3-2-1 Rule

The 3-2-1 Rule is the classic basic rule of data backup: three copies of the data, stored on two different types of media, with one copy at an external location.

The logic behind it: every single protective measure can fail – only the combination of redundancy, media diversity and spatial separation covers the typical loss scenarios, from hardware defects and operating errors to a fire in the data center. In response to ransomware, the extension 3-2-1-1-0 has become established: additionally one copy offline or immutable (for example as an Immutable Backup or behind an Air Gap) – and zero errors in regular recovery tests.

The rule is deliberately kept simple and does not replace a complete backup strategy with retention periods, recovery objectives and tested processes. As a minimum standard and communication formula, however, it remains the most widespread starting point for any data backup planning to this day.