Backup Storage Calculator
Raw storage for the 3-2-1 backup rule from source, copies, and versions.
What it does
The 3-2-1 rule keeps three copies of important data on two media types with one copy offsite. One drive dies, one format ages out, and one building floods without taking the data, because each failure mode meets a surviving copy.
The tool above takes source size with copies and kept versions. It returns raw storage in GB and TB with the offsite share beside it. The default 500 GB at 3 copies and 4 versions needs 6,000 GB (6 TB) with 2,000 GB offsite.
Wording matches CISA backup guidance with NIST ransomware recovery references beside it: three copies of important files, two storage media such as drive plus cloud, and one copy away from the building. Backups run automatically and restore tests prove them.
Raw math ignores compression, deduplication, and RAID overhead on purpose. Those shrink or grow real disk buys, so treat this total as the planning floor and size hardware from measured ratios afterward.
How to use
- Type source data size.
- Pick GB or TB.
- Keep copies at 3 unless policy says otherwise.
- Type kept versions or snapshots.
- Read raw total first, offsite beside it.
- Split media: local drive plus cloud or tape offsite.
Formula
- copies
- 3 in the rule: primary plus 2 backups.
- versions
- Kept generations or snapshots.
- offsite
- One full version set away from site.
TB inputs multiply by 1000 first. Binary TiB shown for hardware shopping: divide GB by 1073.74.
Worked example
Example: 500 GB, 3 copies, 4 versions
Defaults: 500 GB source, 3 copies, 4 versions.
- Per version: 500 x 3 = 1,500 GB.
- Total: 1,500 x 4 = 6,000 GB.
- Terabytes: 6,000 / 1000 = 6 TB.
- Offsite: 500 x 4 = 2,000 GB.
- Hardware view: 6,000 / 1073.74 = 5.59 TiB.
- Tool shows 6,000 GB, 6 TB, 2,000 GB offsite.
Reference table
| Source x copies x versions | Raw total | Offsite |
|---|---|---|
| 100 GB x 3 x 1 | 300 GB | 100 GB |
| 500 GB x 3 x 4 | 6,000 GB (6 TB) | 2,000 GB |
| 1 TB x 3 x 1 | 3,000 GB (3 TB) | 1,000 GB |
| 256 GB x 3 x 2 | 1,536 GB | 512 GB |
Limits
- Raw math; compression and dedup excluded.
- RAID overhead excluded.
- Counts copies, not restore speed.
- Policy gaps like retention law ignored.
Common mistakes
- Counting versions as free.
- Keeping all copies on one media.
- Skipping the offsite leg.
- Buying TB while software quotes TiB.
- Never testing a restore.
Try it yourself
Quick checks. Answers sit tucked under each question.
100 GB, 3 copies, 1 version?
300 GB total, 100 GB offsite.
1 TB, 3 copies, 1 version?
3,000 GB (3 TB).
256 GB laptop, 3 copies, 2 versions?
1,536 GB total.
Questions
What is 3-2-1?
3 copies, 2 media types, 1 offsite.
Default total?
6,000 GB (6 TB) with 2,000 GB offsite.
GB or GiB?
Decimal here; 6,000 GB is 5.59 TiB for drive labels.
RAID instead?
RAID guards drives, not deletions or fire; pair it with offsite copies.
How often?
Automated and regular per CISA, with tested restores.
Versions needed?
Enough to outrun slow noticed ransomware; 4 is a common start.