How Big Should a Time Machine Drive Be?

The most common Time Machine mistake isn't a bad drive — it's a drive that matches the Mac's disk exactly, fills up within months, and starts shedding history. Here's the sizing rule that actually works, why extra space buys you more than safety, and how to share one drive between two Macs.

The rule: 2–3× the data you're backing up

Size the drive against the data on your Mac, not against the size of the internal disk. Check used space in System Settings → General → Storage, subtract anything you plan to exclude, then multiply:

• 2× is the practical floor — enough room for months of version history plus headroom for big reorganisations, when Time Machine briefly keeps both the old and new copies of everything you moved.
• 3× is comfortable — a year or more of history for most people, with no babysitting.
• 1× or less technically works, but the first full backup nearly fills it and history stays shallow forever.

Concrete examples: a MacBook Air with 180 GB used is well served by a 500 GB drive. A 1 TB MacBook Pro with 700 GB used wants 2 TB. Storage prices being what they are, rounding up is rarely the choice you regret.

Drive speed matters less than you'd think. Time Machine works in the background, and after the first pass it only copies what changed — so a cheap spinning hard drive is perfectly fine here. This is the one job where the slow drive in your drawer earns its keep. Just budget a long session for the first backup: hours, not minutes, so start it overnight.

Extra space buys history, not just safety

Time Machine keeps hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for as long as the drive has room. That last clause is the whole game: every spare gigabyte extends how far back "as long as there's room" reaches.

When the drive fills, Time Machine deletes the oldest weekly backups to make room — silently and automatically. Your newest backup is never at risk, but the version of a project from last spring quietly ages out. A drive twice as big roughly doubles how far back you can reach on the day you discover a file was mangled months ago.

There's no setting to cap how much space Time Machine uses; it will happily grow into everything you give it, and that's by design. If the same drive must also hold regular files, give backups their own volume so the two don't fight over the same free space.

One drive backing up two Macs

Modern macOS formats Time Machine drives as APFS, which makes sharing a drive genuinely pleasant: give each Mac its own APFS volume instead of carving fixed partitions.

1. Connect the drive and open Disk Utility on the first Mac.
2. Select the drive's existing APFS volume and click the + button in the toolbar (or Edit → Add APFS Volume). Name the new volume after the second Mac.
3. APFS volumes share the container's free space automatically, so there's no split to guess in advance.
4. On each Mac, pick its own volume in System Settings → General → Time Machine.

Size a shared drive at 2–3× the combined data of both Macs. Because the volumes share space, a Mac that grows faster can borrow room the other isn't using — exactly what rigid partitions can't do.

Slow the growth while you're at it

Backups balloon fastest when caches, virtual machines, and build folders ride along. Excluding them can cut backup size dramatically — see excluding folders from Time Machine.