Plan photo backup space from your own library—not a photo-count estimate

Measure the files you intend to preserve

“Ten thousand photos” is not a capacity measurement. The space needed by ten thousand small screenshots is different from that of a mixed library containing RAW photographs and long videos. Start with the byte size of the files you actually plan to copy.

This planner budgets a new, full copy on one destination. It is not a subscription recommendation or a prediction of phone storage. If a destination already contains part of the same archive, do not blindly add the full library again: first establish what exists and what your backup software will retain. Do not delete that existing copy to make the estimate fit.

Use exported-file sizes where possible. Optimized phone storage or a compressed cloud collection may not represent the size of the originals you want to keep. Export a manageable sample, check its format, then measure the actual collection when feasible.

Make the assumptions visible

Reserve is a chosen planning allowance, not a manufacturer requirement. In this model it is a fraction of the total allocated space budget. A 20% reserve therefore means dividing by 0.8, not simply adding 20% to the data amount.

“Extra retained versions” means additional old or changed files kept alongside the current archive. Enter zero if you are planning only one full version, but recognize that doing so does not budget version history. Software that retains multiple full snapshots could need much more space; this simple model does not simulate deduplication or compression.

Worked case: 120 GB today, a year of growth

Hypothetical inputs, not a measured user library: 120 GB current archive, 4 GB of net additions per month, 12 months, 30 GB of extra retained versions, and a 20% reserve.

1. Growth: 4 × 12 = 48 GB.
2. Future archive plus versions: 120 + 48 + 30 = 198 GB.
3. Total budget: 198 ÷ 0.8 = 247.5 GB.
4. If the intended destination reports 200 GB of free space, the gap is 47.5 GB.

The answer is not “buy the next advertised size.” Compare with usable free space after formatting and other stored files. If you intend two independent destinations, each needs its own suitable budget; two copies competing for the same free space do not create two independent devices.

Run these numbers in the backup space planner. It also doubles only your growth assumption for a sensitivity check: 120 + 8 × 12 + 30 = 246 GB of data, requiring 307.5 GB with the same reserve. The 60 GB difference shows why a small change in monthly capture habits can matter more than rounding a current reading.

Keep GB and GiB separate

This tool uses decimal GB: 1 GB = 1,000,000,000 bytes. A GiB is 1,073,741,824 bytes. Consequently, 247.5 GB is about 230.5 GiB. These are two unit labels for the same number of bytes, not evidence that files disappeared.

If a file manager reports GiB, use the unit-conversion field before entering capacity numbers. If its label is ambiguous, prefer an exact byte count or the operating system’s documentation. Mixing a decimal budget with a binary free-space reading can produce a false shortage or surplus.

Replace the estimate with observations

Write down the collection size and the date at two comparable points. Net additions divided by elapsed months give a starting growth estimate. A month with an overseas trip or a video project may not represent the rest of the year; test a higher-growth scenario separately.

Revisit the plan after changing camera quality, adding RAW files, keeping edited copies or changing retention settings. A plan can be arithmetically correct and operationally wrong if those assumptions change. Space is only one part of a backup: finish by checking copy integrity and whether you can actually retrieve the files.