When first installing your raspberry pi you mostlikey used the linux tool ‘DD’ to copy over your operating system image over to your SD card, as I did in this guide.
The problem with this is most of the images are from 2GB SD cards, and this is reflected in the default partition sizes. Basically even if you have a larger SD card there will only be 2GB of usable partitioned spaced until it’s resized.
Warning: Any changes to your file system may cause dataloss, keep backups of your data!
The people with Raspbian are fairly lucky as there is a really simple tool that will automatically do this for you. Type in the following command from either a local terminal session or via SSH.
Then go down to ‘expand_rootfs’ and follow the onscreen instructions, after thats done and a quick reboot you should have all your SD card space usable, cool!