What does the percentage calculator do?

This calculator is built around four practical math modes: percent of a number, part-to-total percentage, percentage change, and increase/decrease by a percentage. Each mode uses a different base value, so choosing the right mode is part of getting the correct answer.

• Find X percent of a value and see the value after increase or decrease.
• Find what percent A is of B.
• Calculate percentage change from an old value to a new value.
• Apply a percentage increase and decrease to a base value.

What is a percentage?

A percentage expresses a value out of 100. For example, 25% means 25 parts out of 100. Percentages make it easier to compare different quantities on the same scale.

For example, 15% of 200 is 30. In the opposite direction, 30 is 15% of 200. The first statement finds a percentage of a number; the second finds the ratio of a part to a whole.

How do you calculate a percentage of a number?

To calculate a percentage of a number, multiply the number by the percentage and divide by 100. The first mode of this calculator performs that operation.

This mode also shows what the value becomes after adding or subtracting the same percentage, which is useful for quick increase and decrease checks.

What percent is A of B?

The second mode calculates the ratio of a part to a whole. Use it when you want to know what share one value represents within another value.

How is percentage change calculated?

Percentage change measures how much a value has changed relative to the old value. The calculator also labels the direction as increase, decrease or no change.

This is useful for price changes, sales growth, score differences, measurements and many other comparisons over time.

How does increase/decrease calculation work?

The increase/decrease mode applies a percentage to a base value and shows both the increased and decreased result. Adding 18% to 500 and subtracting 18% from 500 are different operations and produce different results.

• After increase = value × (1 + percent / 100)
• After decrease = value × (1 − percent / 100)
• The percentage input is not negative in this mode; increase and decrease are shown as separate results.

Percentage vs percentage point

Percentage change and percentage points are often confused. If a rate rises from 10% to 15%, the increase is 5 percentage points. But the relative percentage change is 50%, because the 5-point difference is half of the original 10% rate.

Which mode should you use?

• Use % of Number to find X percent of a value.
• Use What % is A of B to compare a part with a total.
• Use % Change to compare an old value with a new value.
• Use Increase/Decrease to add or subtract a percent from a base value.

This distinction matters because the same numbers can produce different meanings depending on the chosen base value.

Common percentage mistakes

• Assuming a 20% increase followed by a 20% decrease returns to the original value.
• Entering the part and total in the wrong order.
• Using the new value instead of the old value as the base for percentage change.
• Confusing percentage points with percentage change.
• Using a general percentage calculation in place of a specialized tax, interest or profit-margin formula.

When is this calculator not enough?

This tool is suitable for general percentage math. Some fields, however, use percentages with additional rules. VAT, discounts, profit margin, interest, ROI and statistical rates may require a more specialized calculator or interpretation.

Example calculations

• 15% of 200 is 30.
• 30 is 15% of 200.
• Changing from 100 to 125 is a 25% increase.
• Adding 10% to 400 gives 440.
• Subtracting 10% from 400 gives 360.

How should you read the result?

Read the result together with the mode you selected. A 25% result can mean a part-to-total ratio in one mode, or a relative change from an old value in another mode.