HaatBeatHow it's calculated
X% of Y = X ÷ 100 × Y
X as % of Y = X ÷ Y × 100
Change = (new − old) ÷ |old| × 100
Percentage change is measured against the starting value. Its size (|old|) is used so that a rise always shows as an increase, even when the starting value is negative.
Assumptions
- Results are shown to at most two decimal places; the calculation keeps full precision.
- A percentage of 0, or a change from 0, has no meaningful value, so none is shown.
Worked example
15% of 2,400 is 360, and 360 is 15% of 2,400. Going from 2,000 to 2,400 is a 20% increase, but going back from 2,400 to 2,000 is a 16.67% decrease, because the starting value is different.
Questions
Why is a rise and then an equal fall not back to zero percent?
Each change is measured against its own starting value. A 20% rise from 2,000 reaches 2,400; a 20% fall from 2,400 reaches 1,920, not 2,000.
What is the difference between percent and percentage points?
A rate moving from 10% to 12% rises by 2 percentage points, which is a 20% increase in the rate itself.
Can I use negative numbers?
Yes. Percentage change uses the size of the starting value, so going from −50 to −25 is shown as a 50% increase.