Your disk space usage is the sum total of all data in your account including emails, databases and files (website).
The server will automatically send disk space warnings at 80%, 90%, 98% and 100% usage to the contact email specified in cPanel under "Preferences -> Update Contact Details".
Checking Disk Space Usage
- Log into cPanel.
- On the left hand side, you can see a summary of your current usage
- For details, please go to "Files -> Disk Space Usage"
- Scroll down the page, then expand the folders wit the "+" sign to the left of each folder. This will show exactly where your disk space is being used.
Using Awstats
In your site's cPanel, click on Logs -> Awstats 
- At the top, check the "Viewed" vs "Not Viewed Traffic". Not Viewed Traffic is access by bots which may indicate a problem with spiders accessing your site too frequently.
- Further down, you can also check "File Type" to see which type of content generates the most bandwidth usage. If you spot any unusually high values, this may indicate an area for optimisation
Commmon Problems
EMAILS USING UP DISK SPACE
If emails are stored on the server, then these count against your disk space usage. You can reduce the disk usage with the following strategies:
Outlook - If you are using Outlook (with POP3) then you can set it to either delete the emails from the server after download or to delete after X days or to delete when deleted from your Outlook "Deleted Items" folder. To modify the settings, go to "Advanced -> Delivery" under your email account in Outlook. 
POP3 - POP3 is the most common method of accessing your email on the server. Check your mail program if you can set it to delete emails from the server after download.
IMAP - if you are using IMAP, then deleting emails from your mail program's inbox will reduce disk space.
ERROR LOGS
Error log files can build up quickly. Check your folder for "error_log" files. It's safe to delete them via FTP or File Manager in cPanel. When cPanel encounters a new PHP error or warning, it will simply recreate a new (empty) file.
DATABASES
Some websites are configured to track extensive visitor behaviour date which can build up quickly on a busy site. Which tables you can truncate (empty) will depend on your software, so in the first instance please check with your software provider.
Disk space calculations
Databases
Please note that PhpMyAdmin and cPanel read the size of the database differently. PhpMyAdmin basically uses the SHOW TABLE STATUS command and uses Data_length and Index_length which is the size of those two files only, while cPanel looks at the database files in their entirety and not just the index/data indexes. To see the accurate size of your db, please therefore use the cPanel "Files -> Disk space usage" log.
