Backup Retention & 3-2-1 Calculator
Enter your data size, backup frequency, and retention period to estimate total backup storage needed — plus the full footprint required to follow the 3-2-1 backup rule.
Source: the 3-2-1 backup rule as documented by CISA. See the full methodology page. Reviewed: July 2026.
What the 3-2-1 rule actually means
3 total copies of your data, on 2 different media/storage types (e.g. disk + tape, or on-prem NAS + cloud object storage), with 1 copy kept offsite. The production data itself counts as one of the three copies — this calculator estimates the storage you need to purchase or provision for the other two: a local backup copy and an offsite copy.
Full vs. incremental storage sizing
Full backups repeat the entire dataset every time, which is simple but storage-heavy over a long retention window. Incremental backups store a single full backup plus only the changed data on each subsequent run — usually far more space-efficient for daily backups with a low daily change rate, at the cost of longer, more complex restores (each restore needs the full backup plus every incremental since).
Choosing a daily change rate
3-5% daily change is typical for general file servers and databases with moderate write activity. Highly transactional databases or media-heavy workloads can run 10%+ — check your actual backup job reports for a real figure rather than guessing, since this number drives most of the incremental storage estimate.
Does the 3-2-1 rule protect against ransomware?
Only if at least one copy is immutable or air-gapped/offline. A live-mounted "offsite" copy that's still network-accessible can be encrypted by the same ransomware that hits production. Consider adding immutability (object lock, WORM storage) as a "3-2-1-1" extension.
Should retention period match compliance requirements?
Often not by default — many regulatory frameworks (financial records, healthcare, etc.) require longer retention than typical operational backup policy. Check your specific compliance obligations separately; this calculator only models storage for operational recovery purposes.