Quick answer
Take the new value minus the old value, divide by the old value, then multiply by 100. A positive result is an increase and a negative result is a decrease.
Why this matters in day-to-day decisions
Percentage change is one of the most common pieces of business maths, but it becomes slippery when the base value is not clear. People often know the two numbers, but not which one should be treated as the starting point.
The three-part way to think about it
Starting value
The original number you are measuring from.
Ending value
The new number you want to compare with the original.
Change
The difference between the two, measured against the starting value.
Worked example in plain English
Imagine a figure moves from 100 to 120. The absolute change is 20, but the percentage change is measured against the original 100, not against the new 120.
That is why the base value matters so much. Once the base is wrong, the final percentage will be wrong too.
Where this shows up in business
- Checking how much a cost has risen.
- Comparing an old price with a new one.
- Reviewing changes in revenue or performance.
- Explaining increases or decreases in a cleaner way.
Common mistakes to avoid
- Dividing by the wrong value.
- Mixing up which number is old and which is new.
- Presenting the final percentage without the underlying numbers.
- Using the result without checking whether the change is an increase or decrease.
Best practical habit
Keep the old value and the new value visible next to the final percentage. That makes the result easier to trust, interpret, and explain to someone else.
FAQs
Why do people get percentage change wrong?
Because they often divide by the wrong number or forget which value is the starting point.
What does a negative result mean?
A negative result means the value has decreased from the starting point.
When is percentage change most useful in business?
It is especially useful for comparing old and new prices, cost increases, revenue movement, and simple trend checks.