Quick answer
Percent change measures how much a value moved up or down compared
with its original value. It answers a very specific question:
how large was the change relative to where you started?
This is why percent change is used so often for prices, salaries,
revenue, sales, traffic, grades, population, and investing. A raw
change by itself does not always tell the whole story. A $10
increase can be huge in one situation and small in another.
Main percent change formula:
((new value − original value) ÷ original value) × 100
Positive result
A positive percent change means the value increased.
Negative result
A negative percent change means the value decreased.
What percent change means
Percent change tells you how big a change is compared with the
starting value. That starting value is the baseline.
This matters because the same raw change can represent very
different percentages depending on what you started with.
Example A
Going from 50 to 100 is a change of 50, which is a
100% increase.
Example B
Going from 500 to 550 is also a change of 50, but only a
10% increase.
The absolute change is the same in both examples, but the percent
change is very different because the original value is different.
How to calculate percent change step by step
Suppose a value changes from 80 to
100.
Step 1: Find the difference
100 − 80 = 20
Step 2: Divide by the original value
20 ÷ 80 = 0.25
Step 3: Convert to a percent
0.25 × 100 = 25%
Result: the value increased by 25%.
Percentage increase examples
Example 1: Price increase
A product price goes from $40 to $50.
((50 − 40) ÷ 40) × 100 = 25%
The price increased by 25%.
Example 2: Revenue increase
Revenue goes from 1,200 to 1,500.
((1500 − 1200) ÷ 1200) × 100 = 25%
Revenue increased by 25%.
Example 3: Website traffic increase
Traffic goes from 2,000 visits to 2,600 visits.
((2600 − 2000) ÷ 2000) × 100 = 30%
Traffic increased by 30%.
Percentage decrease examples
Example 1: Salary decrease
Income drops from $60,000 to $54,000.
((54000 − 60000) ÷ 60000) × 100 = -10%
The salary decreased by 10%.
Example 2: Product discount
A product falls from $120 to $90.
((90 − 120) ÷ 120) × 100 = -25%
The price decreased by 25%.
Example 3: Score decline
A test score goes from 92 to 83.
((83 − 92) ÷ 92) × 100 ≈ -9.78%
The score decreased by about 9.8%.
Reading the sign:
Negative = decrease. Positive = increase.
Why the original value matters
The original value matters because it defines the scale of the
change. A change of 20 units means something very different when
the starting value is 40 than when the starting value is 400.
40 to 60
Change = 20
50% increase
400 to 420
Change = 20
5% increase
That is why percent change is often more informative than just
reporting the raw difference.
Percent change vs percent difference
These are not the same thing.
Percent change
Use when one value clearly comes first and the other is the
updated value.
Percent difference
Use when comparing two values without treating either one as
the official baseline.
Percent difference uses a different formula:
|A − B| ÷ ((A + B) ÷ 2) × 100
Use percent change when the question is:
How much did this change from where it started?
What happens when the original value is 0
This is one of the most common trouble spots.
If the original value is 0, standard percent
change is undefined because the formula requires division by zero.
((new − 0) ÷ 0) × 100
Since division by zero is not defined, there is no normal percent
change result.
Practical rule:
If the original value is 0, you can still report the absolute
change, but not standard percent change.
Common mistakes
- Dividing by the new value instead of the original value
- Forgetting to multiply by 100
- Ignoring the negative sign on decreases
- Mixing up percent change with percent difference
- Trying to use the formula when the original value is 0
- Rounding too early before finishing the math
Good habit:
Do the full calculation first, then round the final percentage.
When to use percent change
Percent change is useful whenever you want to compare a new amount
with an old amount.
Common uses
Prices, revenue, salary changes, sales growth, grades,
investing, and traffic changes.
Best question
How much did this go up or down compared with where it
started?
Bottom line
Percent change measures movement relative to the original value.
Find the difference, divide by the starting value, and multiply by
100.
Main formula to remember:
((new value − original value) ÷ original value) × 100