Percentage Decrease Calculator
How much a number fell (or rose) as a percent of the start. Click any i for detail.
Values
Starting value
i
From
Original amount before the change. |
|
|---|---|
Ending value
i
To
Amount after the change. |
How to use this calculator
- Enter the starting value.
- Enter the ending value.
- Read the percent decrease (or increase) and absolute change.
Results explained
Percent decrease is (start − end) / |start| × 100 when the value falls. If the end is higher, the page reports a percent increase instead. A start of zero cannot define a percent change. This is relative change, not percentage points (which are a different idea when comparing two percents).
Quick reference: percent change
Relative change = (end − start) / |start| × 100.
| Item | Detail |
|---|---|
| 100 → 80 | 20% decrease |
| 80 → 100 | 25% increase |
| Percentage points | Different: 40% to 50% is +10 points, not +25% |
| Cousin | Percentage increase calculator |
Division by zero is rejected when the start is 0.
How the estimate is built
pct = (to − from) / |from| × 100; decrease amount = max(from − to, 0) / |from| × 100.
Example scenario
100 → 80 → 20% decrease.
FAQ
Is a drop from 50% to 40% a 10% decrease?
As relative change on the values 50→40, it is a 20% decrease. People often mean 10 percentage points — say which one you need.
What if the start is negative?
The formula still uses absolute value in the denominator so the sign of the change stays readable. Interpret carefully in accounting contexts.
How is this different from percent of?
“Percent of” asks what fraction one number is of another. This page asks how much something changed relative to its start.
Can I reverse a decrease?
To undo a 20% decrease you need a 25% increase (80 × 1.25 = 100), not another 20%.