How the percentage calculator works
Five calculation modes cover common percentage questions: finding a percentage of an amount, finding what percentage one amount represents, measuring change, and recovering an original value before an increase or decrease.
Calculation method
Choose the question that matches your problem before entering values. The calculator keeps full numeric precision during the calculation and displays up to ten decimal places, avoiding premature rounding in intermediate steps.
Part = percentage ÷ 100 × whole. Percentage = part ÷ whole × 100. Percentage change = (new − original) ÷ |original| × 100.Assumptions and counting rules
- Percentage change uses the original value as the comparison base.
- A positive change is an increase and a negative change is a decrease.
- Reverse percentage and percentage-change calculations cannot use zero as their denominator.
Practical examples
15% of 240
15 ÷ 100 × 240 = 36.
Useful for discounts, tips, and allocations.
30 is what percent of 200?
30 ÷ 200 × 100 = 15%.
Useful for completion rates and shares of a total.
Increase from 80 to 100
(100 − 80) ÷ 80 × 100 = 25%.
Useful for price, traffic, or performance comparisons.
Common mistakes
- Dividing by the new value instead of the original value when calculating change.
- Confusing a percentage-point change with a percentage change: 20% to 25% is five percentage points but a 25% relative increase.
- Applying a decrease and equal increase as if they cancel: decreasing 100 by 20% gives 80, and increasing 80 by 20% gives 96.
Edge cases
- A starting value of zero has no finite percentage change.
- Negative original values can be meaningful in accounting, but interpretation depends on context.
- Repeating decimals are displayed with limited decimal places even though the internal calculation uses full floating-point precision.
Questions specific to this tool
What is the difference between percent and percentage points?
Percent describes a ratio; percentage points describe the arithmetic difference between two percentages.
Can the calculator use negative numbers?
Yes, but results involving losses, deficits, or sign changes require contextual interpretation.
Why is 15% of 240 equal to 36?
Fifteen percent means fifteen hundredths. Multiplying 0.15 by 240 produces 36.
Last reviewed: August 7, 2026 · Method version 1.0.0