What This Tool Does
Choose the mode that matches your question: % of a number, Is what percent of, Percent change, or Increase / decrease by %. Each mode shows the formula, substituted values, and result.
How to Use the Percentage Calculator
- Choose a mode: select % of a number, Is what percent of, Percent change, or Increase / decrease by %.
- Enter your values: fill in the fields shown for that mode.
- Calculate: click Calculate to see the formula, steps, and result.
- Save the result: use Copy result or Download result (.txt). The download uses the filename
percentage-calculator-result.txt. - Start over: click Reset to clear every field, return to % of a number, restore Increase by, hide errors, and hide the result.
Calculation Modes and Formulas
% of a number
To find X% of a number Y, convert the percentage to a decimal by dividing by 100, then multiply by Y:
X% of Y = (X ÷ 100) × Y
For example, 15% of 200 is (15 ÷ 100) × 200 = 30.
Is what percent of
To find what percent a part is of a whole, divide the part by the whole and multiply by 100:
percent = (part ÷ whole) × 100
For example, 30 is what percent of 200? (30 ÷ 200) × 100 = 15%. The whole cannot be zero, since dividing by zero is undefined.
Percent change
To find the percent change from an old value to a new value, subtract the old value from the new value, divide by the old value, and multiply by 100:
percent change = ((new − old) ÷ old) × 100
A positive result is a percentage increase; a negative result is a percentage decrease. For example, going from 50 to 65 is ((65 − 50) ÷ 50) × 100 = 30% increase, while going from 65 to 50 is a -23.08% change (a decrease). The old value cannot be zero, since percent change is undefined when starting from zero.
Increase / decrease by %
To increase a number by a percentage, add that percentage of the number to itself:
result = base × (1 + percent ÷ 100)
To decrease a number by a percentage, subtract instead:
result = base × (1 − percent ÷ 100)
For example, 80 increased by 25% is 80 × 1.25 = 100, and 80 decreased by 25% is 80 × 0.75 = 60.
Inputs, Validation, and Limitations
Enter signed digits with an optional decimal, such as -12.5. The accepted format matches /^-?\d+(\.\d+)?$/. Blank values and unsupported formats show an error.
- Is what percent of: Whole cannot be zero.
- Percent change: Old value cannot be zero.
- Negative numbers and negative percentages are supported. In Increase / decrease by %, a negative percentage reverses the selected direction: Increase by -X behaves as a decrease by X, and Decrease by -X behaves as an increase by X.
- The percent-change result is labeled Increase when the computed percentage is nonnegative and Decrease otherwise. With negative inputs, that sign-based label may feel counterintuitive.
- If clipboard access is unavailable, Copy result displays "Could not copy - select and copy manually."
Privacy & Security
Calculator calculations run locally in your browser. The page also loads Ahrefs and Google Analytics scripts. If you use the optional newsletter form, it sends the submitted email address to /assets/subscribe.php.
Frequently Asked Questions
How do I calculate X% of a number?
Multiply the number by the percentage divided by 100: X% of Y = (X ÷ 100) × Y. For example, 15% of 200 is (15 ÷ 100) × 200 = 30. Use the "% of a number" mode and enter the percentage and the number.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100: percent = (part ÷ whole) × 100. For example, 30 is what percent of 200? (30 ÷ 200) × 100 = 15%. The whole cannot be zero, since dividing by zero is undefined.
How is percentage increase or decrease calculated?
Percent change = ((new value − old value) ÷ old value) × 100. A positive result is a percentage increase, a negative result is a percentage decrease. For example, going from 50 to 65 is ((65 − 50) ÷ 50) × 100 = 30% increase. The old value cannot be zero.
Why is percent change from 0 undefined?
Percent change divides by the starting (old) value. If the old value is 0, that division is undefined, so the calculator reports an error instead of a result.
How does Increase / decrease by % work?
Enter a Number, choose Increase by or Decrease by, and enter a Percentage. A negative percentage is treated as the opposite direction. For example, Increase by -10% behaves as a 10% decrease.