View Full Version : Folder Size in windows properties

21-03-2007, 03:04 PM
Why do the "size on disk" and "size" not match up

for instance i have a folder when i click on properties of that folder

it says

size 29Mb
size on disk 81.9Mb


Speedy Gonzales
21-03-2007, 03:29 PM
Most probably coz 1 folder doesnt include all files on that hdd?

21-03-2007, 03:42 PM
Size refers to the files actual byte count. Size on disk refers to the amount of cluster allocation the file is taking up.

Drives are organized by fixed-size units called clusters. The size of a cluster depends on several factors, the most crucial being file system and partition size.

Some background to help you understand:
Back in the days of DOS and Win95, the only usuable file system was FAT16 (also known as FAT). Because hard drives were so small back then (1GB was considered HUGE), FAT's limitations were minor, one of which was the limitation that the maximum partition size was 2GB.
With FAT16, the cluster size within a partition was determined by the size of the partition. Here is a chart showing the default cluster size for various FAT16 partition sizes:


Drive Size FAT Type Sectors Cluster
(logical volume) Per Cluster Size
----------------- -------- ----------- -------
0 MB - 15 MB 12-bit 8 4K
16 MB - 127 MB 16-bit 4 2K
128 MB - 255 MB 16-bit 8 4K
256 MB - 511 MB 16-bit 16 8K
512 MB - 1023 MB 16-bit 32 16K
1024 MB - 2048 MB 16-bit 64 32K

Now, with small drives, this wasn't a problem because partitions were small. Once drives started growing, the cluster size issue became problematic.

Here's why:
When you write a file to disk, the smallest unit it can take up is 1 cluster. If you have a 1024MB (1GB) partition, and the default cluster size is 32KB, a 1KB file will take up a 16k cluster.
Since you can't have more than one file occupying a cluster, the remainder of that cluster is considered wasted space. In this example, a 1KB file is taking up 32KB of disk space, and wasting 31KB of that space.
Say you have a 68KB file. That file will be written acrosss several clusters, since a cluster is smaller than the actual file. Thus, a 68KB file will take up 2 full 32KB clusters, and 4KB of a third cluster, while also wasting 28KB of that third cluster. While the file is only 68KB, it's taking up 96KB of disk space.

As you can see, with a lot of small files, you can waste a ton of space on such a drive.

To remedy the situation, MS unveiled FAT32 with the introduction of Windows 98. FAT32 not only allowed for significantly larger partition sizes, but also reduced the default cluster size dramatically. Under FAT16, a 2GB partition used a default cluster size of 64KB. Under FAT32, that was reduced to 4KB. A 16GB partition only used 8KB clusters, and up to 32GB uses only 16KB clusters.
As you can see, the savings on such large drives can be significant, resulting in more usuable drive space.

With Windows XP, the home user was introduced to NTFS. Under NTFS, a 1GB partition uses 2KB clusters, and anything above that up to 2TB (terabytes) only uses 4KB clusters.

So, the reason you're seeing that difference is because when you select properties, the Space On Disk reading is accounting for the wasted space attributable to the cluster size on the disk.

Note: MS has a chart with the cluster sizes listed on TechNet.

Borrowed from here


21-03-2007, 04:02 PM
Thanks for that, I had wondered about it for years and realised it had something to do with the clusters but never really realised what