Day to day: a few gigabytes
iOS needs working room for caches, temporary files and the space apps use while running. Below roughly two gigabytes free, things start to degrade: photos fail to save, apps close unexpectedly, and the system spends effort evicting caches it would rather keep.
Five gigabytes free is a comfortable floor for normal use. Below one, an iPhone becomes genuinely unpleasant and can refuse to record video or take photos at all.
For an update: roughly twice the download
A major iOS release wants around 15 GB free. That is far more than the download size, because the installer unpacks the new system alongside the version you are running before switching over. It checks at install time, not download time, which is why the failure so often arrives after a long download completes.
A point release needs much less. If you are repeatedly short, updating from a computer sidesteps most of the requirement — our computer update guide covers it.
What actually happens when it fills
iOS purges what it can — caches, offloadable app binaries, optimised photo versions — without telling you. That is why a phone showing very little free space often keeps working. Once there is nothing left to purge, saving fails, the camera refuses, apps crash on launch and updates will not install.
Nothing is lost when this happens; the phone simply stops accepting new data. But it is a poor state to leave a device in.
Which capacity to buy
The honest answer depends almost entirely on video. Photos and apps are manageable at 128 GB with iCloud Photos and Optimise Storage on. Recording video regularly, particularly at high resolution and frame rate, will fill any capacity you buy.
64 GB is no longer realistic for a primary phone in 2026 — a major update alone wants a quarter of it. If you are choosing between two tiers and shoot video at all, take the larger one; storage cannot be added later.
Making a smaller iPhone workable
Turn on Optimise iPhone Storage for Photos, turn on Offload Unused Apps, set Messages to keep one year rather than forever, and check streaming downloads periodically. Those four together are worth a great deal on a 64 or 128 GB device.
Our storage guide covers each of them, ordered by how much space they actually return. Our device pages list the storage tiers each model shipped in, which is worth checking before buying used.
Frequently Asked Questions
How much free space should an iPhone have?
Around five gigabytes for comfortable day-to-day use, and roughly 15 GB free to install a major iOS update.
Why does an iOS update need so much more than its download size?
The installer unpacks the new system alongside the one you are running, so it needs room for both at once.
What happens when an iPhone runs out of storage?
iOS purges caches automatically first. Once there is nothing left to purge, photos fail to save, apps crash and updates will not install.
Is 128 GB enough for an iPhone?
For most people, yes, with iCloud Photos and Optimise Storage on. If you record video regularly, choose more — storage cannot be added later.