The is-ought fallacy occurs when an arguer uses empirical facts alone to conclude that something is morally right or good, without supplying a premise that generally defines morality in way that matches up to the asserted facts. The fallacy lies in thinking that merely because something is the case, therefore it is morally right. Ordinarily, this argument is made when something is prevalent or very commonly the case, so that the fallacy resembles an appeal to popularity.

Here are some examples:

"Alcohol is legal in some form in almost every country, and is consumed by millions of people, therefore it most be wrong to condemn alcohol consumption as morally wrong."

"The F-word is one of the most commonly used words, and has been added to every major dictionary. Therefore, we should not think it taints one's virtue to use the F-word."

The problem in these examples is that it is entirely possible for a practice to be immoral, irrespective of how widespread the practice is.

Two particular forms of the is-ought fallacy are the naturalistic fallacy and the need makes right fallacy.