Which JPEG quality setting should you use?

Every program that saves JPEG files asks for a quality between 0 and 100, and most people leave it wherever it was. That number decides how much of the photo is thrown away, and the trade between size and quality is far from linear. We saved six photos at twelve quality settings and measured what each step actually changes.

What the quality number does

JPEG splits a photo into blocks of 8 by 8 pixels and describes each block as a mix of simple patterns, from flat to finely striped. The quality setting decides how precisely those patterns are stored: high quality keeps the fine stripes, low quality rounds them away. Fine detail and sharp edges suffer first, which is why a heavily compressed JPEG shows blocks and halos around edges.

The number itself is not a percentage of anything, and it is not the same between programs. Quality 80 in Photoshop, in a phone camera and in a web tool can give quite different files. What follows uses MozJPEG, the encoder in our image compressor, which makes smaller files than the standard libjpeg at the same setting.

Size and quality at each setting

We saved six different photos, each 2048 pixels wide, at every setting from 20 to 100 and measured the file size and the SSIM score, which describes how similar the result looks to the original on a scale from 0 to 1. Averaged over the six photos:

Going from 30 to 75 more than doubles the file and visibly cleans up the picture. Going from 75 to 95 quadruples it for an improvement most people only notice when they zoom in. The last five steps, from 95 to 100, multiply the size by 2.6 again for a difference no one can see at normal size.

What it looks like

Neon letters on a dark background are a hard case for JPEG: bright, saturated colour next to near black. Here is the same part of a sign at four settings, enlarged twice so each pixel is visible. The sizes are for the whole photo.

Quality 30, 114 KBQuality 95, 1,069 KB

Why sizes jump at 80 and 90

Two steps in the table are much bigger than their neighbours: 75 to 80 adds 37 percent and 85 to 90 adds 67 percent. The quality of the patterns does not change suddenly there. Something else does.

Our eyes are much less sensitive to fine detail in colour than in brightness, and JPEG exploits it with chroma subsampling: it can store the colour information at half the width (4:2:2) or at half the width and half the height (4:2:0), keeping brightness at full resolution. MozJPEG, set to choose automatically, uses 4:2:0 below quality 80, 4:2:2 from 80 to 89 and full colour (4:4:4) from 90. On the macaw photo, quality 79 gave 174 KB and quality 80 gave 220 KB; quality 89 gave 339 KB and quality 90 gave 469 KB.

For most photos the halved colour is invisible. It shows mainly on sharp edges between saturated colours, such as red text on blue, or neon. If you save screenshots or graphics as JPEG (which you usually should not), a setting of 90 or more avoids colour fringes around text.

Not every photo reacts the same

At quality 75, our photos ranged from 90 KB (the cat, soft fur on a plain background) to 394 KB (the lake, with thousands of trees). Detailed photos need more bytes at any setting, and they also show artefacts later, because busy detail hides small errors. Smooth areas such as sky, skin and studio backgrounds show blocks and banding first. When you choose a setting for a whole batch, check a photo with large smooth areas, not the busiest one.

Which setting to use

• Photos for a website or a social network: 70 to 80. Remember that most social networks recompress whatever you upload.
• Photos sent by email or chat: 75 is plenty, and it keeps attachments small.
• Photos for print, or that you will edit and save again: 85 to 90. Every save as JPEG loses a little more, so leave room.
• Archiving originals: keep the camera file. If you must convert, 95 is a reasonable ceiling; 100 wastes space.
• Anything with text, lines or flat colour: PNG instead of JPEG. See JPG, PNG, WebP or AVIF.

If you need a file under a specific size, for an upload form for example, the image compressor can search for the highest quality that fits a target such as 100 KB and shows you the result before you download. It runs in your browser, so your photos stay on your device.