File Compression Ratio Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The compression ratio is calculated with original size ÷ compressed size (the ratio), and (original size − compressed size) ÷ original size × 100 for the percentage saved. For a 100 MB file compressed to 25 MB, the ratio is 4:1, or 75% of space saved.
Explanation
Compression ratio expresses how many times a file has been shrunk by a compression algorithm (ZIP, JPEG, MP3, H.265 video...), generally presented in two complementary forms: the compression ratio (for example 4:1, meaning the original file is 4 times larger than its compressed version), and the percentage of space saved (75% in the same example), more intuitive for evaluating a storage or bandwidth gain. The achievable compression ratio strongly depends on the type of content and the algorithm used: plain text or a file that's barely compressed to begin with generally compresses well (high ratios), while an already-compressed file (an H.265 video, a JPEG image, a ZIP file) compresses very little further, since most of the redundancy has already been eliminated by the first compression. There's also a distinction between lossless compression (like ZIP or PNG, which restores the exact original data) and lossy compression (like JPEG or MP3, which sacrifices some quality for a higher ratio): this calculator gives the numerical ratio in both cases, without judging the perceived quality of the result. To estimate a video file's size from its bitrate rather than measure a compression ratio, see our video file size calculator; to convert the resulting file size between bytes, kilobytes, and other data units, our data conversion calculator.
Example: a 100 MB file compressed to 25 MB
Inputs
Original size: 100 MB. Compressed size: 25 MB.
Calculation
Ratio = 100 ÷ 25 = 4 (i.e. 4:1). Percentage saved = (100 − 25) ÷ 100 × 100 = 75%.
Result
This file was compressed with a 4:1 ratio, saving 75% of space.
Frequently asked questions
Why do some files compress much better than others?
Because compression works by eliminating redundancy and repetitive patterns in the data. A plain text file or an image with uniform colors contains a lot of redundancy and therefore compresses very well; an already-compressed file (JPEG, MP3, H.265 video, ZIP) or one containing near-random data (already-encrypted data) has very little redundancy left, and so compresses little, if at all.
What is the difference between a 4:1 ratio and a ratio of 4?
It's exactly the same information, expressed in two equivalent forms: a ratio of 4 means the original file is 4 times larger than the compressed file, which is also written 4:1 (4 to 1). The two notations are interchangeable.
Does recompressing an already-compressed file give an additional gain?
Generally very little, if any: once redundancy has already been eliminated by an effective first compression, little remains to compress further. Recompressing a ZIP file into another ZIP, for example, typically gives a near-zero gain, and can even slightly increase the size because of the second format's header.