How do you calculate percent change with zero?

Percent change represents a change as a portion or a multiplier of the base number. So if you start with n=10 and end up with m=15, your percent change is the difference (5) divided by the base (10), which is 0.5, or 50%.

When your base gets really, really small (close to zero), your percent change gets really, really large (in mathematical terms, it approaches infinity). When your base is actually zero, the percent change has no meaning, since there is no such thing as a portion (or a multiplier) of nothing.