MrTools guides
How to calculate percentage change
Percentage change compares a new value with an old baseline; the denominator and zero case matter.
Use the old value as the baseline
Subtract old from new, divide by the absolute old value and multiply by 100: (new − old) ÷ |old| × 100. A positive result indicates an increase and a negative result a decrease relative to the baseline.
Separate three common questions
“What is x% of y?”, “x is what percent of y?” and “what is the percentage change?” use different formulas. Choose the matching mode before entering numbers.
Handle zero and rounding honestly
Change from an old value of zero is undefined because division by zero is impossible. MrTools reports that case instead of infinity. Results use JavaScript floating-point arithmetic and display up to eight localized decimal places.
02 · Answers
Frequently asked questions
Why is the old value inside absolute bars?+
Using |old| keeps the result sign tied to the direction of change while treating the baseline magnitude as positive.
Can percentage change start from zero?+
No. There is no finite percentage change from a zero baseline; report the absolute change instead.