What Are Main Differences Between Size VS Size On Disk

Plenty of folks have been puzzled when they try to look at the properties of a folder, file, or drive of the system. The properties size vs size on disk do not usually exactly the same. The value of size in the properties page you see indicates the real size of the file in a system. However, the size on disk indicates the number of bytes that it actually takes up on the hard drive.

Ever look at a file or folder of your OS and wonder why the heck the “size” and “size on disk” are different? Yeah, me too. Until recently I got no idea why these two are different and just brushed it off as being a specialized mumbo jumbo. I decided it is time to learn just what these two are. Why they are different from each other.

Why size vs size on disk Differences Exist

How the size vs size on disk differences come? In reality, the file system of the FAT32 you use needs to keep an eye on where each file is stored. If it were mainly used for the purpose of maintaining a set of every single byte for storing, the table you use (just like an address book) would grow. It will increase at the same velocity as the data. Therefore, you will waste a great deal of space. What exactly they do is use “allocation units.” It is also known as the “cluster size.”

size vs size on disk 1

The particular volume is mainly divided into these allocation units. When you see the file system, they cannot also be subdivided. Basically, these are the smallest blocks it can deal with. A person has a house number, but the postman doesn’t understand how many bedrooms that person has or who lives in them.

So what will happen if you do have a very small file size? Actually, the file system of your OS doesn’t care if the file is 2 kB, 0 kB, or even 20 kB. Your OS will give it the minimum space it can, which is 32 KB in the example above. The file you use is only using a small amount of this space, which is 32 KB here. Therefore, the rest of the space is actually wasted but still goes to the file – much like a bedroom you leave unoccupied.

The allocation units of your OS, as a general rule, will waste materials a lot of room if you have plenty of small sized files. Right now there usually isn’t a valid reason to go above 4 kB for general use.

Understand size vs size on disk Differences

The discrepancy of the size vs size on disk comes from the way the file system stores the file on the drive. File systems treat a number of bytes as a single cluster in order to reduce the number of addresses that are being used. Depending on the file system of your OS, common cluster sizes can vary from 2 KB to as large as 32 KB.

Also See, Simple Tip to Enable File and Folder Compression

The file that is written to disk requires a discrete number of clusters regardless of each actual size. So your 1 KB file when you save in a file system with 2 KB clusters would take up 2KB however in a file system with 32 KB clusters, it would occupy 32KB. Also, your 33 KB file would take up 17 2KB clusters (34 KB) or 2 clusters in a 32 KB file system (64 KB). The amount of wasted space for each file would not exceed the cluster size.

Why Size of Disk Is Greater

You would expect that the size on disk of your OS would be more significant than the exact actual size by no more than the cluster size of the operating system you use. Though this is often true, some factors are able to affect these beliefs. A folder furnished with a lot of files inside, the difference can be much more significant because each individual file on the OS can have wasted room and it all could sum up for the folder.

In some of the situations, the size on the disk can be smaller than the actual size of the file. It is apparently impossible, but it can take place when you are using certain features like computerized file compression that is proposed by the operating system. The displayed size you see on the OS is the actual size of the file, but since the os compresses it, space occupied would often be substantially smaller.


The worth labeled as “size” is the actual, literal size of the file(s)/folder(s) you have selected. Therefore if dotTech.jpg is listed as a size of 1. 25 MB, it is literally 1.25 MB large. When I was to download dotTech.jpg, I might be downloading it 1. 25 MB really worth of data.


It is the amount of disk space that the file(s)/folder(s) you have selected uses. Therefore while dotTech.jpg may be 1.25 MEGABYTES, it takes up – for example – 1.3 MB space on the hard drive.

Why size vs size on disk is Different?

The answer of the size vs size on disk comes down to file systems you use. Almost all of the file systems, available these days store data in clusters. They are blocks of the area on the hard drive. Your data is stored in them.

The 1 KB of data in a cluster is enough to reserve that the whole cluster of the system for the file in question, even if the file doesn’t fully utilize that cluster. Thus that distinction between the total size of the last cluster utilized by a file and the total amount of the data kept in that cluster is what results in the difference between size vs size on disk. Confused? Let’s look at an example to understand the size vs size on disk.

Let’s say you have a file, having the size of 10.5 MB. Let us suppose that the file is stored on a disk that utilizes clusters of just one MB. It means that this file needs 11 clusters of space to be entirely stored. It is true that a 10.5 MB of data cannot be kept in 10 MB of clusters. Therefore, it needs 11. The 11 clusters equal eleven MB. Thus, since that last 0.5 MB of data occupies a one MB cluster. As a result, the “size on disk” for the file is 11 MB.

Are these claims a trick in order to make us buy bigger hard drives?

Yes. No. Or maybe. Really don’t know. Juicy gossip avoids my inbox. However, being the ultra-rational logic wise man I am, I will go out on an arm or leg and say, no. Right now there is technical justification at the rear of using data clusters. Believe in me when I say brilliant people have developed our digital world. They know what they are doing. And if they don’t really know what they are doing, I don’t want to hear about this. Keep in mind ignorance is bliss.

Do I need to worry?

Simply no. You don’t even need to know the size vs size on disk difference exists. It really makes no difference in your personal life, particularly if you are a reasonable person and not a programmer. Knowing the difference between size vs size on disk is purely for self-knowledge purposes.

What Happens When You Transfer Files from one Drive to Another?

What about it? The “size” of a file is the actual size of that file. It is the amount of data you move when moving files. You are not moving clusters. The available space on a drive is the amount of data – in clusters – that is available. So long as the size of a file is smaller than the available space, you are excellent to go. There will be enough clusters for storing the file(s) you are moving.

What Are Clusters

Files on your system are stored on the disk in what are called clusters. They are the number of disk sectors of the storage device. Size on disk is the term refers to the amount of cluster allocation a file is taking up. However, the file size is a real byte count.

The lowest cluster size that you find in FAT32 is 1. It is really possible that the actual file you wish to save only needs a small portion of that cluster. In this case, the size on disk for that specific file will reflect the whole cluster as being used. So, when you check the properties tabs of a file in the Microsoft Windows, you will usually examine that the size on disk size is bigger than the file size.

Summary of size vs size on disk:

  1. Size is the actual byte count number of the file while size on disk is the actual byte depend that it occupies on the disk.
  2. The size on the disk you see in your OS is usually more significant than the real size of the file.
  3. Size on disk has the capacity to be smaller than the actual size for drives that use data compression.

Here are the main differences you can see in the size vs size on disk. If you know more differences between the size vs size on disk, share with us.