Do you know Google offers you free 15 GB of storage capacity which you can use for Google account such as Gmail, Google plus photos, Google drive ?
Type the url https://www.google.com/settings/storage
Now enter the user name and password of your Google account for which you want to find out the space used.
In the picture above you can see that I have used 84% of my storage capacity which is 12.73 GB.
If you point out the pie chart you can see how much space is used by each Google account such as Gmail, Google drive, Google plus photos.
This is how you are able to find out how space you have consumed of 15 GB. If you want to upgrade then you can upgrade for 100 GB for $1.99/month . 1 TB for $9.99/month.