Lightroom's previews can be larger than the photos they preview
Standard and 1:1 previews, plus Smart Previews, are stored beside the catalog. What each is for and which to discard.
Lightroom keeps its catalog small and its previews large. The catalog is a database of edits, which is text sized. The previews are rendered images, and on a library of tens of thousands of photos with 1:1 previews built, they can exceed the size of the originals.
The files beside the catalog
du -sh ~/Pictures/*.lrlibrary ~/Pictures/*/*.lrcat* 2>/dev/null | sort -h
du -sh ~/Pictures/*/*Previews.lrdata 2>/dev/null | sort -h| File | What it is | Safe to delete |
|---|---|---|
.lrcat | The catalog: edits, keywords, collections | No. This is your work |
Previews.lrdata | Standard and 1:1 previews | Yes, rebuilt on demand |
Smart Previews.lrdata | Editable proxies for offline work | Yes, if originals are attached |
.lrcat-data | Newer catalog data | No |
| Backups folder | Catalog backups | Keep at least the recent ones |
1:1 previews are the ones that grow
A standard preview is a screen sized JPEG. A 1:1 preview is a full resolution render for zooming, and it is built per photo. Lightroom has a setting to discard 1:1 previews automatically after a day, a week or thirty days, and setting it is the difference between a preview folder that stabilises and one that only grows.
Smart Previews are a deliberate trade
Smart Previews let you edit with the originals disconnected, which is exactly what you want when the photos live on an external drive. They cost around a tenth of the original size each. Keeping them for an archive you edit occasionally is sensible; keeping them for every photo you have ever imported is not.
The rebuild cost
Deleting a preview file loses nothing but time: Lightroom rebuilds previews when you next browse those photos, which is slow for a large catalog and completely safe. The catalog itself is the file to protect, and it is small enough to back up easily.
If the originals rather than the previews are the problem, moving them is the answer, and what moves safely to an external drive covers doing it without breaking the references. For the Apple side of the same question, the Photos library guide has the equivalent numbers.
Common questions
Can I delete Lightroom preview files?
Yes. Previews.lrdata and Smart Previews.lrdata are rebuilt on demand, at the cost of time. The catalog file itself holds your edits and must not be deleted.
Why are Lightroom previews so large?
Because 1:1 previews are full resolution renders built per photo, and they are kept until discarded. Setting Lightroom to discard them automatically after a week or a month stops the folder growing indefinitely.
What are Smart Previews for?
Editing when the original files are disconnected, such as when photos live on an external drive. They cost roughly a tenth of the original size each, which is worth it for an active archive and wasteful for everything.
How do I find Lightroom's files on a Mac?
They sit beside the catalog, usually in ~/Pictures, as .lrcat files with matching Previews.lrdata and Smart Previews.lrdata folders. A du command across ~/Pictures shows the split immediately.