Allocation Unit Size – Which is best for Your Drive?

If you’ve ever formatted a hard drive or USB thumb drive, you might have noticed the Allocation Unit Size setting. This is set by default, but you can change it if you desire. 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 not-so-simple and straightforward. The two-word response for any of them could be it depends.

Allocation Unit Size Exfat
Allocation Unit Size Exfat

What Does Allocation Unit Size Mean?

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

Even a blank file will be the size of your allocation unit size. Every time a file increases, 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 usually depend on which operating system you’re working and how big the drive is. For example, Microsoft has a list of the default sizes for different Windows versions available on its website.

Generally, utilizing whatever is the default for your system is the best choice, but this isn’t always the case. If, for instance, 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.

They are using more extensive than required 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 due to fragmentation.

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

Should You Work Different Sizes for SSD or Hard Drives?

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

The answer is likely no. So far, there haven’t been any real-world cases 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 extra wear and tear on your SSD.

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

Allocation Unit Size Fat32
Allocation Unit Size Fat32

How Is the Default Allocation Units Size Determined?

Which allocation unit size is best depends on several factors, from the total size of the drive to the specific operating system that you’re using. The allocation unit size has to be taken so that there’s a good balance between drive performance and efficient space use.

However, the kinds of files that are stored on a system drive may be very different from those on a drive that will only be used to store media files, as one example.

Then we have the problem of solid-state drives, which don’t suffer from performance loss when file fragmentation happens. Fragmentation is also in element a function of allocation size. So the default allocation size that you’re given when formatting a drive is a general-purpose size that should run for most people, most of the time.

Allocation Unit Size Usb
Allocation Unit Size Usb

What Impact Does Allocation Unit Size Have on Performance?

Allocation unit size does affect drive performance—especially mechanical hard drives. Essentially, the bigger you make the allocation unit size, the fewer the total number of allocation units. This makes sense because your plots of drive real estate are bigger. So when your computer has to look up your data’s physical location, the address book is much thinner.

This reduces the “seek time” of the drive. How long it takes to look up the file’s location in the file allocation table and then to access the correct allocation units. Again, on mechanical drives, this is a significant issue because it physically has to move the read/write heads of the hard drive to the location of the allocation unit you want it to access.

Having a small allocation size can also lead to extreme fragmentation. That’s because any files that are larger than the allocation unit will be written to several units. The problem here is that open units can be scattered across the drive as files are written and deleted over time.


Q: What is the most suitable allocation unit size for exFAT?
A: The simple answer is to reformat in exFAT with an allocation unit size of 128k or less. Then everything fits since not so much wasted space in each file.
Q: Does allocation unit size affect speed?
A: If, for instance, 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. You are utilizing more extensive than necessary allocation unit sizes can create unnecessary fragmentation on your drive.
Q: Does allocation unit size matter?
A: If you are a Standard User by Microsoft’s definition, you should have the default 4096 bytes. The allocation unit size is the block size on your system hard drive when it formats NTFS. If you have lots of tiny files, then it’s a good idea to keep the allocation size small, so your hard drive space won’t be wasted.
Q: What is the fittest allocation unit size for FAT32?
A: According to Hitachi, the suggested cluster size for a 16GB FAT32 partition is 4KB, the recommended cluster size for a 64GB FAT32 partition is 16KB, the recommended cluster size for a 128GB FAT32 partition is 32KB, and the suggested cluster size for a 256GB partition is 64KB.


As you’ve apparently noticed, you’re generally safe sticking with the default allocation unit size. Unless you’re preparing a computer for a particular 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 regarding wearing out your SSD early, this isn’t going to have much to do with it. Instead, please take a look at our list of things you shouldn’t do with solid-state drives.

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