3-2-1 Backup Storage Calculator

Frequently Asked Questions

What is the 3-2-1 backup rule?

Keep 3 copies of data, on 2 different media, with 1 copy offsite. The calculator sizes total storage across those copies plus retention and growth.

How does retention drive storage?

Full + incremental schemes accumulate: storage ≈ full size + (daily change × retention days), times the number of copies. Longer retention multiplies cost.

How do I account for data growth?

Apply the monthly growth rate compounded over the retention horizon; backup targets should be sized for end-of-period data, not today's.

Does deduplication change the numbers?

Yes - dedup/compression can cut backup storage 2–10×. This is a pre-dedup worst case; treat real usage as lower.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How This Calculator Works

The 3-2-1 rule means 3 copies of data, on 2 different media, with 1 copy offsite. This tool sizes the storage that rule actually consumes. It compounds your monthly change/growth rate over the retention window, then for each retained restore point counts a full backup at the grown primary size or an incremental at only the change delta (change% × primary). Summing the chain gives storage per location; multiplying by the number of copies gives the total footprint, and the offsite figure is the chain that must replicate off-premises.

Worked Example

1,000 GB primary, 5% monthly change, 12-month retention, 1 full + 11 incrementals, 3 copies. The full near end-of-window grows to 1000 × 1.05¹² ≈ 1,796 GB. Each incremental is ~5% of the grown size (~50–90 GB) across 11 months. Per location totals roughly 2.4 TB; with 3 copies (3-2-1) the grand total is ~7.3 TB, and ~2.4 TB of that must live offsite. Without compounding growth you would badly under-provision the array.

Real-World Use

IT teams use this to budget NAS/SAN capacity, cloud object-storage spend, and tape libraries before a backup project. It answers the procurement question directly: "given our data and retention policy, how much raw storage must we buy - on site and off?" It also frames the cost difference between a full-every-week scheme and an incremental-forever scheme.

Common Mistakes

Ignoring growth: sizing to today's data guarantees you run out before the retention window completes.

Counting only one copy: 3-2-1 means the footprint is roughly 3× the chain, not 1×.

Treating incrementals as free: they accumulate; long chains plus high change rates can rival full backups.

Forgetting offsite bandwidth: the offsite copy must replicate within the backup window, not just fit on disk.

Limitations & Related Tools

This is a capacity estimate - it does not model deduplication or compression, which can cut real usage 2–10× depending on data type, nor synthetic-full or GFS rotation schemes precisely. It assumes a steady monthly change rate; bursty data (databases, VMs) varies. Pair it with a bandwidth calculator to confirm the offsite copy fits the replication window, and always add headroom for retention-policy changes and unexpected growth.

3-2-1 Backup Storage Calculator

Frequently Asked Questions

Related Networking & IT Calculators