How to use this calculator
Choose the calculation you need, then enter both values. Use Percentage of for a portion of a number, What percent? to compare a part with a whole, or Percentage change to compare a starting and ending value.
The math behind it
Percentage of = (percent ÷ 100) × value What percent? = (part ÷ whole) × 100 Percentage change = ((new − original) ÷ original) × 100
A worked example
15% of 200 is 30. Going from 80 to 100 is a 25% increase; going from 100 to 80 is a 20% decrease.
Assumptions & precision
The whole cannot be zero. Percentage change requires a positive starting value to avoid ambiguous negative baselines. Signed numbers are allowed in the other modes. Results show up to 6 decimal places, with scientific notation for very small nonzero values. Calculations use unrounded values internally; displayed magnitudes are limited to 10¹⁵.
A common question
Why are an increase and a decrease different percentages?
Each percentage uses its own starting value. Adding 20 to 80 is 25% of 80; subtracting 20 from 100 is 20% of 100.