Does converting a file lose quality?
Sometimes — and it depends entirely on which two formats. The rule is short: you lose quality whenever the destination is a lossy format. And the corollary is the one nobody wants to hear: converting to a lossless format restores nothing. An MP3 turned into a FLAC is a bigger file containing exactly the same damaged audio.
Lossless and lossy, in one paragraph each
Lossless formats — PNG, TIFF, BMP, WAV, AIFF, FLAC, ALAC — compress the way a ZIP file does. Clever packing, nothing discarded. Decompress a FLAC and you get back the exact samples that went in, bit for bit. Same for a PNG's pixels. The file is smaller; the data is complete.
Lossy formats — JPEG, HEIC, WebP (in its lossy mode), AVIF, MP3, AAC/M4A — get their much larger savings by throwing information away. The encoder models what human eyes and ears will not notice, deletes it, and keeps the rest. This is why a JPEG is a tenth the size of the equivalent PNG, and it is a genuinely brilliant trick. It is also irreversible. The discarded data is not compressed, or hidden, or recoverable. It is gone.
The four cases
Every conversion you will ever do is one of these.
1. Lossless to lossless — no loss at all.
WAV to FLAC. FLAC to AIFF. PNG to TIFF. BMP to PNG.
The output is bit-identical to the input. A FLAC made from a WAV decodes back to precisely the same samples; a PNG made from a BMP holds precisely the same pixels. You can do this a thousand times in a row with no degradation whatsoever. The only thing that changes is the file size and what software will open it.
2. Lossless to lossy — loss, once.
WAV to MP3. PNG to JPG. TIFF to WebP.
This is the encoder doing its job, and this is the good case for a lossy conversion: it is working from a complete original, so it makes its decisions with full information. The damage is controlled by your quality or bitrate setting, and at a high setting it is genuinely hard to see or hear. This is a first-generation encode, and it is the best a lossy file can ever be.
3. Lossy to lossy — loss, twice. This is the one that bites.
M4A to MP3. HEIC to JPG. JPG to WebP. JPG to JPG at a different quality.
The second encoder is handed audio or pixels that have already been damaged — including the artefacts the first encoder introduced — and it applies its own, different rules about what to discard. It cannot tell the difference between real detail and the first encoder's mistakes, so it faithfully compresses both. This is generation loss, and the result is measurably worse than the source and worse than a first-generation encode from the original would have been. Unavoidable, in every app, on every platform. It is a property of the formats, not of the software.
4. Lossy to lossless — no loss, and no gain. The big misconception.
MP3 to FLAC. JPG to PNG. AAC to WAV.
Keep reading.
Converting an MP3 to FLAC does not improve it. It cannot.
The audio the MP3 encoder deleted is not in the MP3. Nothing you convert the MP3 into can put it back, because the destination format has nothing to read it from. What you get is a perfect, lossless, bit-exact copy of a damaged recording — every artefact preserved with total fidelity, in a file three or four times the size.
Identical logic for images: a PNG made from a JPEG contains the JPEG's compression artefacts, losslessly, in a much larger file. It is not "restored to PNG quality". There is no such thing.
The one legitimate reason to do this is as an editing workflow: if you must repeatedly edit and re-save, working in a lossless format stops each save from adding new damage (see case 3). It does not undo the old damage. Nothing does.
The re-saving trap
This is case 3, quietly, over and over.
Open a JPEG. Crop it. Save. Open it again next week. Adjust the brightness. Save. Send it to someone who crops it again and saves it again. Each save re-runs the lossy encoder on an image that already went through it, and each pass compounds the artefacts. Enough rounds and you get the crunchy, blocky, over-saturated look of a meme that has been through fifty phones — that is generation loss, made visible.
Do your editing in a lossless format and export to a lossy one once, at the end. That is the entire discipline, and it is the only real defence.
Things that feel like quality loss but are not
Changing a container. MOV and MP4 are containers — boxes holding a video stream and an audio stream — and the same H.264 video can sit in either. Repackaging it ("remuxing") touches no pixels and costs nothing. However: whether an app remuxes or re-encodes is up to the app, and many re-encode. Convexy re-encodes, because it uses Apple's export presets — so MOV to MP4 in Convexy does cost you a generation. We would rather say so than let you assume otherwise.
Stripping metadata. Removing EXIF, GPS and timestamps deletes information about the photo, not the photo. The pixels are untouched.
A smaller file at the same quality setting. Sometimes a format is simply more efficient. HEIC really is about half the size of JPEG at the same visual quality — compression doing its job, not a trick.
The practical rules
- Always convert from the best original you still have — not from a copy of a copy. If you have the WAV, encode the MP3 from the WAV, not from the M4A you made from it.
- Never chain lossy conversions. Each one costs you. Go from the source, once.
- Keep the original. Convert a copy. The original is the only thing you cannot recreate.
- Choose the quality setting deliberately. On a second-generation encode, be generous — the extra bits are cheap insurance against compounding artefacts.
- Do not sweat a single high-quality conversion. One careful HEIC-to-JPG will not ruin your photograph. The compounding is what gets you, not the first step.
Common questions
Does converting MP3 to FLAC improve the quality?
No. FLAC is lossless, so it perfectly preserves whatever it is given — and what it is given is an MP3 that has already had audio permanently removed. You end up with a bit-exact copy of a damaged recording in a file three to four times the size. The deleted audio is not in the MP3, so no conversion can retrieve it.
Does converting JPG to PNG improve quality or remove artefacts?
No. PNG is lossless, so it faithfully stores exactly what the JPEG contained — including every compression artefact — in a much larger file. It does not restore lost detail and it does not clean anything up. The only reason to do it is if you are about to edit and re-save repeatedly, where working losslessly prevents further damage.
Is converting HEIC to JPG lossless?
No. Both are lossy formats, so the JPEG encoder re-compresses an image that HEVC has already compressed — a second generation of loss. At a high quality setting you are unlikely to see it, but it is real, and it is why you should convert a copy and keep the HEIC original.
Does converting MOV to MP4 lose quality?
It depends on whether the app re-encodes or simply repackages. MOV and MP4 are both containers, and the same H.264 stream can live in either, so a pure remux costs nothing. Many apps re-encode instead, which costs a generation. Convexy re-encodes, because it uses Apple's AVFoundation export presets — so in Convexy, MOV to MP4 is not free.
Which conversions are genuinely lossless?
Lossless to lossless pairs. For audio: WAV, AIFF, FLAC and CAF (with PCM) convert between each other with bit-identical samples. For images: PNG, TIFF and BMP hold identical pixels. You can convert back and forth as many times as you like with no degradation at all.
Does converting the same file over and over make it worse?
Only if the destination is lossy. Each lossy save re-runs the encoder on an already-compressed image or recording and compounds the artefacts — this is generation loss, and it is what makes a much-shared meme look crunchy. Lossless conversions can be repeated indefinitely with no effect at all.