The short answer
Check Settings › General › iPhone Storage to see what's using the space. On most phones it's Photos. Clear exact copies with the built-in Duplicates album, delete old screenshots and long videos, then empty Recently Deleted, because iOS only gives the space back once photos leave that album.
"Storage Almost Full" usually means one thing: the camera roll. Apps and messages add up too, but on most iPhones photos and videos are the biggest item by far, and they're the one that keeps growing every week.
First, see where the space went
Open Settings › General › iPhone Storage. The bar at the top splits your storage by kind, and the list below sorts apps by size. Give it a few seconds to finish calculating.
If Photos is near the top, everything below will help. If it's an app you don't use, offloading or deleting that app is the quicker win.
The quick wins in the Photos app
1. Merge exact duplicates
Photos finds identical copies on its own: the same picture saved twice, or the same shot at two sizes. Open Photos, find Utilities (under Collections, or Albums on older versions of iOS) and open Duplicates. Tap Merge on a set, or Select to merge many at once. Photos keeps the best version, with all the details combined, and moves the rest to Recently Deleted.
It won't find bursts or ten retakes of the same group photo, because those aren't identical. Our guide to deleting duplicate photos covers those.
2. Delete screenshots you've finished with
In Photos, open Media Types › Screenshots. Most of them were useful for a minute: a map, a code, a receipt you've since filed. Tap Select, then slide a finger across the grid to pick many at once. More on screenshots.
3. Go after big videos
A single minute of 4K video can take as much space as a hundred photos or more. Open Media Types › Videos and start with the longest clips: the length shows on each thumbnail. Delete the ones you'll never watch. For the ones you want, make a smaller copy.
4. Look at Live Photos
A Live Photo saves a short video with every still, which often doubles its size. If you don't use the motion, turn Live Photos off for new shots.
Then empty Recently Deleted
This is the step most people miss. Deleted photos and videos sit in Utilities › Recently Deleted for 30 days, and they still count against your storage the whole time. To get the space back now, open the album, unlock it with Face ID or your passcode, tap Select, then Delete All.
Once they're gone from there, they're gone for good. If you aren't sure, leave them for the 30 days instead. How Recently Deleted works.
If you use iCloud Photos
Turn on Optimize iPhone Storage: open Settings, tap your name, then iCloud › Photos. Your full-size originals stay in iCloud and the iPhone keeps smaller versions, downloading the full one when you open it. It can save a lot of space on a big library, as long as your iCloud plan has room for everything.
Remember that with iCloud Photos on, deleting a photo on your iPhone deletes it from iCloud and your other devices too. How to keep photos in iCloud but off your iPhone.
Other places space hides
- Messages attachments. In Settings › General › iPhone Storage › Messages, review large attachments: videos and photos people sent you are stored a second time inside Messages.
- Downloaded media. Podcasts, music, and shows saved for offline viewing in streaming apps.
- Apps you never open. Offload App in iPhone Storage removes the app but keeps its data, so reinstalling it later picks up where you left off.
Keep it from filling up again
A camera roll refills every week, so a big cleanup once a year leaves you back at "Storage Almost Full" by spring. A few minutes a week on the newest photos is less work than one long weekend, and you're deleting while you still remember which shot was the good one. That's the idea behind cleaning your camera roll as a habit.