• Windows
  • Photos

Lightroom previews can be larger than the photos they preview

Catalogs, previews and scratch files across the common editors. Which are derived and which are your work.

5 min read

Photo software 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 they can exceed the originals.

Where the files are

Get-ChildItem "$env:USERPROFILE\Pictures" -Recurse -Include *.lrcat,*.lrdata,*.cosessiondb -EA SilentlyContinue |
  ForEach-Object {
    $len = if ($_.PSIsContainer) {
      (Get-ChildItem $_.FullName -Recurse -File -EA SilentlyContinue | Measure-Object Length -Sum).Sum
    } else { $_.Length }
    [PSCustomObject]@{ MB = [math]::Round($len/1MB); Name = $_.Name }
  } | Sort-Object MB -Descending | Select-Object -First 12
FileWhat it isSafe to delete
.lrcatThe catalog: edits, keywords, collectionsNo. This is your work
Previews.lrdataStandard and 1:1 previewsYes, rebuilt on demand
Smart Previews.lrdataEditable proxies for offline workYes, if originals are attached
Backups folderCatalog backupsKeep the recent ones
Editor cache foldersPreviews and undo historyYes, once the app is closed

1:1 previews are the ones that grow

A standard preview is a screen sized image. A 1:1 preview is a full resolution render for zooming, built per photo. Lightroom has a setting to discard them 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

They let you edit with the originals disconnected, which is what you want when photos live on an external drive, and they cost roughly a tenth of the original size each. Keeping them for an active archive is sensible; keeping them for every photo ever imported is not.

Scratch space while editing

Layered editors use scratch space on a chosen drive, defaulting to C, and it can be many gigabytes while the app is open. If space vanishes while you work and returns when you quit, that is the mechanism rather than a fault, and pointing it at a second drive is the fix.

The originals

Previews rebuild and exports re-export, so the only category that grows without limit is the originals. Moving those to another drive is the real answer on a machine with a small system disk, and the catalog should stay on the fast drive because it is small and written to constantly.

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 be kept.

Why are Lightroom previews so large?

Because 1:1 previews are full resolution renders built per photo and kept until discarded. Setting Lightroom to discard them after a week or a month stops the folder growing indefinitely.

Where should a photo catalog live on Windows?

On the fast system drive, because it is small and constantly written to. The originals belong on a larger drive, with a second copy elsewhere.

Why does my disk fill while editing photos?

Layered editors use scratch space on a chosen drive, which defaults to C and can reach many gigabytes on a large document. It is released when the application closes cleanly.

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