How much backup storage do you actually need?

Count the files you would miss

Check the combined size of your documents, photographs, projects and other personal files. Avoid double-counting a folder inside another folder. Include files on external working drives. A nearly empty 2 TB computer may have less to protect than a 512 GB computer with several attached drives.

Give special attention to things that are inconvenient to recreate: an assignment, a family photo library, a home video edit, or the only export of a project. Re-downloadable installers and disposable caches can often be excluded if you are comfortable replacing them. Decide what belongs in the backup before deciding what to buy.

Allow for growth over a sensible period

Choose a planning period, perhaps two or three years. A growth estimate is an assumption, not a prediction. If your folders grew from 300 GB to 360 GB over a comparable year, 20% is a useful scenario to test. Starting a video course could change that pattern completely.

With compound growth, 400 GB growing by 20% each year becomes 480 GB after one year, 576 GB after two and 691.2 GB after three. Real additions need not follow this neat pattern. Recalculate after a large project instead of treating the original estimate as a permanent allowance.

Separate older versions from spare room

Keeping yesterday’s version of a document needs more space than keeping one current copy. The extra amount depends on what changes, the software’s storage method and how long old versions remain. Editing small documents and repeatedly replacing large videos can produce very different backup sizes.

For planning, add a version-history allowance, then leave unused capacity. An illustrative 50% history allowance means multiplying projected files by 1.5. Keeping 20% of the destination free means dividing that result by 0.8. These are adjustable planning choices, not guarantees about a particular backup app.

A complete 400 GB example

Suppose you have 400 GB of important files, expect 20% annual growth for three years, allow 50% extra for history, and want 20% of the destination free. The target is about 1,296 GB, or 1.296 TB using decimal units. A 1 TB drive is below that target; a 2 TB drive is a common capacity above it.

Compare a few assumptions rather than relying on one answer. Lower growth and shorter retention may change the result. Do not shorten retention simply to force a preferred drive size to fit: choose the recovery history you need, then find a suitable destination.

Size each backup destination separately

Two backups should remain two recoverable copies, not two halves of one copy. If you plan a local drive and remote backup, each must cover the files assigned to it. Dividing one drive into two folders does not protect against that drive failing.

The Canadian Centre for Cyber Security recommends an original plus two backups, across two types of media, with one copy off site. Your storage budget should reflect the whole arrangement. Another recovery location may be more useful than making the first destination much larger than necessary.

Check before buying, then check after a month

Drive makers generally label capacities in decimal GB and TB. Some software displays binary units, making 1 TB appear as about 931 GiB. Compare matching units; a smaller displayed number is not, by itself, evidence of a defective drive.

After the first backup, check the space actually used and restore a sample. Look again after a month of normal changes. That gives you a more useful history estimate than a generic multiplier and makes the next capacity decision easier.

• Check the connection and file-system compatibility.
• Confirm the software’s supported destinations and retention settings.
• Keep encryption credentials recoverable outside the device being protected.
• Review capacity after major changes to your files.