Guetzli is a JPEG encoder from Google that targets high visual quality at small file size. Files come out 20 to 30 percent smaller than libjpeg at equal quality according to the project. Encoding needs about 300 MB of memory and about one minute of CPU time per megapixel. Its owner archived the repository on April 18, 2026.
Large photos slow down pages and many sites look for smaller JPEG files. Guetzli trades encoding time for size and the project says so openly.
Facts follow the project README on GitHub. Size claims come from the project and this guide did not test them.
What is Google Guetzli?
Guetzli is a JPEG encoder. README text says it aims for excellent compression density at high visual quality. Output files are sequential JPEGs and never progressive ones.
What does Guetzli Cost in Time and Memory?
| Item | README lists |
|---|---|
| Memory | About 300 MB per megapixel of input |
| CPU time | About one minute per megapixel |
| File size | Typically 20 to 30 percent smaller than libjpeg at equal quality |
| Inputs | PNG or JPEG |
By those figures a 20 megapixel photo needs about 6 GB of memory and about 20 minutes on one core.
How do You Use Guetzli?
guetzli [--quality Q] [--verbose] original.png output.jpg- quality. Sets quality in libjpeg equivalent units.
- verbose. Prints a trace of encoding attempts.
- JPEG input. Syntax is identical for a JPEG file.
Which Limits should You Know?
- Color. Guetzli assumes sRGB with gamma 2.2 and ignores color profile metadata.
- Source quality. Use high quality uncompressed sources. Re encoding compressed images gives poorer results.
- Transparency. JPEG has no alpha channel so transparent PNG files flatten onto a black background.
- Status. Its owner archived the repository on April 18, 2026 so nobody can push changes.
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Frequently Asked Questions
What is Guetzli?
Google built this JPEG encoder for small files at high visual quality.
How Much Smaller are Guetzli Files?
20 to 30 percent smaller than libjpeg at equal quality according to the project. This guide did not test that claim.
How Slow is Guetzli?
About one minute of CPU time per megapixel according to the README.
Does Guetzli Keep PNG Transparency?
No. JPEG has no alpha channel so transparent areas become black.
Is Guetzli Still Maintained?
No. Archiving on April 18, 2026 ended new work.
Key Takeaways
- Google built Guetzli as a JPEG encoder.
- It needs about 300 MB of memory per megapixel.
- Encoding takes about one minute per megapixel.
- Transparent PNG files flatten to black.
- Maintenance ended with the archive.