How much space does Time Machine need?

Short answer: plan on a backup disk two to three times the size of the data on your Mac. A Mac using 400 GB is comfortable on a 1 to 2 TB drive.

The extra room holds history: the hourly, daily and weekly copies that let you get back a file from last month.

A quick size table

Space used on the Mac	Smallest that works	Drive to buy
Up to 250 GB	500 GB	1 TB
250 to 500 GB	1 TB	2 TB
500 GB to 1 TB	2 TB	4 TB
1 to 2 TB	4 TB	6 to 8 TB

To see how much your Mac is using, open Apple menu > System Settings > General > Storage. The number at the top is the one to use.

Why it needs room for more than one copy

Time Machine keeps hourly backups for the past 24 hours, daily backups for the past month and weekly backups for as long as there is room. Each one stores only what changed, but the changes add up, especially with photos, video and virtual machines.

When the disk is full, Time Machine deletes the oldest backups first. A drive that only just fits your Mac still works, but it may only reach back a few weeks.

How to make a smaller drive last longer

Exclude what you can download again. In System Settings, General, Time Machine, Options, add folders such as downloaded video, game libraries and virtual machines.
Keep big changing files out. A virtual machine image that changes daily gets copied again every day.
Back up just your files. A tool that writes to an ordinary folder, such as Copy Shelter, needs roughly what your files take, and deduplication keeps repeated files from being stored twice.

If Time Machine already refuses your disk, the guide on Time Machine saying there is not enough space covers that error and the network disk limit.

Common questions

Is a 1 TB drive enough for Time Machine?

For a Mac using up to about 350 GB, yes. If more than about 500 GB is in use, a 1 TB drive will fill quickly and hold only a few weeks of history, so 2 TB is the better buy.

Does Time Machine need the same size drive as my Mac?

It needs room for what is in use plus history, so size it by the data on the Mac, then double or triple that. A drive the same size as your internal disk works but leaves little history once the Mac fills up.

What happens when the Time Machine disk is full?

Time Machine deletes the oldest backups to make room for new ones. Your newest backup is kept, and what you lose is how far back you can go.

Can I use a drive I already have for other files too?

Yes, by adding a separate APFS volume for Time Machine on that drive in Disk Utility. You can give that volume a limit so Time Machine does not take over the whole drive.