TECTechnology
Lossy vs Lossless Compression: What You Give Up and What You Keep
Published 5 min read
A colleague emails a product photo that looked fine on Monday. By Friday it has been cropped, saved, attached, downloaded, tweaked and saved again, and now the edges of the lettering on the box look smudged. Nothing broke. The file simply went through lossy compression several times, and each pass quietly removed a little more detail. Knowing the difference between lossy and lossless compression explains that smudge, and it helps you pick the right format before it happens.
The short version
Lossless compression makes a file smaller in a way that can be fully reversed: decompress it and you get back exactly the data you started with, bit for bit. Lossy compression makes a file much smaller by discarding information judged less important, usually detail that human eyes or ears pick up poorly. The discarded data is gone for good, so a lossy file can never be turned back into the original.
How lossless compression works
Lossless methods look for repetition and predictability. A run of identical pixels in a blue sky can be stored as "this colour, this many times" rather than listing every pixel. A word that appears hundreds of times in a document can be swapped for a short code. Nothing is thrown away; the file is just written down more efficiently.
The catch is that the savings depend on the content. A spreadsheet full of repeated labels or a screenshot with large flat areas shrinks well. A photograph full of fine texture and noise has little repetition to exploit, so lossless versions of photos stay fairly large.
Familiar lossless formats include:
- ZIP and similar archives for documents and folders
- PNG for screenshots, diagrams and graphics with transparency
- FLAC and ALAC for audio
- TIFF (with lossless options) and RAW-style formats for photo masters
How lossy compression works
Lossy methods go further by deciding what can be dropped. An image encoder might keep brightness information in full but store colour more coarsely, because eyes are more sensitive to light and dark than to subtle shifts in hue. An audio encoder might drop sounds masked by louder sounds at a similar pitch. Video codecs reuse whatever stays the same from one frame to the next and spend their data on what moves.
Most lossy formats let you choose a quality level. High settings remove so little that the result is hard to tell from the original at normal viewing size; low settings produce tiny files with visible blockiness, smeared edges, ringing around text or a watery quality in music.
Familiar lossy formats include JPEG for photos, MP3 and AAC for audio, and the video codecs used by streaming services. Some newer image formats, such as WebP, AVIF and JPEG XL, offer both modes in one format.
Side by side
| Lossless | Lossy | |
|---|---|---|
| Can the original be restored exactly? | Yes | No |
| Typical file size | Moderate reduction, varies with content | Large reduction, adjustable by quality setting |
| Effect of saving again | None; repeated saves are identical | Detail can degrade a little each time |
| Good for | Text, code, spreadsheets, screenshots, masters, archives | Photos on web pages, music for listening, streamed video |
| Bad for | Sending large photo or video libraries over slow links | Anything that will be edited many more times |
The re-saving problem
That smudged product photo is an example of what is often called generation loss. Each time a lossy file is opened, changed and saved again in a lossy format, the encoder makes fresh decisions about what to discard, and small errors stack up. Cropping or resizing between saves makes it worse, because the blocks the encoder works with no longer line up with the previous ones.
The fix is a habit rather than a tool: keep a lossless master, edit the master, and export a lossy copy only at the end, for the place it will be used. Portraits are a good test case, because faces show compression damage quickly; if you are preparing a profile picture, the advice in our guide to turning selfies into polished headshots pairs well with exporting the chosen version once, at a high quality setting.
Which one should you use?
- Will the file be edited again? Keep it lossless until the editing is finished.
- Must every bit survive? Text, code, financial records, contracts and database exports always need lossless compression. A single changed character can matter.
- Is it a photo going onto a web page or into an email? A good-quality lossy version is usually the sensible choice, because visitors and recipients benefit from the smaller download.
- Is it a logo, chart or screenshot? Lossless raster or a vector format is usually better; our comparison of SVG vs PNG sets out when each fits.
- Is it audio for listening or for production? Lossy files suit everyday listening; recordings that will be mixed or mastered belong in a lossless format.
Where you meet it without noticing
Streaming is lossy compression at work all day long. Films, live sport and music reach a phone or a TV only because codecs strip out detail that would otherwise need far more bandwidth. When a connection slows, many services drop to a more heavily compressed version rather than stopping, which is why a picture can turn soft for a few seconds before sharpening again. Our explainer on bandwidth for internet TV looks at how resolution, codec and the number of screens add up.
Image formats keep improving the balance between size and quality. If you are curious about a format that handles both lossy and lossless modes and can repack existing JPEGs without changing a pixel, the article on JPEG XL and image compression covers it.
Is lossless always better quality?
It is always faithful to the source, which is not quite the same thing. A lossless copy of a blurry photo is still blurry. And a lossy file at a high quality setting can look identical to the original at the size people will see it.
Does zipping a JPEG make it smaller?
Usually only slightly. A JPEG is already compressed, so a ZIP archive finds little left to squeeze. Archiving is still handy for bundling many files into one download.
Can I turn an MP3 back into a lossless file?
You can convert it to a lossless format, but the result only preserves the MP3 as it is. The detail removed when the MP3 was made does not come back.
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