See whether a number went up or down, and by how much, by comparing an original and a new value.
How to Use the Percentage Change Calculator
Enter the original value and the new value; the result is labeled "Increase" or "Decrease" automatically, depending on which direction the number moved.
percentage change = ((new value − original value) ÷ original value) × 100The denominator here is always the original value — never the new one, never an average of the two — and that choice has a genuinely counter-intuitive consequence. Going from 50 to 100 is a 100% increase (an extra 50 measured against the original 50). Reverse it, and going from 100 back down to 50 is only a 50% decrease (the same 50, but now measured against the bigger 100). The distance traveled is identical both times; the percentage isn't, because the base changed.
That asymmetry is exactly why a price that rises 50% and then falls 50% never returns to where it started. $100 up 50% is $150. $150 down 50% is $75 — not $100 — because the second calculation runs against $150, a bigger number than the original $100 ever was.
This shows up anywhere something gets compared before and after — stock prices, page views, a discount reversed later — and it's the same reason a store can't undo a "40% off, then another 40% off" sale by later marking things up 80%.
An original value of zero breaks the math outright — there's no percentage relative to nothing — so the calculator needs a non-zero starting figure.