Metallica's 1991 album had sold 16.4 million copies in the United States by 2016 according to SoundScan-era figures.
That total made Metallica the best-selling album in the United States since Nielsen SoundScan began tracking sales in 1991. The album passed the previous benchmark held by Shania Twain's Come On Over and became the first SoundScan-era album to exceed 16 million copies.
Its performance was unusual because sales continued for decades rather than being concentrated only around release week. The album never sold fewer than 1,000 copies in a week during the cited 2016 period, and it remained a regular presence on the Billboard 200.
The figure refers to US sales, not worldwide circulation. Global estimates are higher, and later certification updates use different measures such as units shipped. This distinction explains why sources may quote several different totals for the same album.