Calculator
Backup storage calculator
Enter the size of your data, the share that changes each day, the days of history to keep and the number of copies. The calculator estimates storage for one full backup plus daily changes, per copy and in total, and it ignores compression and deduplication.
The tool
Estimate: one full backup plus one daily incremental of the changed share, kept for the chosen number of days, multiplied by the number of copies. It ignores compression, deduplication, versioned file systems and periodic new full backups, so treat it as a starting size and add headroom. A 3-2-1 plan keeps three copies on two kinds of media, one of them offsite.
How to use it
- Enter the size of the data you want to back up.
- Enter the percentage of that data that changes each day.
- Set the number of days of history to keep.
- Set the number of copies, three for a 3-2-1 plan.
- Read the storage per copy and the total across all copies.
How the estimate is built
The estimate starts with one full backup, which holds the whole data set once. Each day adds an incremental backup of the data that changed, sized at the daily change percentage. Over the retention period, those daily changes add up, so one copy needs the full size plus the daily change multiplied by the days of history. For 500 GB of data with 2 percent changing each day and 30 days kept, one copy needs 500 GB plus 30 times 10 GB, or 800 GB. The daily change percentage is the share of the data that changes in one day.
The total multiplies the per-copy figure by the number of copies. A 3-2-1 plan has three copies, so the example above needs 2,400 GB across all three. The total is a planning figure, not a number any particular backup tool will report, and it grows with the change rate, the days kept and the copies made. Changing the number of copies changes the total in direct proportion, which makes it easy to compare a two-copy plan with a three-copy plan.
What the estimate leaves out
The estimate ignores compression and deduplication. Compression shrinks files that pack well, and deduplication stores repeated data once. Backup tools that use either feature may store less than the calculator shows, so the figure is a planning estimate rather than a promise. A cautious plan sizes storage from the estimate, then checks the result when the first backup runs. Compression depends on the data. Photos, video and archives tend to compress less well than plain text, so the saving varies from one set of files to another.
The 3-2-1 rule asks for three copies, two kinds of media and one copy off-site. The calculator sizes the copies, but it does not test them. Use the figures to choose drive and cloud sizes, then run a restore test on the software you pick, because a backup that has never been restored is an assumption. A restore test from the off-site copy is worth running as well, since that copy matters most after a fire or theft at home. A copy that stays connected to the computer can be lost in the same event, so keep at least one copy apart from it.
Rankings behind this tool
- Best backup software
Arq BackupTop pick91.5then Carbon Copy Cloner, Kopia - Best cloud storage services
Google DriveTop pick91.5then Dropbox, Proton Drive - Best file encryption software
VeraCryptTop pick92.5then Cryptomator, GnuPG - Best file archivers
7-ZipTop pick93.5then PeaZip, Keka - Best media server software
JellyfinTop pick94.0then Plex, Immich
Articles that go with this tool
All articles- Guides · 4 min readHow to choose a backup strategy: 3-2-1, restores and toolsBuild a backup plan with the 3-2-1 rule, local and cloud copies and restore tests. Compare backup software, from Arq and Kopia to restic and Backblaze.
- Guides · 4 min readHow to choose cloud storage for files and devicesChoose cloud storage by free space, platform support, encryption and self-hosting. Learn why sync is not backup, and compare Google Drive, Dropbox and Proton Drive.
- Explainers · 5 min readWhat end-to-end encryption means and what it does not coverEnd-to-end encryption means only the sender and recipient hold the keys. Learn how it differs from in-transit and at-rest encryption, and how to check claims.
Questions about the backup storage calculator
- How much storage does a backup need?
- A full backup needs about the size of the data. Each day of history adds the daily changes, so the total grows with the change rate and the number of days kept.
- What is a 3-2-1 backup plan?
- It keeps three copies of your data, on two kinds of media, with one copy off-site. The backup strategy guide explains how to apply the rule to your devices.
- Does the calculator account for compression?
- No. The estimate ignores compression and deduplication, so a backup tool that uses either may store less than the figure shown.
- What if I do not know the daily change percentage?
- Try a few values to see how much the total changes. The result is a planning estimate, so a range of figures is more useful than a single number.
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