NAS, RAID and Backup Storage
3-2-1 backup storage calculator.
A 3-2-1 plan keeps 3 copies of your data: the original plus 2 backups. Each backup copy needs (full backups kept x data size) plus (incrementals kept x data size x daily change rate). Weekly fulls and 30 days of history for 2 TB with 2% daily change need 11 TB per copy and 24 TB in all. Runs in your browser; nothing is sent or stored.
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Worked example
2 TB of data, 2% daily change, a full backup every 7 days and 30 days of history: fulls kept = 30 / 7 rounded up = 5, so 5 x 2 TB = 10 TB; incrementals kept = 30 - 5 = 25, so 25 x 2 TB x 2% = 1 TB. Each backup copy needs 11 TB (5.5 x the data), the two backup copies need 22 TB, and with the original the three copies hold 24 TB.
How it works
Number of fulls kept = history days / days between fulls, rounded up. Incrementals kept = history days - fulls kept. Per copy = fulls x data + incrementals x data x change rate. Two backup copies plus the original make the three copies of the 3-2-1 rule.
This is a simple planning model: it assumes every full backup is the whole data size, every incremental is the same size, and nothing is compressed or deduplicated. Real backup software that stores synthetic fulls, compresses or deduplicates can use much less. Then work out the time each run needs with the backup window calculator and compare ongoing cost with the cloud backup vs NAS calculator.
Estimates only. RAID is not a backup: it keeps an array running through a drive failure but does not protect against deletion, malware, fire or theft. Real arrays and drives vary, so use this for planning and check your own drive datasheets.
Sources and basis (opened 2026-10-08):
- Data Backup Options (CISA, Ruggiero and Heckathorn): The 3-2-1 rule: keep 3 copies of any important file (1 primary and 2 backups), on 2 different media types, with 1 copy offsite. The size model on this page is our own arithmetic.
FAQ
What is the 3-2-1 backup rule?
Keep 3 copies of any important file (1 primary and 2 backups), on 2 different media types, and store 1 copy offsite.
How much storage do I need for backups?
Each backup copy needs the fulls you keep plus the incrementals. For 2 TB of data with weekly fulls, 30 days of history and 2% daily change, that is 11 TB per copy.
Do I count the original in the three copies?
Yes. The rule counts the primary copy plus 2 backups, so the example totals 2 TB + 11 TB + 11 TB = 24 TB.
How do I find my daily change rate?
Your backup software usually reports how much data each incremental backed up. Divide that by the total data size and enter it as a percentage.
Is RAID one of the three copies?
No. RAID is redundancy inside one device and protects against drive failure only. A mirrored array is still one copy of your data.