How big should my Time Machine backup drive be?

My 13" MacBook Pro has 500GB internal hard drive space, and I bought a 1TB external HDD to use as my Time Machine backup. I only read from many articles that the backup should be about 2 to 3 times the size of my hard drive, but I feel like 1TB will never be filled up, and in fact, leave a lot of free space.

I understand that Time Machine takes a snapshot of my whole system and stores it in the backup drive. But say, for example, only about 120GB of my internal HDD is in use. Then it'll only take a snapshot that's about 120GB in size, and while that same snapshot will grow over time as I take more recent snapshots of my system, it will never go beyond 500GB because that's the maximum size of my internal HDD. Or do I have it wrong? How does Time Machine work, really?

I'm asking this because I want to know if I should use a separate external HDD for storing my actual files.

You probably want to check out this website for the most information, but here is the answer to your question, quoted from the website above:

1. How big a drive do I need for Time Machine?

A general "rule of thumb" is, to keep a reasonable "depth" of backups, Time Machine needs 2 to 4 times as much space as the data it's backing-up (not necessarily the entire size of your internal HD). Be sure to add the size of the data on any other drives/partitions you want to back up.

But this varies greatly, depending on how you use your Mac. If you frequently add/update lots of large files, then even 5 times may not be enough. If you're a light user, you might be able to get 1.5 times to work, but that's subject to problems any time a large backup is needed.

And, of course, the larger the drive, the more old backups Time Machine can keep for you. A drive that's too small may only have room for a few weeks (or even days) of backups.

Unfortunately, it's rather hard to predict, and most of us have a tendency to add more and more data to our systems over time, so if in doubt, get a bigger one than you think you need now.

Also, there are some OSX features and 3rd-party applications that take up large amounts of backup space, for various reasons. See question 9 for details.

This is a trade-off between space and how long Time Machine can keep its backups, since it will, by design, eventually use all the space available. But it won't just quit backing-up when it runs out: it starts deleting the oldest backups so it can keep making new ones. Thus, the more space it has, the longer it can keep your backups.

If your backup disk is on the small side, and Time Machine needs to do a very large backup, either because you've added or changed a lot or done something like an OSX update since the previous backup, you may get one of the messages in Troubleshooting item #C4 (which one depends on exactly what happened, and which version of OSX you're on.)

My Answer

Then it'll only take a snapshot that's about 120GB in size, and while that same snapshot will grow over time as I take more recent snapshots of my system, it will never go beyond 500GB because that's the maximum size of my internal HDD.

No, not quite. The 1 TB hard drive will be filled up because Time Machine keeps your backups and deletes them once your hard drive is filled. There is more than one backup stored on that hard drive. As stuffe pointed out as well, Time Machine allows you to restore to a previous backup, since Time Machine may keep 7 or 8 backups on that 1 TB hard drive.

I'm asking this because I want to know if I should use a separate external HDD for storing my actual files.

Yes, you should. You should be dedicating a drive to Time Machine. Here is the quoted answer from 3 in the website I linked above:

3. Can I use my Time Machine disk for other stuff?

Yes. Time Machine will not delete anything you put there. But it's not a good idea to put anything else important on the same physical drive, unless you back it up elsewhere. When (not if) that drive fails, you risk losing it.

If you want to do this anyway, it's much, much better to partition an external drive into 2 (or more) parts, also called volumes. Assign one to Time Machine, for its exclusive use for backups; use the other partition(s) however you want. To use a new drive, or one you don't mind erasing, see question #5. To add a partition to an existing drive that already has data on it, see question #6.

The size of your drive doesn't really matter, it's the size of your data. If you are really only using 120Gb, then potentially an external drive of 240-360Gb could well be enough (until such time as you start to use more, obviously...)

As for how TM works, it doesn't just snapshot the current state of your data, but it's historical changes. This is why it can use more total space than the actual amount of used space on your drive. Let's say that you have your 120Gb current system, and some months down the line you install OS X Mavericks when it's released - this may up your used space to (all figures are estimates!) 130Gb, but it's not just 10Gb worth of extra files, it might be 35Gb of changed files, and 5Gb of new files, resulting in a 40Gb change for Time Machine. This way, when you restore, it's not just a case of being able to restore to the current most recent backup on your TM, but being able to go back in time to any point that it still covers and restore to that point, which may be as far back as prior to OS upgrades etc.

Now, for more everyday changes, here's a good example of how you can consume a good amount of space in your backup drive without changing anything. TM backs up at a file level. If a file changes, the new version is backed up in full. Let's say you have a couple of films of a Gb each, but they are named incorrectly (either at a filename or META data level) - changing the title of the films doesn't really affect the space used on your machine, but it will result in both files being backed up again because they have changed.