The brain's "capacity"

It's a common expression: "Well, we only use X percent of our brain's capacity." Sometimes it's "fifteen percent"; sometimes it's a more scientific-sounding "seven percent" or a similarly precise number. Here's an inconvenient question: how do we know what the brain's "capacity" actually is? Has anybody measured it? What are the units of measure? Gigabytes? Gigahertz? RPM? Furlongs per fortnight? Could this be just another of those popular slogans that fall apart when you think about them?