How to buy space in iPhone

You cannot buy space on the phone. It will always have the amount of storage it came with. To make space you must delete photos, messages, apps, etc. Be sure to save important photos and data to your computer or external drive. Check these links...

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

I would like you to realize and know one important thing and that is that there is NOTHING that you can do to increase the physical storage capacity of your iPhone. Your iPhone's storage capacity will forever remain the same as the day you bought it.

You can purchase ➔ iCloud Storage from Apple where you create and save your iPhone backups and synch (Not Store) your Messages, Photos, Contacts, Calendars, Notes and Reminders. Note that iCloud Storage is NOT an extension of storage capacity of your iPhone. Just wanted you to know some facts. Thanks.

Axel F.

While the above answers are technically true, you can use iCloud Storage to offload idle data to iCloud, thus freeing up space on your phone for new content.

Idle data may include things like photos or music tracks you haven't viewed/played in a while, or applications that you haven't run in a long time. These are replaced with placeholders so you can still see the content (e.g. a low-resolution copy of the photos, an application icon, etc.) and any time you actually access that content, the full version is automatically and transparently downloaded from iCloud.

Check Settings -> General -> iPhone Storage to enable this.