Reference

Backup space

Add up the storage your devices use against the free amount in your iCloud, Google or OneDrive account, and find the smallest Time Machine drive Apple suggests for a Mac.

Device 1: storage used, GB (from its storage screen) Device 2: storage used, GB Device 3: storage used, GB Device 4: storage used, GB Already in the same account, GB (for Google, Gmail and Drive count too) Free amount in the account Other free amount, GB (only if you chose Other)

Time Machine drive for a Mac

Apple suggests a Time Machine drive at least twice the storage capacity of the Mac (Apple, Back up your Mac with Time Machine). Type the Mac's storage capacity; the Mac's About This Mac window names the model.

The Mac's storage capacity, GB

Assumptions behind the numbers

The total adds the figures you read off your own devices' storage screens. A phone backup is usually smaller than its storage used, because apps and data already synced to the cloud are not copied again (Apple and Google list what their backups include), so the total is an upper bound.
Free amounts as the companies state them on 2026-10-19: iCloud 5 GB, shared with photos and mail (Apple); Google 15 GB, shared across Gmail, Drive and Photos (Google); OneDrive 5 GB (Microsoft).
The calculator observes the difference. It does not know what any company charges and is not a recommendation to buy storage.
Time Machine: Apple suggests a drive at least twice the storage capacity of the Mac (Apple, Back up your Mac with Time Machine).