Which Allocation Unit Size Is Best for Your Drive?

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If you’ve ever formatted a hard drive or USB thumb drive, you might have seen the “Allocation Unit Size” setting. This is set by default, but you can change it if you want. Should you? If so, what should you set it to? What does allocation unit size even mean?

There are multiple answers to the above questions, both simple and not-so-simple. The two-word answer for any of them could be “it depends.”

What Does Allocation Unit Size Mean?

Depending on who you’re talking to, allocation unit size may also be referred to as “cluster size.” Either way, it’s pretty simple. This is the smallest possible chunk of data on your drive.

Best Allocation Unit Size Choosing Size

Basically, even a completely empty file will be the size of your allocation unit size. Every time a file grows, no matter how big or small that file may be, it will be by at least this much.

What Allocation Unit Size Should You Use?

The optimal allocation unit size for your drive will often depend on which operating system you’re using and how big the drive is. As an example, Microsoft has a list of the default sizes for various Windows versions available on its website.

Generally, using whatever is the default for your system is the best choice, but this isn’t always the case. If, for example, you have tons of smaller files, a larger allocation unit size will eat up your drive space slightly faster. For everyday computing, this isn’t likely.

Using larger than necessary allocation unit sizes can create unnecessary fragmentation on your drive. This is more of an issue for hard drives since solid state drives (SSDs) aren’t as prone to performance issues as a result of fragmentation.

Best Allocation Unit Size Fragmentation

In most cases, Microsoft recommends an allocation unit size of 4 KB. This is what the company says works best for “standard users.” If you need a non-default size, you probably already know that and why you need it.

Should You Use Different Sizes for SSD or Hard Drives?

As mentioned above, fragmentation doesn’t present the same problems on an SSD as it does a hard disk. Because of this, you could theoretically use larger allocation unit sizes without the performance hit. Would this actually speed things up?

The answer is likely no. So far there haven’t been any real-world examples of larger allocation unit sizes leading to any performance changes on SSDs. Larger unit sizes can lead to more writes over time, which would lead to more wear and tear on your SSD.

Best Allocation Unit Size Format Drive

Some of the same examples that we’ve already seen with standard hard drives apply here as well. Games and other applications that frequently read and write very small files (less than 4 KB) could benefit from a smaller cluster size. Even so, you’re unlikely to see any measurable difference in performance.


As you’ve probably noticed, you’re generally safe sticking with the default allocation unit size. Unless you’re preparing a computer for a very specific use case, it’s not even something you should spend much time thinking about. There are always exceptions, but that’s generally the rule.

If you’re worried about wearing out your SSD early, this isn’t going to have much to do with it. Instead, take a look at our list of things you shouldn’t do with solid state drives.

Kris Wouk Kris Wouk

Kris Wouk is a writer, musician, and whatever it's called when someone makes videos for the web.