How to do it
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Open iPhone Storage
Open Settings, tap General, then tap iPhone Storage. The bar at the top shows what is using space, and the list below shows each app.
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Read the Recommendations first
If your iPhone shows recommendations, tap Show All, read each one, then tap Enable or open it to see what you can delete. These are the system’s own suggestions for freeing space.
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Tap the biggest apps
Tap an app in the list to see its size. Cached and temporary data might not count in that number. You can offload the app (keeps its documents and data) or delete it (removes the app and its data).
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Clear Messages attachments
In Messages, tap the contact or group icon at the top of a conversation, open a category such as Photos, then tap Edit, tap Select, choose attachments, and delete them. You can also set Keep Messages to 30 Days or 1 Year in Settings.
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Check storage again later
After you delete something, the number can take a moment to update. System Data can also shrink by itself when your iPhone clears cached and temporary files to make room.
What is System Data on iPhone?
Apple’s page on checking iPhone storage has no row called “System Data”. It lists Other as non-removable mobile assets such as Siri voices, fonts, dictionaries, non-removable logs and caches, the Spotlight index, and system data like the Keychain and CloudKit database. System is the space the operating system takes, and it varies by model (Apple: How to check the storage on your iPhone and iPad). If your Settings shows a System Data row, it is this kind of content.
Can I delete it?
Not directly. Apple says that to free up space your iPhone removes cached and temporary files on its own, so you don’t have to (Apple). Apple’s page names no setting that clears System Data on request. So a big System Data number is a reason to look at everything else.
Is the Photos number part of System Data?
No. Photos and videos have their own row. If the Photos row looks different from what you expect in the Photos app, see Documents and Data in Photos. If photos and videos are what fills your iPhone, free up iPhone storage walks through that side.
What actually frees space?
Apple’s Manage storage on iPhone (iOS 27 edition) names the usual big items: photos and videos (back them up to iCloud, then remove them from the device), Messages conversations and attachments, downloaded videos in streaming apps, files in the Files app, and apps you no longer use. It also describes Offload Unused Apps (Settings, Apps, App Store), which removes unused apps when storage is low and keeps their data. Menu names differ in older iOS versions, so use the Storage list on your own iPhone as the guide.