.

Use Cases

  • Calculating a discount amount during shopping.
  • Finding percentage growth or decline between two business metrics.
  • Figuring out a test score as a percentage of total marks.

What Your Result Means

Each section answers a different question: "What is X% of Y" gives a direct amount, "Percentage Increase/Decrease" shows the rate of change between two values (negative means a decrease), and "What % is X of Y" gives X's share of the whole as a percentage.

Tips

  • A negative percentage change result means a decrease, not an increase.
  • For discount calculations, "X% of Y" gives the discount amount — subtract it from Y for the final price.
  • Double-check which number is "old" and which is "new" in the change calculator, since swapping them flips the sign.

Common Mistakes

  • Using the wrong section for the question being asked — e.g. using "% of" when you actually need "% change".
  • Swapping old and new values in the change calculator, producing a sign-flipped result.
  • Forgetting a percentage increase result is relative to the old value, not the new one.

FAQs

How do I calculate a percentage increase?

Subtract the old value from the new value, divide by the old value, then multiply by 100 — a positive result means an increase, negative means a decrease.

What does "X is what % of Y" mean?

It answers questions like "20 is what percent of 80?" by dividing the part (20) by the whole (80) and multiplying by 100 (giving 25%).

Can percentages be negative?

Percentage change can be negative, indicating a decrease — but the "X% of Y" and "reverse percentage" results are typically shown as positive, unless you enter negative values yourself.

How is a discount amount calculated?

Use the "What is X% of Y" section with X as the discount percentage and Y as the original price to find the discount amount, then subtract it from the original price.

Why do I get different answers going from A to B versus B to A?

Percentage change is not symmetric — a change from 50 to 100 is a 100% increase, but going from 100 back to 50 is only a 50% decrease, because each is calculated relative to a different starting value.

Educational calculator for everyday percentage calculations.

Last updated: July 25, 2026