Basic Math Guide
Percent Change Calculator Guide: Increase and Decrease Without Guessing
Learn how percent change compares an old value with a new value, and why the starting value matters.
Quick answer
Percent change measures how much a value moved compared with where it started. The formula is (new value - old value) / old value x 100. If a value rises from 80 to 100, the increase is 25%. If it falls from 100 to 80, the decrease is 20%. The direction matters because the base is different.
Why this matters
Percent change is useful because raw differences can be misleading. A 20 unit increase is huge if the starting value is 40, but modest if the starting value is 2,000. Percent change puts that movement into context by comparing it with the original value.
Example
Suppose a small newsletter had 800 subscribers in January and 1,000 in February. The difference is 200 subscribers. The percent change is 200 / 800 x 100 = 25%. That tells you the list grew by one quarter relative to its starting size, which is more meaningful than saying it gained 200 people.
How to use the calculator
Enter the old value and the new value. Positive results mean increase, negative results mean decrease. If you are comparing repeated periods, keep the time window consistent. Comparing one week to one month will make the percent change look dramatic but not very useful.
Common mistakes
The biggest mistake is reversing old and new values. Another is assuming a 50% decrease followed by a 50% increase returns to the starting point. It does not. A drop from 100 to 50 is -50%, and a rise from 50 to 75 is +50%, leaving the value below the original 100.
When not to rely on this estimate
Percent change is sensitive to very small starting values. Moving from 1 to 3 is a 200% increase, but the actual difference is only 2. Always read percent change alongside the raw numbers when the starting value is small.
FAQ
Why is the old value important?
It is the base of the comparison.
Can percent change be negative?
Yes. A negative value means the new value is lower than the old value.