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Percentage Change Calculator

Work out the percentage increase or decrease between any two values — and see why the order of the numbers changes the answer.

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How it works

A compact workflow from input to download.

1

Enter your figures

Fill in original value and new value. Every value stays in your browser — nothing is sent to a server.

2

Read the result

The result updates as soon as your inputs are valid, with the headline figure highlighted and the supporting numbers broken out beneath it.

3

Change the inputs and compare

Adjust any value to see immediately how it moves the result — the quickest way to understand which input the outcome is actually most sensitive to.

Frequently asked questions

What is the formula?
Subtract the old value from the new value, divide by the old value, and multiply by 100. The old value is always the denominator, because you are asking how much it changed relative to where it started.
Why is a 50% fall not undone by a 50% rise?
Because the two percentages are taken of different bases. Falling 50% from 100 gives you 50; rising 50% from 50 gives you 75, not 100. To get back to 100 from 50 you need a 100% rise. This asymmetry is why investment losses hurt disproportionately — a 50% drawdown requires a doubling just to break even.
What if the original value is zero?
Then percentage change is undefined, because you would be dividing by zero. There is no meaningful percentage increase from nothing to something — the correct thing to report is the absolute change, or to say the value went from zero to whatever it is now.
Are my numbers sent anywhere?
No. Every calculation runs in JavaScript inside your own browser — nothing is uploaded, logged or stored. Your figures, including financial and health details, never leave your device.

Direction matters, and so does the base

The most common error in percentage change is swapping which value is the base. Going from 80 to 100 is a 25% increase, because the change of 20 is measured against the starting value of 80. Going from 100 to 80 is a 20% decrease, because the same change of 20 is now measured against a starting value of 100. Identical absolute movement, two different percentages, and both correct. The base is always where you started from. This is why you cannot simply reverse a percentage change to undo it, and why comparing 'percentage growth' figures between two organisations of very different sizes tells you far less than it appears to.

How percentage change gets used to mislead

Because a percentage hides the base, it is a superb instrument for making numbers look like whatever you want. A startup announcing 300% growth may have gone from one customer to four. A drug that reduces your risk of a disease by 50% sounds dramatic until you learn the risk went from two in a million to one in a million — a relative reduction of 50% and an absolute reduction of one in a million. Newspapers routinely report the relative figure because it is the bigger number. The defence is simple and always the same: whenever you are given a percentage change, ask what the base was. If the answer is not offered, that is usually deliberate.

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