Percentage Increase Decrease Calculator

This calculator works out the percentage change between two numbers: you enter an original value and a new value, and it returns the percentage increase or decrease, along with the absolute difference. It uses the standard formula (new − original) ÷ original × 100, so a positive result means an increase and a negative result means a decrease.

Formula

The percentage change is calculated as:

Percentage change = ((New value − Original value) ÷ Original value) × 100

• Original value (a) – the starting amount, before the change.
• New value (b) – the amount after the change.
• Percentage change – the result, expressed as a percentage. A positive number is an increase; a negative number is a decrease.

For example, if a price goes from $50 to $60, the change is (60 − 50) ÷ 50 × 100 = 20%, so it is a 20% increase. If it goes from $60 to $50, the change is (50 − 60) ÷ 60 × 100 = −16.67%, a 16.67% decrease.

Worked Example

Using the default values from the calculator: original value = 100, new value = 120.

1. Subtract the original from the new: 120 − 100 = 20.
2. Divide by the original: 20 ÷ 100 = 0.2.
3. Multiply by 100: 0.2 × 100 = 20%.

The calculator shows 20.00%, which matches. The absolute difference is 20.

When to Use This Calculator

You might use a percentage change calculator for:

• Comparing prices – a product went from $80 to $95, what is the increase?
• Tracking growth – your savings grew from $1,000 to $1,250, what is the percentage increase?
• Measuring decline – a stock dropped from $50 to $40, what is the percentage decrease?
• Analyzing data – test scores, population figures, or sales numbers over time.

The key is that you have two numbers: a starting point and an ending point. The calculator works for any units – dollars, kilograms, liters, or plain counts – because the percentage is unitless.

How to Read the Result

The result is a percentage that tells you how much the new value differs from the original relative to the original. A result of 25% means the new value is 25% larger than the original. A result of −10% means the new value is 10% smaller.

Be careful: a 50% decrease followed by a 50% increase does not bring you back to the original. For example, start with 100. Decrease by 50% → 50. Then increase by 50% → 75. You are still 25% below the start. This happens because the second percentage is applied to a different base.

Common Mistakes and Misconceptions

• Dividing by the new value instead of the original. The formula always uses the original value as the denominator. If you divide by the new value, you get a different number.
• Confusing percentage points with percentage change. If an interest rate rises from 2% to 3%, that is a 1 percentage point increase, but a 50% increase relative to the original rate. The calculator gives the relative change (50%).
• Rounding too early. If you round intermediate steps, the final result can be off. Keep full precision until the end.
• Ignoring the sign. A negative result means a decrease. Some people mistakenly read a negative as an increase.

Limitations and When Not to Use It

This formula works well when the original value is positive and meaningful. If the original value is zero, the formula divides by zero, so the percentage change is undefined – you cannot calculate a percentage change from zero. In that case, you might just report the absolute difference.

Also, the formula treats all changes as linear. It does not account for compounding over multiple periods. If you want to find the average annual growth rate over several years, you need a compound annual growth rate (CAGR) calculation, not a simple percentage change.

For very small original values, the percentage change can be huge even if the absolute change is tiny. For example, going from 1 to 2 is a 100% increase, but the actual difference is just 1 unit. Always consider the absolute difference alongside the percentage.

Frequently Asked Questions

How do I calculate percentage increase manually?

Subtract the original value from the new value, divide that difference by the original value, then multiply by 100. For a decrease, the result will be negative; you can report it as a positive percentage decrease.

What if the original value is zero?

You cannot calculate a percentage change from zero because division by zero is undefined. In that situation, just state the absolute difference or describe the change without a percentage.

Is percentage change the same as percentage difference?

No. Percentage change uses the original value as the base, while percentage difference uses the average of the two values as the base. They give different results unless the two values are equal.

Can I use this for negative numbers?

It can be mathematically computed, but the interpretation becomes tricky. For example, going from −10 to −20 is a decrease of 100% by the formula, but the absolute value increased. It is usually clearer to work with absolute values or use a different measure when negative numbers are involved.

Why does a 50% decrease need a 100% increase to get back to the original?

Because the base changes. After a 50% decrease, the remaining value is half the original. To double it back to the original, you need a 100% increase on the new, smaller base.

Does the calculator work for currency or other units?

Yes, as long as both values are in the same unit. The percentage is unitless, so it works for dollars, euros, kilograms, liters, or any other measure.

Related

• This File Came From Another Computer and Might Be Blocked
• Mike Holt Load Calculator: How It Works
• Free PC Optimizer: What It Really Does & If You Need One

in a | Original value | - | 100 | 0
in b | New value | - | 120 | 0
out pct | Percentage change | % | ((b - a) / a) * 100 | 2
out diff | Absolute difference | - | b - a | 2