Size the backup space a phone needs, including photos, apps, messages and several retained snapshots.
A phone backup is not one copy of the phone. The first snapshot holds the full set — camera roll, app and system data, messages and attachments — and every retained snapshot after that stores only what changed, which for a phone is typically 5-10% between backups. Three retained snapshots of a 67 GB phone therefore need about 75 GB rather than 201 GB. A 20% headroom allowance is added because a backup destination that fills is a backup that silently stops running.
Phone Backup Storage
Total = full snapshot + (snapshots retained - 1) x full snapshot x change rate
Sizing
Recommended plan = total x 1.2 to leave headroom
The three data categories are summed into one full snapshot, then each additional retained snapshot adds only the changed share of that figure. Twenty percent headroom is applied so the destination does not run to the limit.
Cloud backup plans are sold in fixed tiers and a phone that outgrows its tier stops backing up, usually quietly. Sizing for retained snapshots rather than for a single copy is what keeps restore points available when you actually need one.
5-10% between daily backups is typical for a phone in normal use. Heavy video capture or a large app update pushes individual snapshots far higher, so use the upper end if you shoot a lot of 4K footage.