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What is ‘Size’ vs ‘Size on Disk’ on Windows

Have you ever wondered why Windows shows both Size and Size on Disk in the file and folder properties window? While both appear the same, there is a slight difference between Size and Size on Disk. Usually, the difference is a few bytes or kilobytes, and the Size on Disk will always be higher than the actual size. In fact, if you take a careful look at bytes in the properties window, you can see the difference.

size vs size on disk

In this article, let’s learn the difference between Size and Size on Disk and why Windows shows both in the properties window.

Size vs Size on Disk

In simple terms, the “size” represents the actual size of the contents of the file or folder, and “size on disk” is the actual amount of space occupied by the file or folder including the file system’s allocation unit size and disk storage format. That is the reason why the Size on Disk is always more than the Size.

Here’s a more detailed explanation:

First of all, Size. It’s the straightforward one. It tells you the amount of space that file or folder’s content takes up. So, if you have a text document that’s 5 MB in size, that’s what it shows.

Size on Disk is a bit different from Size. It goes beyond the file’s content and takes into account the details such as the file system’s allocation unit size (cluster size) and the format of your storage disk. So, what is allocation size and why does it make “size on disk” bigger than “size”? Let’s break it down further.

Windows divides your storage into fixed-size chunks called allocation units or clusters. Each allocation unit or cluster is of a predefined size and depends on the file system. For example, the default cluster size of the NTFS file system is 4KB. That means each allocation unit is the size of 4KB. So, any time you save a file, Windows uses up a minimum of 4KB of storage space. Windows cannot further divide a cluster or save two different files in the same cluster. Any leftover space within a cluster is wasted and contributes to the larger “size on disk” compared to “size.”

For instance, if you are saving a 2KB text file, Windows uses 4KB of storage space because of the cluster size. That means the “size” of the file is 2KB and the “size on disk” is 4KB. So, the actual storage space used by the file is 4KB. The excess 2KB is wasted space. As another example, if the file you are saving is 10KB then Windows shows the “size” as 10KB and “size on disk” as 12KB (4KB multiplied by 3).

I hope this article helped you understand the differences between “size” and “size on disk.” I know it’s a bit confusing at first glance but all you need to remember is that “size” is nothing but the actual size of the file or folder contents whereas “size on disk” is the size of the file or folder contents plus the wasted size of allocation unit.

If you are stuck or need help, send an email, and I will try to help as much as possible.

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