Percentage Change Calculator: Increase and Decrease Explained

Percentage change is one of the most useful everyday calculations. It tells you how much a value has grown or shrunk in relative terms — whether that is a price rise, a drop in website traffic, a salary bump, or a change in test scores. A percentage change calculator does the math instantly, but knowing the formula helps you interpret the result and avoid common traps.

What Is Percentage Change?

Percentage change measures the size of a change relative to the starting value, expressed as a percentage. A rise from 100 to 150 is a 50% increase; a fall from 100 to 75 is a 25% decrease.

The reason we use percentages rather than raw differences is context. A £10 increase means something very different on a £20 item than on a £2,000 item. Percentage change captures that proportion.

The Formula

Percentage change = ((new value − old value) ÷ |old value|) × 100

• A positive result means an increase.
• A negative result means a decrease.

We divide by the absolute value of the old number so the sign of the result reflects the direction of change correctly.

A Worked Example

A subscription rises from $40 to$52:

• Difference = 52 − 40 = 12
• Percentage change = (12 ÷ 40) × 100 = 30% increase

Now suppose website visitors fall from 5,000 to 4,000:

• Difference = 4,000 − 5,000 = −1,000
• Percentage change = (−1,000 ÷ 5,000) × 100 = 20% decrease

How to Use the Percentage Change Calculator

1. Enter the original (old) value — where you started.
2. Enter the new value — where you ended up.
3. The calculator shows the percentage change, labels it as an increase or decrease, and displays the absolute difference.

The Subtlety of Reversing a Change

A common surprise: a percentage increase and the decrease that undoes it are not the same number. If a $100 item rises by 50$150, it must then fall by 33.3% (not 50%) to return to $100. This is because the base changes — the increase is measured against 100, but the decrease is measured against the larger 150. Understanding this prevents a lot of confusion in finance and analytics.

Where Percentage Change Is Used

• Retail and pricing: measuring markups, discounts, and price rises.
• Business analytics: tracking growth in revenue, users, or conversions.
• Personal finance: comparing salary changes or investment returns.
• Science and statistics: expressing measured differences in relative terms.

Common Mistakes to Avoid

• Swapping old and new. The starting value goes in the denominator. Reversing them gives the wrong base and the wrong percentage.
• Dividing by the new value. Always divide by the original value, not the final one.
• Confusing percentage change with percentage points. Going from 10% to 15% is a 5 percentage-point rise but a 50% relative increase.
• Starting from zero. If the old value is 0, percentage change is undefined — there is no base to compare against.

Percentage Change vs Percentage Difference

Percentage change assumes a clear “before” and “after.” If you are instead comparing two independent values with no natural starting point, you may want percentage difference, which divides by the average of the two numbers. For most everyday questions — prices, scores, metrics over time — percentage change is the right tool.

Conclusion

Percentage change turns a raw difference into a meaningful, comparable figure by measuring it against the starting value. Remember to divide by the original number, watch the direction of the sign, and keep percentage change separate from percentage points. A calculator makes it effortless.

Try our free Percentage Change Calculator for instant results.