Percentage Calculator
Select the question first, then enter the two numbers using the mapping shown in that option. The result repeats the formula so percent-of, ratio, and percentage-change calculations cannot be confused.
Arithmetic tool only. Percentage change from a zero starting value is undefined, and changes involving negative starting values can be mathematically valid but require careful interpretation.
Enter your scenario
Use the details that match your situation. Your estimate stays in this browser.
How this estimate works
- Percent of uses P ÷ 100 × X.
- What percent uses A ÷ B × 100, so B cannot be zero.
- Percentage change uses (new − starting) ÷ starting × 100, so the starting value cannot be zero.
Official references
- NIST Special Publication 330: percent means parts per hundred
- NIST SI unit rules: percent and percentage-difference notation
NIST defines percent as parts per hundred and emphasizes making the quantities and operations clear. The selected mode and displayed formula make those roles explicit.
Frequently asked questions
What is the difference between percent of and what percent?
Percent of applies a known percentage P to a base X. What percent divides a part A by a whole B and converts that ratio to percent.
Why can percentage change not start at zero?
The starting value is the denominator in the formula. Division by zero is undefined, so there is no finite percentage-change result.