Backup drive size calculator
Work out how much space your files, future growth, and backup history could need.
A rounded-up planning capacity for one backup destination.
Your figures stay in this browser. The optional share link stores the numbers in a URL fragment. This estimate covers one backup destination—not a complete backup strategy.
Room for the files you haven’t made yet.
A backup drive needs space for more than today’s files. This planner compounds annual growth, adds your chosen version-history allowance, and reserves free space before rounding up.
Required capacity = projected files × (1 + history allowance) ÷ (1 − free-space share)
Example: 800 GB of files, no growth or version history, and 20% free space needs 1,000 GB. The free space is 20% of the destination, so dividing by 0.8 is different from adding 20% to your files.
A planning estimate, with limits.
We round up using a capacity ladder: 500 GB, 1, 2, 4, 8, 12, 16, 20, 24, 32, 48 and 64 TB, then whole TB above that. These are planning sizes, not a specific product recommendation. Formatting, encryption, backup software and retention settings use space too.
GB and TB here are decimal: 1 TB = 1,000 GB. Version allowance is a configurable estimate, not a prediction of how much your backup application will retain.
Read the backup sizing guide, then plan where your other copies will live.
A few useful answers
Why not just double my data size?
That can be a useful rough starting point, but it hides the assumptions. A growing photo library and a mostly unchanged document archive need different allowances.
Is one backup drive enough?
A drive is one destination. It does not protect against every failure. Keep independent copies and consider an off-site or offline copy; test that you can restore a file.
Does compression reduce the estimate?
This planner does not assume compression savings. Already compressed photos and videos may shrink very little. Use measurements from your own backup application if you have them.