JPG (JPEG): the format for photos you share

JPG or JPEG?

They are the same format. JPEG is named after the Joint Photographic Experts Group, which published the standard in 1992. The three-letter .jpg extension comes from old Windows and DOS file names, which allowed only three characters after the dot. .jfif files — which Windows sometimes produces when you save an image from a browser — are also ordinary JPEGs in a slightly different wrapper. Any tool that opens a .jpg opens them, and renaming one to .jpg usually works too.

How JPEG makes photos small

JPEG compression is a series of steps, each of which throws away something the eye is bad at noticing:

1. Brightness and colour are separated. People see detail mostly in brightness, so the colour information is usually stored at half the resolution (chroma subsampling). On a photo you cannot tell. On thin red text on white, you can.
2. The picture is cut into 8×8-pixel blocks, and each block is described as a mix of smooth and fine patterns.
3. Fine patterns are rounded off. This is where the quality setting acts. Lower quality rounds more aggressively, which removes texture first and then starts to show the block grid.

That is why JPEG routinely stores a photo in around a tenth of the space its raw pixels would take and still looks the same at normal viewing size, while a screenshot of text gets fuzzy halos around every letter. Photos are made of smooth transitions JPEG can approximate. Text is made of hard edges it cannot.

Quality settings are not a percentage

The 0–100 quality number is not "percent of the original kept", and it is not consistent between programs — quality 80 in one app is not the same file as quality 80 in another. As a practical guide in ImageDoctor:

Rather than guessing, the Image Compressor can aim for a file size — 200 KB, say — and find the highest quality that fits.

Every save costs a little

Opening and viewing a JPEG changes nothing. Saving it again runs the lossy steps again, so a JPEG that is edited and re-saved many times gets gradually worse — like photocopying a photocopy. Keep your original, edit from it, and export a JPEG once at the end.

Hidden data in JPEG files

Camera and phone JPEGs usually carry EXIF metadata: the device model, exposure settings, the date and often the GPS position where the photo was taken. It travels with the file when you share it. The Metadata Viewer shows what a file contains, and the Metadata Remover strips it from a JPEG without re-encoding the picture, so there is no quality loss.

When to use JPEG

• Photos you hand to someone else — email attachments, job and government upload forms, print shops. JPEG is the one format they will all accept.
• Photos where transparency is irrelevant and small size matters more than perfect fidelity.
• Anything going into older software or hardware that predates WebP and AVIF.

When not to

• Logos, icons and anything with a transparent background. Use PNG, WebP or SVG.
• Screenshots, text and line art. The edges ring and blur. PNG or WebP keeps them crisp.
• Images on a website you control. WebP is typically 25–34% smaller at the same visual quality, and AVIF smaller still.
• Files you are still editing. Use a lossless format for the working copy and export JPEG last.