Percent error calculator
Percent error, with the signed version and the other denominator shown alongside — because two definitions are in circulation and a figure you have been handed rarely says which it used. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Which number goes on the bottom
Percent error divides by the accepted value. That is the convention, and the reason is that the accepted value is the thing you are comparing against — dividing by your own measurement makes the answer depend on how wrong you were.
A fair number of sources divide by the measured value anyway. For small errors the two agree to several decimal places, which is exactly why nobody notices; for large ones they separate. This page shows both so you can tell which one a figure you have been handed was computed with.
Signed or absolute
The usual school definition takes the absolute value, so percent error is never negative. That throws away the direction, which is often the useful part — a thermometer reading 2% low every time is a different problem from one that is 2% out at random.
Both are given above. Neither is wrong; they answer different questions.
And it cannot be done against zero
If the accepted value is zero there is no percent error, because there is no percentage of nothing. That is not a limitation of this page, it is the definition — and a calculator that returns Infinity there is giving a number where there is not one.