How to Check Your iPhone's Storage Capacity
When you're ready to sync, make sure you have enough room on your iPhone before actually hitting the Sync button. Your iPhone's storage capacity depends on which version of iPhone you have.
Considering that the operating system itself takes up about one gigabyte of memory and another half a gigabyte is kept as a reserve for apps running in the background, that leaves you with 14.5 gigabytes on a 16GB phone, 30.5 on a 32GB phone, or 62.5 on a 64GB phone.
That sounds like a warehouse of storage, until you start loading apps, movies, TV shows, podcasts, games, photos, and videos, when you find out it fills up fast. There are two ways to know how much space you have on your iPhone:
On iPhone: From the Home screen, tap Settings→General→Usage. The list shows you how much memory is occupied by Videos, Photos & Camera, Music, and other apps with the biggest space hogs at the top of the list and tells you how much storage is Available and Used.
On iTunes: When your iPhone is connected to iTunes and selected in the source list, you see a colored bar near the bottom of the iTunes window that illustrates how much space each type of data will occupy on your iPhone when you perform the sync you've set up.
If you are in the Music pane and you deselect the Sync Music box, the blue section of the bar that represents audio files shrinks. If your iPhone is nearly full, you can play with your syncing options and see how they affect the storage capacity.
Click once on one of the titles under the chart and the number of items in each category is displayed. Click a second time and it displays how long it will take to listen to all the audio and video in your library. Click a third time to return to the gigabyte numbers.
If you have more media than memory, you have a couple of choices for managing and choosing which data to sync:
Manually select which music, videos, and podcasts you want on your iPhone and sync only those.
Subscribe to iTunes Match to listen to music in streaming.