Optimize Mac Storage in Photos: Save Disk Space

A serious photo library outgrows a laptop SSD years before it outgrows an iCloud plan. Optimize Mac Storage is Photos' answer: full-resolution originals live in iCloud, your Mac keeps lightweight versions, and originals stream down when you actually open a picture. Here's how to turn it on — and what you're trading away.

Turn it on

1. Open Photos and choose Photos → Settings… (⌘,).
2. Click the iCloud tab and make sure iCloud Photos is on — optimisation only exists when the library syncs, since iCloud is where the originals will live.
3. Select Optimize Mac Storage (the alternative radio button is Download Originals to this Mac).

Nothing dramatic happens immediately. macOS reclaims space lazily: as the disk fills, Photos quietly swaps full-resolution originals for device-sized versions, oldest and least-viewed first. You keep seeing every photo in the library — thumbnails and previews stay local — and any photo you open, edit, or export pulls its original from iCloud on the spot. The setting is per-Mac: your desktop can hold everything while the laptop optimises.

What it actually saves

The savings scale with library size and how much of it is video. Optimised versions are a small fraction of originals — a 400 GB library can settle under 40 GB locally, though the exact footprint depends on how much macOS decides it needs to evict. To watch it work, check the library size on disk (~/Pictures/Photos Library.photoslibrary, Get Info) over a few days, or the storage bar in System Settings → General → Storage.

The trade-offs, honestly

• Offline access dies. On a plane, an evicted photo is a preview you can look at but not export, print, or edit at full quality. If you shoot on trips and edit en route, this will bite.
• Editing gets a loading step. Opening an evicted RAW or 4K video means waiting for the download — seconds on fibre, painful on hotel Wi-Fi.
• Your Mac stops being a backup of the library. With originals only in iCloud, Time Machine backs up only what's downloaded. For irreplaceable photos, keep one machine on Download Originals so a full copy exists somewhere you control — this is the most important line in this article.
• Bulk exports get slow. Migrating away from Photos or exporting a year's shoot means re-downloading everything first.

Switching back to full originals

Reversing is the same radio button — select Download Originals to this Mac in Photos → Settings → iCloud. Photos then downloads every original, which needs two things in quantity: disk space (the library returns to full size — check you have room first, or the download stalls) and time (days, for big libraries on ordinary connections; the progress readout sits at the bottom of the Library view). Plug in, stay on Wi-Fi or Ethernet, and let it grind. If the download seems stuck, the usual suspects are a full disk, Low Power Mode, or the paused-sync states covered in our uploading guide.

Related space-savers

If the library is huge even optimised, shrink the library itself — deduplicate and prune your Photos library, or move the whole thing to an external drive and let the boot disk breathe.