The ratio, and the percentage it maps to
These are two views of the same result. The ratio is how many times smaller the file got; percent saved is how much space you got back. A 4:1 ratio and 75% saved describe the exact same compression. Confusing the two is the usual mistake — a "50% compression" is only a 2:1 ratio.
Ratio and percent saved, side by side
| Ratio | Space saved | Ratio | Space saved |
|---|---|---|---|
| 1.5:1 | 33.3% | 5:1 | 80% |
| 2:1 | 50% | 10:1 | 90% |
| 3:1 | 66.7% | 20:1 | 95% |
| 4:1 | 75% | 50:1 | 98% |
Percent saved climbs toward 100 but never reaches it, and each extra point costs a much bigger ratio: going from 90% to 95% doubles the ratio from 10:1 to 20:1.
What ratio to expect by format
Ratio depends on the data and the codec, not just the format. Lossless formats keep every bit and stay modest; lossy formats discard imperceptible detail and go much further.
- ZIP / GZIP (lossless). Around 2–3:1 on text and documents, near 1:1 on data that is already compressed.
- PNG / FLAC (lossless). Roughly 1.5–3:1; nothing is thrown away, so the ceiling is low.
- JPEG / MP3 (lossy). About 10:1, tunable by quality; the loss is permanent.
- H.264 video (lossy). 50:1 and beyond, which is why raw video is impractical to store or stream.
Common questions
How do I calculate a compression ratio?
Divide the original size by the compressed size. 100 MB down to 10 MB is a 10:1 ratio. The two files must be in the same units before you divide.
What is the difference between the ratio and the percent saved?
The ratio compares sizes (original divided by compressed); percent saved is the space you got back, figured as 1 minus compressed over original, times 100. A 2:1 ratio is 50 percent saved. A 10:1 ratio is 90 percent saved.
Why did my file get bigger after compressing it?
Already-compressed data (JPEG, MP4, ZIP) and truly random data have little redundancy left to remove, and the archive format adds a small header. On tiny or pre-compressed files that overhead can outweigh any saving.
