Android "Internal Storage" used but not reported As an owner of a Nexus One running Gingerbread, I'm continually fighting to keep free space in my phone's Internal Storage. Today, I noticed something odd. 

The bottom of Andorid's "Manage applications" screen shows a bar graph that reports the phone's "Internal storage" (I have 29M free). When I click into an app, I can see storage for the "Application", and for the app's "Data". The application, in most cases, is stored on the SD Card. The application's "data" is stored on the phone (I presume).

What I'm finding is that I'm not seeing the whole picture of what is taking up Internal Storage.  For example, I have 29M free in Internal Storage. The Angry Birds app, which is stored on the SD card, is 3.14M. It's Data is 4.0K. I then delete the Angry Birds (gasp!), and my free Internal Storage jumps up to 31M. So it looks like it's freed up 2M, when I only would have expected it to free up 4.0K. Any idea what's going on here?
The free space difference can be accounted by three relevant factors that may end up giving you some misleading information about the real space available on your internal storage:


The Free space reported

The free space reported at the end of the "Manage Applications" screen is rounded to MB, and it gives you a general free space estimation based on the moment you opened said screen, and it gets updated from time to time (by my tests about every minute).

I've mentioned "rounded" because if you access the "Storage Settings" panel, you'll get a clear picture regarding the free space available, down to KB.
The App Allocation

As already mentioned by t0mm13b, an application may be entirely moved to the SDCard or leave a portion behind, this issue is more related with the development of the application itself and there's no standard rules to follow.

I've performed an exercise with "Angry Birds", and while the download mentioned 20MB, after installed it consumed 24MB. After moving it to the SDCard using the option for this end under "Manage Applications" screen, 1.85MB were left behind on the Internal Storage, plus 4KB of data.
Temporary System Files from other applications or running services

While I was writing this answer, the free space reported varied between 2MB, this tells us that the OS is constantly creating and freeing up space depending on the stuff running.


There may exist other factors that concern the ext4 File System specifications used by Gingerbread , but I don't believe they better account for the 2MB difference.



By the three points mentioned above, while performing an exercise with the application you've mentioned using a device with a stock Gingerbread ROM, I've managed to account for 2MB of difference, just like the space you've mentioned.



Determine Internal and External Space Occupied

Following up on the comments, regarding a way to accurately know how much an app occupies on the device, and being able to view it by external or internal storage, the most accurate application I'm currently using is SanDisk Memory Zone by SanDisk Corp. available for free from Google Play Store.


Open the application
Select the desired storage
Check the details about it