How to Calculate Percentage Change: Formula, Examples & Common Mistakes

The Percentage Change Formula

The formula for percentage change is:

Percentage Change = [(New Value − Old Value) / Old Value] × 100

A few things to keep in mind:

• A positive result means an increase.
• A negative result means a decrease.
• The "Old Value" — the number you started with — always goes in the denominator, never the new one.

If you'd rather skip the manual math, you can plug your numbers straight into our percentage change calculator and get an instant result.

Step-by-Step Example

Let's say a product's price went from $80 to $92.

1. Subtract the old value from the new value: 92 − 80 = 12
2. Divide by the old value: 12 / 80 = 0.15
3. Multiply by 100: 0.15 × 100 = 15% increase

Now let's try a decrease. Say a company's revenue dropped from $50,000 to $42,500.

1. 42,500 − 50,000 = −7,500
2. −7,500 / 50,000 = −0.15
3. −0.15 × 100 = −15%, or a 15% decrease

Notice that even though the dollar amount changed in both examples, the percentages tell a very different story depending on what the original number was — which brings us to the most common mistake.

Common Mistakes to Avoid

1. Using the wrong number as the "old value"

This is by far the most frequent error. If you accidentally divide by the new value instead of the old one, your answer will be wrong — sometimes significantly. Always ask: "What was the starting point?" That number belongs in the denominator.

2. Confusing percentage change with percentage point change

If an interest rate moves from 5% to 7%, that's a 2 percentage point increase, but a 40% percentage change (2/5 × 100). These two numbers get mixed up constantly in news and finance, so it's worth double-checking which one you actually need.

3. Forgetting the negative sign on decreases

A drop from 100 to 80 is a −20% change, not a plain "20%." Dropping the negative sign can flip the meaning of your data, especially in spreadsheets or reports where the sign matters.

4. Rounding too early

If you round an intermediate step (like the decimal in step 2 above) before multiplying by 100, you can introduce small errors that compound across a dataset. It's best to keep full precision until the final step — something our fraction to decimal calculator can help with if you're working from fractional values.

5. Averaging percentage changes incorrectly

You can't just average two percentage changes and expect an accurate combined result, especially over multiple periods — the base value shifts each time. For more on how averages and deviations behave, see our guide on mean and mean absolute deviation.

When Would You Use Percentage Change?

• Comparing prices before and after a sale
• Tracking investment or portfolio performance
• Measuring growth or decline in business metrics (revenue, users, traffic)
• Analyzing changes in scientific or survey data
• Comparing unit prices after a unit conversion

Percentage change tells you how much a value has grown or shrunk relative to where it started — not how much it changed in absolute terms. Always anchor to the original value.

Quick Recap

Percentage change tells you how much a value has grown or shrunk relative to where it started. The formula is simple — (New − Old) / Old × 100 — but getting the old and new values in the right place, and keeping the sign correct, is where most errors happen.

For a fast, error-free result every time, try the percentage change calculator.