Backup Storage Calculator: How Much Space Your Backups Need

Backup storage runs out for the same reason almost every time: people size it to their live data, then keep weeks of daily restore points and a year of monthly copies on top. With a weekly full backup and daily incrementals, the retained full copies usually take up most of the space, and the daily changes add far less. Enter your data, how much of it changes each day and how long you keep restore points, and this calculator works out the space you need per backup copy and across every copy you keep.

How the calculation works

The tool assumes the common grandfather-father-son pattern: a full backup once a week, an incremental backup on the other six days, and some of the full backups held back as weekly, monthly and yearly restore points.

• Data size: your data grown by the annual rate over the planning period, compounded each year.
• Full copies: the number of weekly fulls you keep (or enough to cover your daily restore points, whichever is more), plus your monthly and yearly fulls. Each one is the size of your data.
• Incrementals: every daily restore point that is not a weekly full is an incremental (six in every seven), each the size of one day's changes.
• On disk: the total divided by your compression ratio, multiplied by the number of copies, then topped up with headroom.

This is a planning estimate, and it errs on the generous side for fulls. Software that builds synthetic fulls or uses block cloning on the target (Veeam on ReFS or XFS, for example) stores repeated fulls far more cheaply, and image backups of whole servers include system files your file count leaves out. Run a few weeks of real backups and check the repository's used space against this figure before you buy for five years.

Why the number matters for cyber insurance

Cyber insurers care less about how much storage you buy than whether a clean copy survives an attack. Ransomware attackers often look for backups and try to delete or encrypt them, so at least one of your copies should be offline or immutable, and it needs enough space to hold your full retention period. If the result above is more than your current target can hold, the usual fixes are to keep fewer monthly fulls on fast local storage and push the long-term copies to cheaper cloud or offline storage. Once you know the size, pair it with our RTO and RPO calculator to check how long a restore of that much data would take.