How to use
Choose the question you need to answer: a percentage of an amount, one number as a percentage of another, or change from an original value to a new value. Enter the two values for that mode. The tool keeps each mode’s inputs separate so you can compare questions without turning rounded answers into new source data.
Use the selected number format consistently. Inputs should describe the same units when comparing amounts. This is a general arithmetic calculator; it does not automatically round currency to cents or apply exchange rates.
Calculation method
Percentage of an amount = amount × percentage / 100. The first mode also shows the amount after adding or subtracting that percentage. A part as a percentage of a whole = part / whole × 100. Percentage change = (new − original) / original × 100, alongside the absolute difference.
The comparison base must be positive in the second and third modes. In particular, percentage change from zero is undefined. Negative percentages and a negative new value can be valid arithmetic within the supported ranges.
Worked example
15% of 200 is 30. Adding it gives 230; subtracting it gives 170. A value of 30 compared with 200 is 15%. A change from 160 to 116 is −44 in the original units and −27.5% relative to 160.
A rate moving from 5% to 7% rises by 2 percentage points, but its relative increase is 40%. The calculator’s percentage-change mode answers the relative question.
Precision and interpretation
The tool uses exact scaled integer and rational arithmetic for supported decimal inputs. A rounded display is marked with approximately; changing display format does not change the underlying answer. Repeating decimals are shown to the supported display precision. Results above 100% are not automatically errors, and a negative remainder is not automatically a valid real-world price or quantity.
For exact rational operations use the fraction calculator. For a salary-specific comparison with pay-period and inflation options use the pay raise calculator.
For the measurement and planning details, read percentages, percentage change and percentage points.