You dont until you get going, but you can estimate it if you know how many visitors you get and how big your pages are etc, but if you have a brand new website and havent advertised it anywhere then you wont consume and bandwidth except for your own usage.

Disk space is fairly easy to calculate as you will know what your uploading and Cpanel shows you how much space and bandwidth your using, and emails you with warnings if you get close to your usage limits giving you time to upgrade the account.

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