Compression is a trade between size, quality, and time. Understanding which of the three you are spending makes every file decision easier.
Compression underpins nearly everything digital. Every photograph you view, video you stream, and web page you load arrives compressed. Yet most people only think about it when something will not fit — an attachment rejected, a hard drive full, a website loading slowly. This guide covers what compression actually does and how to apply it deliberately.
The core idea: removing redundancy
All compression exploits predictability. If data contains repetition or patterns, you can describe it more briefly than storing it literally. A row of two thousand identical white pixels can be recorded as "white, two thousand times" instead of two thousand separate values. Real algorithms are vastly more sophisticated, but the principle never changes: compression finds structure and encodes it compactly.
This has a hard consequence. Truly random data cannot be compressed, and already-compressed data has had its redundancy removed. Zipping a folder of JPGs saves almost nothing, and compressing an already-compressed file twice is wasted effort.
Lossless compression
Lossless compression guarantees perfect reconstruction. Decompressing returns exactly the original bytes. It is mandatory for anything where a single altered byte breaks meaning: source code, spreadsheets, databases, executables, archives, and legal documents.
Common lossless formats
- ZIP — the universal archive format, supported natively by every operating system.
- GZIP — dominant for web transfer, compressing HTML, CSS, and JavaScript in transit.
- PNG — lossless image compression with transparency.
- FLAC — lossless audio, typically around half the size of uncompressed WAV.
- 7z and Brotli — higher ratios at the cost of more processing time.
Typical lossless savings on text-like data are dramatic: plain text, code, and structured data often compress by 60 to 90%. On already-dense binary data, savings can be near zero.
Lossy compression
Lossy compression discards information judged imperceptible or unimportant, then compresses what remains. This is how a two-hour film fits in a few gigabytes and a photograph fits in a few hundred kilobytes. The discarded data is gone permanently — you cannot recover it by decompressing.
Common lossy formats
- JPG — photographic images, with a tunable quality parameter.
- WebP and AVIF — modern image formats offering better ratios than JPG.
- MP3 and AAC — audio, discarding frequencies masked by louder sounds.
- H.264, H.265, and AV1 — video, exploiting similarity between consecutive frames.
Lossy compression is the right choice for final-delivery media: what your audience sees or hears. It is the wrong choice for masters, archives, and anything you intend to edit further.
Choosing by file type
Documents
Most oversized documents are heavy because of embedded images, not text. Structural optimisation is usually enough — our PDF Compressor rebuilds a PDF more efficiently without touching the text layer. Removing unneeded pages with Split PDF is even more effective, since the remaining pages stay pristine.
Images
Resize before you compress. Reducing pixel dimensions to what will actually be displayed usually saves more than any quality setting, and it costs no perceptible fidelity. Use the Image Resizer first, then the Image Compressor to tune quality, and the Image Converter to test whether WebP beats JPG for that specific picture.
Photographs from phones
Modern iPhones save HEIC files, which are efficient but awkward to share with Windows users and older software. Converting to JPG with HEIC to JPG trades a little size for universal compatibility — usually the right call when sending photos to someone else.
Collections of files
When sending many files at once, a single archive is easier for the recipient and avoids per-file attachment limits. Note that archiving already-compressed media barely reduces size; the benefit is organisational, not spatial.
Best practices worth internalising
- Always keep the original. Compress a copy, never the master.
- Compress once, from the highest-quality source. Repeated lossy passes accumulate damage.
- Match compression to destination. Web delivery, email, printing, and archiving have entirely different requirements.
- Test on the worst-case content in your file — small text, fine texture, smooth gradients — not the easiest part.
- Resize before compressing images. Dimensions dominate file size.
- Do not compress data you are legally or professionally required to preserve unaltered.
- Remember that compression costs processing time; on large batches this is a real consideration.
Privacy: where your files are processed
Compression tools split into two camps. Server-based tools upload your file, process it remotely, and return a download. Browser-based tools do the work locally using JavaScript, so the file never leaves your machine. For anything containing personal data, client information, financial figures, or unpublished work, local processing is the safer default — there is no transfer to intercept and no server-side copy to worry about.
Every Convert26 tool runs in your browser for exactly this reason. Your files are read into memory, processed, and written back out without ever touching a server.
The mental model
Compression trades three things against each other: file size, fidelity, and processing time. Lossless compression refuses to trade fidelity, so it accepts smaller savings. Lossy compression trades fidelity deliberately, so it achieves far more. Neither is better — they answer different questions. Ask what the file is for, and the right answer follows.