JPG and JPEG are one format

There is no JPG format and no JPEG format sitting beside it. There is one format, specified by the Joint Photographic Experts Group and published in 1992, and two customary spellings of the extension people write on the end of the filename. Open a .jpg and a .jpeg in any hex editor and both begin with the same two bytes. Neither is newer, neither is better, neither compresses more, and converting one to the other is not a conversion — it is a rename.

The reason there are two is MS-DOS. Its filesystem allowed eight characters for a name and exactly three for an extension, so a format whose natural abbreviation was four letters had to lose one, and JPEG became JPG on every PC. Unix and the Macintosh had no such limit and kept all four. When Windows 95 lifted the restriction the habit was thirty years old and stayed. The same history produced .htm beside .html and .tif beside .tiff, and none of those pairs are different formats either.

The extension tells you nothing, but two JPEGs really can differ in ways that matter, and it is worth knowing which questions are the real ones.

Baseline against progressive. A baseline JPEG is stored top to bottom and paints in that order. A progressive JPEG stores several passes of increasing detail, so it appears blurry and sharpens. Both are JPEG, both open everywhere current, and progressive is usually slightly smaller on large pictures. A handful of very old tools and some embedded printers refuse progressive files.

Chroma subsampling. JPEG splits a picture into brightness and colour, and can store the colour at half or a quarter of the resolution because the eye notices less there. 4:4:4 keeps colour at full resolution; 4:2:0 quarters it and is what most encoders do by default. On a photograph it is invisible. On red text against white it produces a visible smear.

Quality. The number an encoder was given when the file was written. It is not stored as a label — a JPEG does not know what quality it was saved at — and it cannot be raised afterwards. Re-saving a quality-40 JPEG at 95 makes a larger file containing the same damage.

Colour profile. A JPEG can carry an ICC profile describing what its numbers mean. Two files with identical pixel values and different profiles are different pictures. Most JPEGs on the web carry none and are assumed to be sRGB.

Upload forms usually list what they accept in an attribute the browser reads, and most list both spellings. When one appears to accept only .jpeg, the fastest fix is to rename the file. On Windows, that requires file extensions to be visible: in Explorer, the View menu, then File name extensions. On macOS, renaming in Finder prompts for confirmation and then does exactly the right thing.

Renaming is safe in that direction. It is not safe in the other: renaming a .png or a .heic to .jpg does not convert anything, and the receiving system will either reject it for having the wrong contents or accept it and fail later. Converting means re-encoding the picture, which is what the converter on this site does.

Which extension should I use?

Whichever the thing receiving the file asks for. If it does not ask, use .jpg — it is the more common of the two and no software prefers the longer form.

The form says it wants JPEG and my file is .jpg. Will it be rejected?

Almost never. Upload forms check the file's contents or accept both spellings. If one genuinely refuses, renaming the file from .jpg to .jpeg is safe: nothing inside the file changes and nothing needs re-saving.

Is .jfif the same thing too?

Effectively yes. JFIF is the interchange format that describes how a JPEG stream is wrapped for storage, and files saved with that extension — Windows used to produce them when saving from a browser — are ordinary JPEGs. Renaming to .jpg works.