Percentage calculator guide
Choose the calculation that matches your question, enter the two values, and select Calculate. Results include a decimal fraction when it is useful.
Use percentage change for comparisons such as price increases, growth, weight loss, or declines.
How to use the percentage calculator
- Choose a mode: Select percent of a number, X is what percent of Y, or percentage change.
- Enter values: Enter numbers without a percent sign.
- Calculate: Press Calculate to update the result.
- Interpret the sign: A positive change is an increase; a negative change is a decrease.
Formula and variables
Percent means per hundred. Percentage change uses the original value as the baseline.
p% of x = (p ÷ 100) × x; change% = ((new − old) ÷ old) × 100- p — Percentage
- A rate expressed per hundred.
- x — Value
- The number to which a percentage is applied.
- old — Original value
- The starting value for a change calculation.
- new — New value
- The ending value for a change calculation.
Find 20% of 150
Calculate a common percentage for a quick discount or allocation.
- Percentage
- 20%
- Value
- 150
- 20 ÷ 100 = 0.20
- 0.20 × 150 = 30
Result: 30
Twenty percent of 150 is 30.
Understanding your results
Percentage result
A percentage is a rate per 100. For a change calculation, compare the signed result with the original value to understand the direction and size of movement.
Assumptions
- Values are treated as ordinary real numbers.
- Percentage change uses the first value as the baseline.
- The result is rounded to two decimal places for display.
Limitations
- Percentage change from zero is undefined.
- A percentage and a percentage point are different measures; use a percentage-point calculator when comparing rates directly.
Common mistakes
- Using the final value instead of the original value as the change baseline.
- Entering 20 instead of 0.20 when doing a manual decimal calculation.
- Confusing percentage change with percentage-point change.
- Forgetting that a negative result means a decrease.
Practical use cases
Shopping and budgeting
Estimate discounts, sales tax, tips, and budget shares.
Reports and analysis
Measure growth, decline, conversion rates, and relative differences.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. For example, 20% of 150 is 30.
How is percentage change calculated?
Subtract the original value from the new value, divide by the original value, and multiply by 100.
What if the original value is zero?
Percentage change from zero is undefined because zero cannot be used as the baseline denominator.
Sources and review
- Percent — NIST Reference on Constants, Units, and Uncertainty. Accessed 2026-07-14.
Reviewed 2026-07-14.