The arithmetic may be simple, but a percentage has meaning only when the baseline, units, and rounding convention are explicit.
State whether the comparison uses the old…
State whether the comparison uses the old value as the denominator, a target value, or another baseline. Different denominators answer different questions.
Keep calculation precision separate from display precision.…
Keep calculation precision separate from display precision. Rounding early can create a different result from rounding the final display.
For money, tax, measurements, or regulated reporting,…
For money, tax, measurements, or regulated reporting, use the rules and software approved for that domain rather than a public convenience calculator.
Use the related tool carefully
This guide is paired with a browser-only utility. The tool is a practical aid for small examples and inspection; it is not a substitute for application testing, a formal data pipeline, or professional review where the decision has higher consequences.
Open the related Number Format and Percentage Calculator