Manatee

Lossy versus lossless: what you lose when you convert a file

· 6 min read

Converting a file is either lossless, meaning every bit of the original content survives, or lossy, meaning some detail is thrown away to make the file smaller. Which one you get depends on the target format and whether the tool copies the data or re-encodes it, not on how expensive the app is. That distinction is the difference between a conversion that costs you nothing and one that quietly degrades the file, so it is worth a few minutes before you click Convert.

What lossy and lossless actually mean

Lossless formats store the content exactly. PNG and TIFF for images, WAV and FLAC for audio, plain text and most document formats for words: open one, edit it, save it, reopen it, and it is identical down to the last pixel or sample. Nothing was approximated.

Lossy formats do the opposite on purpose. JPEG and HEIC for photos, MP3 and AAC (the audio in most M4A files) for sound, and the H.264 or HEVC video inside most MP4 and MOV files all discard the information the eye or ear is least likely to notice, in exchange for a file a fraction of the size. A 40 MB camera photo becomes a 4 MB JPEG. A full-quality song becomes a 3 MB MP3. Done once, at a sensible quality, the loss is usually invisible, which is exactly why these formats won.

The catch is that lossy is a one-way door. Once detail has been discarded, it is gone, and no later conversion brings it back.

Converting “up” does not restore quality

The most common misunderstanding is that turning a lossy file into a lossless one repairs it. It does not. Convert an MP3 to WAV and you get a much larger file that sounds exactly like the MP3, flaws included, because WAV can only store the samples it was handed. Convert a JPEG to PNG and the PNG faithfully preserves every compression artifact in a bigger container.

This kind of conversion is useful only when a program demands a specific format: an audio editor that wants WAV, a print shop that asks for TIFF. It repackages the content into a format the tool accepts. It never improves the content, and it often multiplies the file size for no visible or audible gain.

The three outcomes of a conversion

Every conversion lands in one of three buckets.

  1. A lossless copy. For video and audio, changing only the container while copying the streams inside is called remuxing. The compressed data is moved, not touched, so quality cannot change and the job takes seconds. A same-size output is the signature of this.
  2. A lossless re-encode. Going from one lossless format to another, such as WAV to FLAC or a bitmap to PNG, rewrites the data more efficiently while preserving it exactly. FLAC compresses audio the way a ZIP compresses text: smaller file, identical samples when it plays back.
  3. A lossy re-encode. Going to JPEG, HEIC, MP3, M4A, or a re-encoded video decodes the content and compresses it again. Done once from a pristine source, it is fine. Done repeatedly, each pass adds damage: a JPEG saved from a JPEG, or an MP3 made from an MP3, is measurably worse than the one before. This compounding is called generation loss.

How to tell which one you are about to do

Look at the source format and the target format together, and you can predict the outcome before converting.

  • Lossless to lossless (WAV to FLAC, PNG to TIFF, BMP to PNG): safe, nothing lost.
  • Lossless to lossy (WAV to MP3, PNG to JPEG, camera raw to JPEG): loses detail, but only once. Keep the lossless original as your master.
  • Lossy to lossless (MP3 to WAV, JPEG to PNG): no further loss, but no gain, and a much bigger file. Do it only when a program requires that format.
  • Lossy to lossy (JPEG to JPEG, MP3 to M4A, MP4 to a smaller MP4): a re-encode that adds loss on top of loss. Avoid it unless you have no lossless source to work from.

Doing a conversion on a Mac without needless loss

macOS has built-in tools that handle the common cases. Preview exports images through File then Export, offering JPEG, PNG, TIFF, and HEIC, with a quality slider when you pick JPEG; that export is always a re-encode. The Music app can create an MP3 or AAC version of a track. The honest limits are that Preview will not touch audio or video, the Music app handles only a handful of formats, and none of them tell you plainly whether the step you are about to take throws data away.

A dedicated converter makes the choice explicit and covers every media type. With Manatee:

  1. Open the app and choose Convert.
  2. Pick your target from the fifteen formats it writes.
  3. Drop the file, or a whole folder, onto the window or the menu bar icon.
  4. The new file is written alongside the original, which is never modified.

Because the original is left untouched, converting a copy to a lossy format is always safe to try: if the result disappoints, the master is still there. The rule that follows from all of this is simple. Keep your lossless originals, whether that is camera raw, WAV, or an untouched recording, and make lossy copies from them for sharing. When a lossy file also needs to be smaller, shrink it once, from the best source you have, rather than re-encoding an already-compressed copy.

Questions

Is FLAC really lossless if the file is smaller than a WAV?

Yes. FLAC uses the same idea as a ZIP archive: it packs the data more efficiently and unpacks to the exact original samples when it plays. You get roughly half the size of a WAV with audio that is bit-for-bit identical.

Does converting a JPEG to PNG improve the picture?

No. The PNG stores the JPEG’s pixels exactly as they are, artifacts and all, in a larger file. Detail that the JPEG already discarded cannot reappear.

I converted a video and it looks the same. Was it lossless?

Either it was a stream copy, which is genuinely lossless, or it was a high-quality re-encode that looks identical but technically lost a little. A quick tell: if the output is about the same size as the source, it was likely copied; a noticeably smaller file was re-encoded.

Which is better overall, lossy or lossless?

Neither, they serve different jobs. Use lossless for masters, editing, and archiving, where you may convert again later. Use lossy for sharing and everyday storage, where a small file matters more than perfect fidelity. Keeping a lossless master and making lossy copies gives you both.