Do you lose quality when converting a file?

There are two families of formats, and that is the whole key

Some store the file whole; others throw away information the eye or ear rarely notices, in exchange for weighing far less.

• Lossless: PNG, TIFF, BMP, GIF, FLAC, WAV, ZIP. They keep everything. Converting between them never degrades anything.
• Lossy: JPG, WEBP, AVIF, HEIC, MP3, AAC, M4A. They discard detail to weigh less.

The rule that settles 90% of the doubts

What matters is where you are converting to:

• To a lossless format (PNG, TIFF, FLAC): nothing is lost in the conversion. But note: nothing already lost comes back either.
• To a lossy format (JPG, WEBP, MP3): there is compression. We use high quality by default so the difference does not show at a glance.

The mistake that really does ruin files: converting over and over

Every pass through a lossy format compresses again. A JPG converted to WEBP, then back to JPG, then to AVIF piles up the damage from all four, and there it does start to show.

If you are going to do several conversions, always start from the original file, not from the result of the last one.

Concrete cases, by name

• HEIC to JPG: there is recompression, but at high quality it is imperceptible. It is the normal move to open iPhone photos anywhere.
• PNG to JPG: there is loss, and transparency disappears too (the background fills with white). Fine for photos, bad for logos.
• JPG to PNG: nothing new is lost, but the file usually weighs more, because PNG was not designed for photographs.
• PDF to JPG: the PDF is drawn as an image. The text stops being text and cannot be selected.
• Word to PDF: the text survives intact. What can shift is the layout if the document uses unusual fonts.
• Video to MP3: the picture is discarded and the sound recompressed. At a high bitrate it is indistinguishable from the original on ordinary headphones.

And does compressing ruin the image?

Compressing is, by definition, removing weight, and past a certain point it shows. That is why the compress image tool tries several options, measures them and keeps the best, and why it never returns a file heavier than the original: if there is nothing to gain, it tells you.

What makes no difference

Which site you use. A JPG at quality 90 is a JPG at quality 90 here and anywhere else: the compression is defined by the format, not by whoever runs it. What does differ between sites is whether they watermark your file, cap how many you can do per day, and whether your documents end up on someone else’s server.