Manage Storage Space on iPhone
Space is precious with an iOS devices. You can't add space to an iPhone, iPad or iPod physically. When your disk is full, your iPhone won't let you even snap another photo or your iPad refuses any new app downloads.
How to Manage Storage Space on iPhone?
You need to monitor the usage from time to time on your iPhone. From your iPhone screen, open Settings, go to General > Usage, and wait for a while as your iOS device will calculate the storage used and which apps are using the most space.
To save iPhone space, you can delete those apps that you never used before and do not plan to use them either. . If you see apps that you rarely use but they take up a ton of your precious iPhone space, you may also consider uninstalling them. You can delete these apps from the Usage area under Settings directly.
Usually the Photos & Camera and Music use most of your space on iPhone. You can sync your iPhone with your computer to back up music and those photos and videos on your Mac or PC.
Chances are you do not want to delete your music, photos and videos from your iPhone and if possible you like to keep them all on your iPhone. There is a workaround for this. You can shrink their file size without removing them. Video Slimmer App is the app for that. It can reduce your video size and thus save your iPhone space.
For example, a 5 minutes video recorded with iPhone 5 may need 500 MB space. You can use the video compressor to reduce the file size by up to 80% or even more, then you can free up and save 400 MB space on iPhone.
See the step by step guide here: How to Reduce iPhone Video Size?