Zip is a common type of compressed data format used to store data efficiently and group multiple files and folders into one file. It's sometimes possible to reduce a zip file's size by switching from one zip file utility to another or by adjusting the settings inside a program. If you can't shrink a zip file as much as you'd like, you can use other file formats that can sometimes result in more efficient compression.
How Zip Files Work
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Zip files allow you to take one or more files and combine them into a single file that's compressed to save disk space and internet bandwidth. They're created by replacing repeated portions of a file with a single shorthand notation. With a properly made zip file, no data is lost by compressing and uncompressing the file.
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You can create or open a zip file on a Mac or Windows PC using built-in operating system tools. You can also use specialized software that supports working with zip files and other compressed file formats. Some programs support different styles of zip files than others. Some newer types of zip files are more efficient and can reduce a zip file size beyond what was possible in earlier software.
Reduce Zip File Size
There's usually a limit to how small a particular compression method can make a zip file. You usually can't reduce file size by zipping a file more than once, and certain files don't compress well because they're already compressed. Those include many popular image and music files, such as JPEG files and MP3 files.
Modern Microsoft Office files are already zipped, so they do not compress particularly well in a zip file format, but it may still be useful to zip them to combine multiple files for emailing or storage.
Some programs allow you to split a zip file into multiple files, which may be useful if you are having trouble saving a large file on a disk or sending it over the internet.
Alternative Formats
While the zip format is one of the most popular compression formats, it's not the only one available. The ratio of the compressed zip file size to the original size is known as the zip compression ratio. Other compressed formats such as RAR, BZIP and 7-Zip can sometimes provide better compression ratios with some data. It can be worth trying multiple formats and tools on your data to see which compresses them best.
If you're sending the data to someone, make sure they have access to a tool that can open the file. Free and open source tools are available online for many formats on many operating systems.