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.” 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. This provides a statement of current free space on each backup storage volume, which you can track over a week or more. Then Total space required = initial backup size + (monthly increment × number of months in use).