Measure the starting collection
Find the size of the folders and disks that will actually be protected. Do not automatically count the computer's full advertised capacity if only part is included. Conversely, additional data may live on external drives or inside specialist applications. Check whether cloud material must be locally available before it can be backed up.
Distinguish a collection of small documents from newly created large files. Someone adding video material or substantial project archives each day needs more growth allowance than someone editing mostly unchanged text. Record a realistic monthly growth estimate and review it after several backup runs.
A simple example helps: if 300 GB needs protection today and roughly 20 GB is added monthly, the base collection alone will be substantially larger after a year. Retained versions of changed files are additional. Treat these figures as planning assumptions and replace them with measured usage after initial runs. Usable capacity after setup may also differ from the advertised figure. Check actual available space in the system before deciding that the margin is adequate.
Allow for versions and the storage method
Some applications save only changed portions, while others create complete replacement copies. Compression often saves little on already compressed images or video. Do not assume a promised reduction until the actual method has been tested with your files. Temporary space needed while a backup runs may also matter.
For Time Machine, Apple recommends a backup device with at least twice the Mac's storage capacity. That recommendation belongs to that particular method; it is not a universal law for every file backup. Check your software's retention policy and response to limited space: does it remove older versions, stop the task or ask for a decision?
Combine spare capacity with monitoring
Choose a destination with an understandable margin and check actual usage after the first complete run. Check again after a representative working week. This reveals whether changing large files consumes more space than expected. Assign responsibility for responding to low-space warnings.
Do not manually remove pieces of a managed backup archive merely to make room. Use its retention controls and first establish which recovery dates would disappear. If a larger destination is needed, keep the previous drive until a new complete backup and restore test have both succeeded. The migration itself should not remove your only usable recovery copy.
