Q I want an external drive to support Time Machine backups of my iMac, which currently has 805 GB free space on its 1 TB internal drive. How big should this backup disk be?

A Unfortunately there is no simple formula for working this out, as what is important is not how much is stored on your current disk, but how often and how much it changes.

You currently have around 200 GB to form the initial, complete backup. However if every week half of that storage space changes, then each week Time Machine will have to store an additional 100 GB. Over a few months, it could easily use a total of 1000 GB to contain the full backup and all its incremental changes.

Of course Time Machine will also age out some of the older changes in time, so would not be quite that greedy.

One rough and ready rule is to ensure that your backup drive can contain 3 complete backups – in your case, 600 GB. You might therefore be able to get away with a 500 GB backup drive, although 1 TB should last you as long as the drive works, and is little more expensive.

Comments Every user’s experience will be slightly different.

My current Time Machine backup is on a 2 TB external RAID, which now has around 1.05 TB of backups, representing nearly 3 years. The iMac feeding those backups started with around 800 GB of its 2 TB hard disk used, which has grown steadily to just under 1 TB.

In my case, the rule of thumb would have required my backup drive to have started at 2.5 TB and to have grown to 3 TB, but 2 TB has remained comfortable. Perhaps an alternative rule of thumb is for the backup drive to be no smaller than the drive which it is backing up, provided that the latter is no more than around half full?

There could be a PhD in this for someone…

Updated from the original, which was first published in MacUser volume 30 issue 05, 2014.