Compress a JPEG — artifacts, quality and best settings

How JPEG compression works

The JPEG standard divides an image into 8×8 pixel blocks and applies a mathematical transform called the Discrete Cosine Transform (DCT) to each one. The DCT separates visual information into frequency components: the low-frequency component captures broad colour and tone, while high-frequency components carry fine detail and texture.

The quality slider controls a quantisation table that decides how aggressively those frequency components are rounded. At quality 100, almost nothing is discarded. At quality 50, large frequency coefficients are rounded to zero, making the file much smaller but introducing visible block patterns.

The sweet spot for most photographs sits between quality 60 and 85. In that range, the encoder discards detail the human eye rarely notices — subtle noise, micro-textures in smooth areas — while preserving edges, contrast and colour transitions.

Visible artifacts explained

Understanding artifact types helps you diagnose over-compression and choose the right quality floor for your content.

• Blocking — the 8×8 DCT grid becomes visible as a mosaic of flat squares. Most common in smooth gradients (sky, skin, solid backgrounds) at quality below 60.
• Ringing — false bright or dark halos appear along high-contrast edges (text on a white background, a dark logo). These come from zeroed high-frequency coefficients and are visible at quality 55–70 on graphic content.
• Colour banding — smooth colour transitions break into distinct steps. Happens in illustrations, sky gradients and infographics compressed below quality 65.
• Chroma smearing — colours bleed horizontally because JPEG chroma subsampling (4:2:0) halves colour resolution. Affects saturated subjects like flowers or flags.

Photographs with complex textures (grass, sand, fabric) are very tolerant of compression. Illustrations, screenshots and images with text are not — keep them at quality 85 or above, or switch to PNG.

Recommended quality by use case

• Website photos (hero, blog, product) — quality 75–80. Reduces a 3 MB camera JPEG to 200–400 KB with no visible loss at screen resolution.
• Email attachments — quality 70–75. Keeps the file under 500 KB, fast to send and preview.
• Social media (Instagram, LinkedIn) — quality 80–85. Platforms re-compress uploads; sending a higher-quality source limits cumulative degradation.
• Print (A4, poster) — quality 90–95. At print resolution (300 dpi) artifacts are visible on paper.
• Archiving originals — quality 95–100, or switch to PNG for lossless storage.
• Thumbnails and previews — quality 65–70 is acceptable when the displayed size is small (under 200 px wide).

Converting to WebP as an alternative

When your target is a website or a modern app, converting the JPEG to WebP rather than re-compressing it as JPEG often gives better results. The WebP codec achieves roughly the same perceived quality as JPEG at 25–40% less file size.

WebP uses a more modern prediction algorithm that avoids the hard 8×8 block boundary of JPEG. Blocking artifacts are replaced by subtler directional blur, which the eye tolerates better at equivalent quality settings.

All major browsers have supported WebP since 2020. The only contexts where JPEG remains the safer choice are email clients (Outlook), older desktop software, and print workflows.

Step-by-step: compress a JPEG with Flowfiles

1. Open the Flowfiles image compressor. No account is needed.
2. Drag your JPEG file into the drop zone, or click to browse. You can add multiple files at once.
3. Select the output format. Choose JPEG to stay in the same format, or WebP to switch codec.
4. Set the quality slider. Start at 80 and preview the result. Lower the value if the file is still too large.
5. Optionally set a target file size (e.g. 200 KB) and let the tool find the right quality automatically.
6. Click Compress. The result is processed in your browser — nothing is uploaded.
7. Download the compressed file or the ZIP archive for batch exports.

Frequently asked questions