Image Compressor

Reducing image file size without uploading anywhere

Choose an image and adjust the quality slider to re-compress it as a JPEG using your browser's own image encoder. Lower quality settings produce smaller files at the cost of some visual fidelity - the before/after size comparison updates live, so you can find the sweet spot between file size and image quality for your use case (web pages generally look fine at 70-85% quality for photos).

Why faster-loading images matter beyond just file size

Image weight is one of the biggest contributors to slow page load on most websites - and page speed is both a direct Google ranking factor and a major influence on whether visitors stay or bounce. Optimizing images before uploading them is one of the highest-leverage, lowest-effort ways to improve a page's real-world loading performance.

Why this always re-encodes as JPEG

JPEG compression is what makes quality-adjustable file size reduction possible - PNG uses lossless compression with no quality slider, so converting to JPEG is necessary for this kind of optimization. If your original image needs transparency preserved, converting it to JPEG here isn't suitable, since JPEG has no transparency channel.

Why the size estimate is calculated, not measured directly

The displayed "optimized" size is calculated from the length of the generated image data rather than from an actual saved file - this is an accurate estimate of the real download size, since it reflects the same encoded bytes the browser would write to a file, just computed before the download step itself.

よくある質問

Why does the output always save as JPEG?

JPEG compression is what makes quality-adjustable file size reduction possible - PNG uses lossless compression with no quality slider, so converting to JPEG is necessary for this kind of optimization. If you need transparency preserved, PNG is not suitable for this approach.

Is there a quality level that works for most images?

Around 75-85% quality is a common sweet spot for photos - noticeably smaller file size with minimal visible quality loss. Graphics with sharp edges or text may need higher quality to avoid visible artifacts.

Does smaller image file size really affect my site's ranking?

Yes, indirectly - page load speed is a Google ranking factor, and images are typically the heaviest part of a page's total weight, so optimizing them is one of the most effective ways to improve real-world load times.

Is my image uploaded to a server for this?

No - the file is read and re-encoded entirely in your browser using the canvas API; nothing is sent anywhere, which also means very large images are limited only by your own device's memory.

Can I optimize a PNG and keep transparency?

Not with this tool, since it re-encodes everything as JPEG, which has no transparency support. A dedicated PNG-specific compressor would be needed to preserve transparency while still reducing file size.