Keep 3 copies of anything that matters, on 2 different media, with 1 off-site.
Each rule breaks a different failure mode: device loss, media-type failure, and site-level disasters (fire, theft, flood).
Three copies of a file that was already corrupt is three copies of a corrupt file. The rule counts copies and says nothing about verification or restore testing — an untested off-site backup has failed at the only moment it matters, and nobody finds out until then.
Estimate with the rule, then check it against the calculator that models it properly.
Open 3-2-1 Backup Calculator →