Comparing two compression tools using ratios measured from different starting files, which tells you nothing about either encoder.
Compression
Compression ratio
Also called compression rate or size reduction ratio. Here is what it means, when it changes what you export, and what people get wrong.
CompressionRuns on your deviceUpdated 10 August 2026
The short answer
Compression ratio is the size of the original image divided by the size of the compressed file, written as something like 10:1. A 10:1 ratio means the compressed file is one tenth the original, a 90 percent saving. The same encoder settings give wildly different ratios on different pictures.
Updated
The short version
Compression ratio at a glance
| Quick fact | Detail |
|---|---|
| Formula | Original size divided by compressed size |
| Written as | 10:1, or 90 percent saved |
| Depends on | Image content, format, quality setting |
| Lossless range | Modest on photos, large on flat graphics |
| Diminishing returns | Past roughly 10:1 on most photographs |
Slide the table sideways to see every column.
How do you work out a compression ratio?
Divide the original size by the compressed size. A 12 MB file that comes out at 1.2 MB has a ratio of 10:1. To turn a ratio into a percentage saved, subtract its reciprocal from one: 10:1 saves 90 percent, 4:1 saves 75 percent, 2:1 saves 50 percent.
The two numbers behave very differently as compression gets stronger. Going from 2:1 to 4:1 saves another quarter of the original file. Going from 20:1 to 40:1 saves only another 2.5 percent, while roughly doubling the amount of detail thrown away. Once you are past about 10:1, chasing a bigger ratio buys very little and costs a lot.
Why can't you compare one tool's ratio with another's?
Two reasons, and both make published figures close to meaningless on their own.
The first is the baseline. A ratio is only defined relative to whatever you called the original. Measuring against an uncompressed bitmap gives a huge number. Measuring against a JPEG that was already compressed once gives a small one. A tool advertising 80 percent savings may simply be starting from a bloated source.
The second is image content. A studio product shot on a flat white background compresses enormously, because most of the frame is one colour. A photograph of gravel, foliage or knitted fabric at the same quality setting may barely compress at all. Any honest comparison runs both encoders over the same set of test images and reports the sizes at matched visual quality, not a single headline ratio.
What people get wrong about compression ratio
Each one is a real failure mode, not a style preference.
Quoting a ratio achieved on a flat white product shot as if it will hold for detailed or textured photographs.
Treating a higher ratio as automatically better, when past a point it just means more visible artefacts.
Confusing the ratio with the percentage saved, so 10:1 gets reported as a 10 percent reduction.
Questions people ask
Compression ratio, answered
The follow-up questions people search for once they have the definition.
What is a good compression ratio for a photograph?
There is no single figure, because it depends on the picture. A more useful target is the outcome: get the file under whatever limit you face at a quality setting where you cannot see damage at full viewing size. Whatever ratio that produces is the right one.
Why did compressing my image barely shrink it?
Usually because it was already compressed. Re-encoding an existing JPEG at a similar quality removes very little, since the detail an encoder would discard has already gone. Fine texture across the whole frame also resists compression, as there is little redundancy to exploit.
Does a bigger ratio always mean worse quality?
Not on its own. Some of a large ratio comes from the picture being simple, which costs nothing visually. It only signals damage when you push the quality setting down to reach it, rather than the content earning it.
Same cluster
More from the glossary
Neighbouring entries, so every term in the set is one hop from every other.
Contrast
Contrast is the tonal separation between the light and dark parts of an image.
Compression artefact
A compression artefact is visible damage introduced by lossy encoding: blocky squares, halos around hard edges, muddy colour at boundaries, or stepped gradients.
Cut-out
A cut-out is a photo with its background removed so the subject stands alone, ready to drop onto white, a colour or a new scene.
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