AI is proving to be the biggest threat to personal computing this century. Among other unintended effects, the insatiable appetite of its data centres has driven up storage prices to the point where the cost of keeping good backups is becoming prohibitive. Last year I started moving up from 2 to 4 TB external SSDs, but this year their cost has already risen from £339 to £539 when they’re available, and an OWC Envoy Ultra 4 TB SSD is currently a jaw-dropping £1,129.

Yet for Time Machine backups, Apple still recommends you should “use a storage device with at least twice the storage capacity of your Mac. For example, if your Mac has 1 TB of storage, your backup disk should ideally have at least 2 TB of storage.” So do you really need to spend so much to back up the 2 TB internal SSD of your Mac mini?

In the absence of anything better, the rule of thumb that backup storage should be twice the size of the backup source might appear to err on the safe side. However, there’s no direct relationship between the size of a Mac’s internal SSD and that required for its backups, and there are circumstances when that rule could prove woefully inadequate. If you’re already backing that Mac up, you can use the information from that to arrive at a better estimate.

Basic formula

At its simplest, you should estimate the size of the initial backup, and the average size of new backups each month for the period you intend using that storage. Then work out
initial backup size + (average monthly backups × number of months taking new backups)

Let’s say my Mac has a 2 TB internal SSD, with around 800 GB to be backed up, and each month 40 GB will be added to that. If I want to use that as my backup storage for 2 years, then it will need a capacity of
800 + (24 x 40) = 1760 GB
so a 2 TB drive should do fine for slightly longer.

In practice, Time Machine and other backup utilities are more frugal with the space they require, as they automatically ‘thin’ or remove some older backups. Time Machine’s policy is to keep:

• hourly backups for the previous 24 hours, in addition to hourly local snapshots on each volume being backed up,
• daily backups over the last month,
• weekly backups since the start of that backup series.

Initial backup size

When backing up data-only volumes, like media libraries, almost everything on them is likely to be backed up initially. Significant exceptions include hidden folders containing Spotlight indexes and version databases; the former can be substantial, and you may be able to measure that by showing hidden files in the Finder, selecting the .Spotlight-V100 folder, and inspecting its size in the Get Info dialog. The latter can only be measured in Terminal, and can normally be ignored. Another hidden consumer of free space are snapshots, whose size can be estimated in Disk Utility.

Estimating backup size from a boot volume group, such as that containing your Home folder, is harder. Time Machine and other backup utilities will only back up the Data volume in any case, and it usually holds many files and folders that aren’t backed up, in addition to its Spotlight indexes and version database. You may be able to gain an estimate from the Storage item in General settings, although that tends to lump a great deal into its System Data, some of which will be backed up, and some won’t.

If you use iCloud Drive much, particularly if you keep your Desktop and Documents folders in iCloud Drive and Optimise Mac Storage is enabled so that many files are evicted from local storage, you’ll need to consider how that might change if many of those items are downloaded, and can then be backed up.

Another consideration is whether your backups will preserve sparse files at their native size. They should remain in sparse format when backed up to local storage, for example by Time Machine or Carbon Copy Cloner, but when backed up using SMB over a network, they will be expanded to and stored at their full size. For some virtual machines, that can double the space they take on disk.

Incremental backup rate

If you’re already backing up a volume, and it’s likely to see similar use in the future, use records of recent backups to estimate their daily or weekly total size. For Time Machine, you can do that using my free utility T2M2 (The Time Machine Mechanic). This provides a statement of current free space on each backup storage volume, which you can track over a week or more.

Full backup storage

Another decision to make is what you’re going to do when that storage is full. If you’re using Time Machine, you can’t copy your backup volume to another disk. The only options are to reformat the volume for reuse, or to retain it as an archive. If you’re going to reuse that disk, consider whether it will still be large enough for use when it comes to the end of its backup series.

Summary

• Estimate initial backup size from the total used by all volumes to be backed up.
• Your boot volume group won’t require as much backup space as it’s using, and Storage settings may help you guess how much to reduce that.
• Estimate the rate of growth using current backups, where available, from reports in T2M2.
• Decide how long you want to back up to that storage, thus how much your backups will grow over time.
• Then Total space required = initial backup size + (monthly increment × number of months in use).

I’m currently looking at a utility to make this process both easier and more accurate.