When zipping actually helps, and when it does nothing
Compressing is the reflex when a file is too big, and about half the time it saves nothing at all. One property of the file decides it, and you can predict the answer before you try.
Zipping is the reflex when a file is too big, and about half the time it accomplishes nothing at all. Whether it helps is not a matter of luck — it depends on one property of the file, and you can predict it before you try.
The one rule
Compression works by finding repetition and describing it more briefly. A file full of patterns shrinks; a file whose patterns have already been squeezed out cannot shrink again.
So: if the format is already compressed, zipping saves nothing. That covers most of what people try to send.
| File type | Typical saving |
|---|---|
| Text, CSV, logs, code | 70-95% |
| Word, Excel, PowerPoint | 10-20% (already zipped internally) |
| BMP, TIFF, WAV, RAW | 30-70% |
| JPEG, PNG, HEIC | 0-3% |
| MP4, MOV, MP3 | 0-2% |
| 0-10% | |
| Already a zip, rar or 7z | 0% |
The row people find surprising is Office documents. A modern .docx or .xlsx is already a zip archive with a different extension — rename one and you can open it as a folder. Zipping it again is compressing a compressed file.
Reasons to zip anyway
Size is only one motivation, and often the weakest.
- Structure. This is the best reason. A zip preserves a folder tree, so the recipient gets an organised directory rather than four hundred loose files. See how to send a whole folder.
- One item instead of many. Easier to name, describe and keep track of.
- Encryption, if you use AES-256 rather than the legacy scheme. Note the filenames inside remain readable without the password — see password protection versus encryption.
- Preserving exactness. An archive is a container, so nothing along the way is tempted to re-encode your images.
Reasons not to
- It adds a step at both ends. Small on a laptop, genuinely awkward on a phone.
- Corporate filters quarantine archives. A zip is one of the most commonly stripped attachment types.
- You lose previews. A recipient cannot glance at a thumbnail inside an archive; they have to extract first.
- Partial access disappears. They wanted one photo and now they are unpacking eight gigabytes.
- Multi-part archives cause support calls. Splitting to fit a size limit hands the recipient a job. If you are doing this, the real answer is a service that takes the whole file.
Format, briefly
Zip for anything leaving your organisation. It opens natively on Windows, macOS and Linux with nothing installed, and that compatibility is worth more than a few percent.
7z compresses better and needs software at the other end. Fine internally, a bad default externally.
RAR needs software to extract and is proprietary. Avoid for anything external.
tar.gz is standard among developers and unfamiliar to everyone else.
The rule of thumb: use the format the recipient can open without thinking, unless you know otherwise.
What to do instead when the file is too big
If you are zipping to get under a limit, compression is the wrong lever. Better options, roughly in order:
- Send a link instead of an attachment. This removes the limit rather than negotiating with it.
- Send the right version. A 1080p review export rather than the master; selects rather than the full card; a screen-resolution PDF rather than print.
- Send less. The file they asked for, not the folder it lives in.
- Split by meaning, not by bytes — three transfers for three deliverables is understandable; a five-part archive is not.
The short version
Zip for structure and tidiness, not for size — unless the contents are text, code, or uncompressed media, in which case enjoy the saving. And if you are compressing to squeeze under an attachment limit, stop and send a link.
Related: email attachment size limits and how to send large files.
