used storage in iPhone 5
Use the Manage Storage feature to check what is using the majority of your storage. It is quite common for app data to be using a large amount of storage which can be easily missed.

Also update your device if there is an update available as it may have been downloaded to your device but not installed and can take up around ~2GB.

Keep in mind too, that iOS, when installed generally takes up about 3GB - 5GB of storage and that the actual formatted capacity of devices is usually a bit less.