File Compression Ratio Calculator

Compare original and compressed file sizes

Your result will appear here.

About the File Compression Ratio Calculator

File Compression Ratio Calculator compares original and compressed file sizes to show how much storage was reduced. It is useful for images, archives, audio, video and backups where the effect of compression needs to be quantified.

Compression ratio and percentage reduction are related but not identical. A 4:1 ratio means the original file is four times the compressed size, which corresponds to a 75% reduction rather than 400% savings.

How to Use the File Compression Ratio Calculator

  1. Enter original file size.
  2. Enter compressed file size using the same unit.
  3. Calculate compression ratio.
  4. Calculate percentage reduction separately if you want to describe storage saved.

How the Calculation Works

Compression ratio = original file size ÷ compressed file size. Both sizes must use the same unit before division, so converting one file to MB and leaving the other in GB would produce a meaningless ratio.

Percentage reduction = (original size−compressed size) ÷ original size ×100, while compressed fraction remaining = compressed size ÷ original size ×100. These related measures should be reported separately because a ratio such as 4:1 corresponds to a 75% reduction, not a 400% reduction.

Worked example

File Compression Ratio Calculator Worked Example

800 MB original compressed to 200 MB

Compression ratio = 800÷200 = 4:1. Reduction = (800−200)÷800×100 = 75%.

Result guide

Understanding Your File Compression Ratio Calculator Result

A 4:1 ratio indicates the compressed file occupies one quarter of the original size. It says nothing by itself about whether compression was lossless or whether quality changed. A related measure is space savings, sometimes written as 1−compressed/original. Keeping ratio and savings percentage separate avoids statements such as “4:1 means 400% smaller,” which is not mathematically correct.

Before you rely on the answer

Assumptions and Limitations

  • Both file sizes use the same unit basis.
  • Original size is positive.
  • Files represent the same underlying content or compression operation.

Practical tip

Using This Result in Practice

When comparing compression methods, record both ratio and output quality or recoverability. A smaller file is not automatically better if important detail was discarded.

Related calculations

Useful Calculations to Continue With

If you need an uncompressed image-size estimate before comparing compression, use the Image File Size Calculator.

If the file is video and bitrate determines its final size, use the Video File Size Calculator.

Common questions

File Compression Ratio Calculator FAQ

What does 5:1 compression mean?

The original file is five times the compressed file size.

Is 5:1 the same as 80% reduction?

Yes; the compressed file is 20% of the original, so 80% was removed.

Does compression ratio tell me whether quality was lost?

No. You also need to know whether the compression method is lossy or lossless.