How Big a Backup Disk Do I Need?

Bigger is better, but how much is enough?

It’s a common question. There are several things to take into consideration, including how much data you have to back up, how many backups you will keep and how long you will keep them, how you will use the external backup drive, and price.

It’s also something that changes over time for a variety of reasons.

Let’s look at what you should consider. I’ll also share my rule of thumb.

How big a backup disk

You don’t need a backup drive as big as all your hard drives combined. Focus instead on how much data is being used. As a rule of thumb, start with a backup drive about four times bigger than all your used data. Bigger is better if your budget allows.

Size versus size

In your question, you mention the size of your drives. That’s one good measure, but it’s often overkill. We rarely fill up our hard disks to capacity. More often, we end up upgrading to larger drives or changing what we store on them.

There’s a different number I’d consider instead: the space used on each drive so far.

My C: drive, shown above, is a 1TB disk1. I’m only using a little less than half of it: 438GB. That’s the number that matters. The unused space doesn’t need to be backed up.

If you’re going to rely on any single number, consider the amount of data currently stored on your hard drive.

Image versus image

Different backup programs use different techniques to store — and most importantly, compress — the images they take.

For example, through compression, Macrium Reflect creates an image file that takes up less space than the original data.

The size of the image file is interesting, but not the primary number I’d have you pay attention to.

Full versus incremental

If you take incremental backups instead of only full, your backups will take up even less space.

Note that this incremental backup image, taken the day after the preceding full-image backup, is dramatically smaller. It contains only things that changed that day.

With a monthly full-image/daily incremental backup schedule, a month’s worth of daily backups could take less than 600GB in space, as opposed to a month’s worth of daily full backups, which would take closer to 1.5TB.

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How many for how long?

How long do you expect to keep your backups?

In general, the rule is “until you’re certain you won’t need them”, but that’s a pretty ambiguous guideline. It’s not helpful.

My approach is to keep three months of daily backups (full and incremental, as described above). I discard the incremental backups older than that so I have another three months of monthly snapshots. I cherry-pick some of those to save (perhaps the quarterlies or just the January 1 backup) and discard anything else.

But I’m an anomaly. And I can tell you I’ve rarely needed a backup older than a few days or weeks.

Unless you have other requirements, I’d advise you to keep at least a month’s worth of daily backups, and ideally two or three. Let your backup program manage this for you.

Dedicated drive?

Will your drive be dedicated to only backups, or will it also serve other purposes?

Those “other purposes” could include backups of a different form, like Windows File History. Or it could be manual archives of your own. Or it could be something else entirely.

My preference, however, is that a backup drive be dedicated solely to the task.

How much?

Larger drives cost more. While prices have steadily declined over the years, so has our appetite for data usage.

Larger is generally better since it gives you more breathing room, but it comes at a cost. And while I strongly believe that backing up is worth it, that’s not practical for everyone.

My rule of thumb

So, with all those factors in mind, what to do?

My general rule of thumb is:

Get a drive at least four times as big as the amount of data you intend to back up.

Remember, that’s just a rule of thumb. Depending on your situation, you may want or need to do something different. But it’s at least a starting point if you’re not sure.

Note that I’m referring to the amount of data, not the size of the existing drive(s). If you’re backing up multiple drives, add up the amount of data used on each for this calculation.

With all the competing aspects of the size of the backups you might accumulate over time, I feel this represents a reasonable compromise.

Of course, there’s nothing wrong with going even larger if your budget allows for it.

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