Case 1
- Original size
- 1000MB
- Compressed size
- 250MB
4.00
Open with these valuesCompression Ratio Calculator
4.00:1
Result: 4.00 :1The ratio is original ÷ compressed, so 1,000 MB down to 250 MB is 4:1 and saves 75 % of the space. A ratio below 1 means the file grew — 250 MB up to 500 MB is 0.5:1. Both sizes must use the same unit; the division cancels it out.
Worked examples
Case 2
- Original size
- 1000MB
- Compressed size
- 100MB
10.00
Open with these valuesCase 3
- Original size
- 500MB
- Compressed size
- 500MB
1.00
Open with these valuesHow it's calculated
ratio = original size ÷ compressed size
- StepEnter the size of the file before compression.
- StepEnter the size after compression, in the same unit.
- StepDivide the first by the second — that is the ratio.
- ResultSpace saved is 1 minus the reciprocal, as a percentage.
What this number means
A compression ratio compares two sizes of the same thing, and putting that comparison into a single number is what makes separate results comparable at all. It is the original divided by the compressed size, so it counts how many times the smaller copy fits into the larger one. With the default settings, 1,000 MB coming down to 250 MB is 1000 ÷ 250 = 4, written 4:1. Seen from the other side the same result is the space saved: 1 − 250 ÷ 1000 = 75 %. Both readings are one number, but they move at very different speeds — going from 4:1 to 10:1 lifts the saving only from 75 % to 90 %, because each further step has less left to take away. What the figure does not carry is any account of how it came about. Two sizes go in; the tool, its settings and the material stay outside, and the material is what dominates the result — log files reach 10:1 where a screenshot saved as PNG sits near 3.33:1. That is the limitation worth holding on to: this is a measurement of one file that has already been through the process, not a property of the process. A 4:1 measured on a text archive predicts nothing about the next file you hand it.
The ratio depends on the material
The same tool gives very different numbers depending on what it is fed: log files can reach 10:1, a text archive around 4:1, a screenshot saved as PNG about 3.33:1. A compressor has no single ratio of its own.
Already compressed data barely shrinks
Data that has been compressed once already, such as JPEG or MP3, has little left to remove. The container can even add overhead, which pushes the ratio below 1 and leaves the file larger than it was.
Direction and unit both matter
The ratio is original divided by compressed, so 1,000 over 250 gives 4 and not 0.25. Both fields must use the same unit, which the division then cancels out.
Commonly misread
A ratio of 4:1 means the file shrank by 25 %.
It shrank by 75 %, because the compressed file is a quarter of the original. Space saved is 1 minus the compressed size divided by the original.
A ratio below 1 has to be a mistake.
It means the file grew, which happens with already compressed data where the encoding only adds overhead.
The ratio is the compressed size divided by the original.
It is the other way round: 1,000 MB down to 250 MB gives 4, not 0.25.
Reference table
| Original, compressed | Typical case | Ratio |
|---|---|---|
| 250, 500 | Encoding overhead | 0.50 |
| 500, 500 | Already compressed | 1.00 |
| 1000, 300 | Screenshot as PNG | 3.33 |
| 1000, 250 | Text archive | 4.00 |
| 1000, 100 | Log files | 10.00 |
Questions
How is the compression ratio calculated?
Divide the original size by the compressed size. 1,000 MB becoming 250 MB gives a ratio of 4, usually written 4:1.
How does the ratio relate to space saved?
Space saved is 1 minus the compressed size divided by the original. A ratio of 4:1 saves 75 %, and 10:1 saves 90 %.
Can the ratio be below 1?
Yes, and it means the file got bigger. That happens with already compressed data such as JPEG or MP3, where the container simply adds overhead.
Does the unit matter?
No, as long as both fields use the same one. Bytes over bytes or gigabytes over gigabytes give the same number.
Sources and last check
Information, not professional advice.