Percentage change tells you how much a value moved compared with where it started. It is the most common calculation in business reports: sales growth, cost change, headcount change.
The formula
=(New - Old) / Old
If last month was 80,000 in B2 and this month is 92,000 in C2:
=(C2-B2)/B2
The answer is 0.15. Format the cell as a percentage and it shows 15%.
Increase or decrease
A positive result is an increase and a negative result is a decrease. You do not need two formulas. Use a custom number format or conditional formatting to colour the decreases red.
Avoid the divide by zero error
If last month was 0, the formula gives #DIV/0!. Handle it like this:
=IF(B2=0, "", (C2-B2)/B2)
An empty cell is better than a fake 0% or 100%.
Percentage of a number
This is a different question. To find 15% of 80,000, use =80000*15%. To increase a number by 15%, use =B2*(1+15%).
Percentage change with negative numbers
When the old value is negative, the plain formula gives a misleading sign. A safer version divides by the absolute value of the old number:
=(C2-B2)/ABS(B2)
Percentage points are not percent
If a rate moves from 10% to 12%, it rose by 2 percentage points, but it grew by 20% in relative terms. Say which one you mean in your report.