How to Calculate Percentage Increase and Decrease
Percentage change tells you how much a value moved compared with where it started. The key word is started: the old value is always the base.
The formula
Change % = (New − Old) ÷ Old × 100
A positive result is an increase and a negative result is a decrease.
Increase example
Monthly sales went from 50 to 65. The change is 15, and 15 ÷ 50 = 0.30, so sales rose by 30%.
Decrease example
Now sales fall back from 65 to 50. The change is −15, and −15 ÷ 65 = −0.2308, so sales fell by about 23.08%.
Why the two percentages differ
The base changed. The first move was measured against 50 and the second against 65. This is why a 30% rise followed by a 30% fall leaves you below where you began: 100 becomes 130, and 30% of 130 is 39, which brings it to 91.
Percentage points are different
If an interest rate moves from 4% to 5%, it rose by one percentage point, but the rate itself increased by 25% (1 ÷ 4). Always say which one you mean.
Common mistakes
- Dividing by the new value instead of the old one.
- Forgetting to multiply by 100 at the end.
- Comparing percentages that use different bases.
A real-life example
Rent goes from 300 to 330 per month, an increase of 10%. Next year it drops from 330 to 300. That is a fall of 9.09%, not 10%. Use the same formula each time and always write down which number is the old one before you start. It stops the most common slip, which is dividing by whichever value happens to be larger.
Try it yourself
Use our free percentage calculator to check your numbers instantly.