3-2-1 Backup Storage Calculator
Work out the storage a 3-2-1 (or 3-2-1-1-0) backup plan needs: copies, version history or a daily/weekly/monthly retention schedule, compression and dedup, data growth, drive sizes, cloud cost and upload time.
How it works
Grown data = data × (1 + growth%)^years · Per copy (simple) = grown × (1 + versions%) × (1 − saving%) · Per copy (GFS) = fulls × full + daily points × daily change · Per copy (incremental forever) = full + restore points × daily change · Per copy (all fulls) = restore points × full · Backup storage = per copy × backup copies · Drive ≥ per copy ÷ fill limit
The 3-2-1 rule keeps three copies of your data on two different media types with one copy offsite, which survives drive failure, device loss, theft and site disasters alike. The original counts as copy one, so you provision storage for two backup copies — usually one local drive for fast restores and one cloud or offsite copy for disasters. The stricter 3-2-1-1-0 variant adds an offline or immutable copy against ransomware, and you can also set any number of copies. Version history can be a simple percentage, or a real retention schedule: how many daily, weekly, monthly and yearly restore points you keep, the daily change rate and the backup type — incremental with periodic fulls (GFS), incremental forever and snapshots, or a full every time. Compression and deduplication savings, a fill limit for drives, drive and cloud prices and your upload speed live under More options, so the result includes the drive size to buy, the cloud bill over your planning window, how long the first upload takes and a year-by-year table as your data grows.
Worked example
500 GB growing 20% a year reaches 500 × 1.2³ = 864 GB after three years. With 30% for version history each backup copy is 864 × 1.3 ≈ 1,123 GB, so two copies need about 2.25 TB — keeping the drive no more than 80% full means 1.12 ÷ 0.8 = 1.40 TB, so a 2 TB drive for the local copy, while the cloud copy costs about $3.90 a month today at $6 per TB (650 GB). Switching to a retention schedule of 14 daily, 4 weekly and 12 monthly points with 3% daily change and weekly/monthly fulls, each copy holds 16 fulls plus 14 incrementals: 16 × 864 + 14 × 25.9 ≈ 14.2 TB. Incremental-forever backup keeps one full plus about one day of changes per restore point, 864 + 30 × 25.9 ≈ 1.64 TB, which is why deduplicating and snapshot tools are so popular.
Frequently asked questions
What exactly is the 3-2-1 rule?
Keep 3 copies of your data (the original plus two backups), stored on 2 different media types (for example an internal drive and an external drive or cloud), with 1 copy kept offsite. It is the long-standing baseline that protects against the widest range of failures with the least complexity.
What is 3-2-1-1-0?
An extension for the ransomware era: on top of 3-2-1, keep one copy offline, air-gapped or immutable (a rotated USB drive, tape, or a cloud bucket with object lock), and aim for zero errors by testing restores. In this calculator it means three backup copies — two on local media and one in the cloud.
Why budget for version history?
A backup that only mirrors the latest state cannot save you from a mistake or ransomware you discover days later — the bad version has already overwritten the good one. Retaining older versions lets you roll back, but they take space. Use the simple percentage for a quick estimate, or enter your actual daily, weekly and monthly retention for a detailed one.
How much does the backup type matter?
A lot. Keeping a full copy for each weekly and monthly restore point (classic GFS rotation) can need 10–15 times your data, while incremental-forever or snapshot tools that store one full plus changed blocks often need only 1.2–2 times. Try both under Retention schedule to compare.
Does the cloud copy count as one of the three?
Yes. A reputable cloud backup satisfies both the "different media" and "offsite" parts of the rule in one step, which is why the common modern setup is: live data, a local external drive, and a cloud copy — three copies, two media, one offsite.
Why does my new drive show less space than advertised?
Drive makers use decimal units (1 TB = 1,000,000,000,000 bytes) while Windows and many tools report binary units (1 TiB = 1,099,511,627,776 bytes), so a 4 TB drive appears as about 3.64 TB. Keeping drives no more than about 80% full also leaves room for new versions.
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This calculator is for educational purposes. Double-check important results before acting on them.