Is JPG and JPEG the same? Yes, and here is why both exist

Is JPG and JPEG the same? Yes, one format with two names

JPG and JPEG are the same image format. The registry of internet media types files both under one type, image/jpeg, and its entry lists the file extensions as “jpg, jpeg”. Software tells a JPEG by its first three bytes, FF D8 FF, not by its name. So we tested it: one JPEG saved under five names.

Every name gave the fingerprint 12159f2d13fb60fe, so not one byte differs. The name is outside the image data, which is why a rename can neither help nor hurt a picture.

Are JPG and JPEG the same quality?

Yes, because they are the same bytes. Google shows a forum reply for this question that claims a JPG keeps more quality and takes more space; the hashes above say otherwise. Quality is set when a picture is encoded, by the quality number, never by the extension.

What does JPEG stand for?

JPEG stands for Joint Photographic Experts Group, the committee that wrote the standard. The standard itself, ITU-T Recommendation T.81, defines the name as “the informal name of the committee which created this Specification.” The group formed in 1986. Its standard was approved on 18 September 1992, and the identical text was published by ISO/IEC as 10918-1.

The “joint” is two worlds working together: the ISO/IEC standards bodies and the telecom side, then called CCITT and now ITU-T. The committee’s own page says it does not mean ISO and IEC together. Today the group is ISO/IEC JTC 1/SC 29/WG 1, and it still meets about four times a year.

What is a JPEG?

A JPEG is a photo stored with the JPEG method of lossy compression. The picture is cut into blocks of 8 × 8 pixels, each block is turned into a set of frequencies, and the finest of them are rounded away. The quality number decides how much is rounded. The Library of Congress puts typical settings at 10:1 to 20:1 smaller than the raw pixels. We put our 9.8 MB camera photo, resized to 1600 pixels wide, through our engine at seven qualities:

From quality 95 to 75 the file shrank from 412.8 KB to 94.1 KB, and the picture looks the same at normal size. Below about 40 the blocks start to show. Enlarged four times, here is the same bow of a boat at three qualities:

A JPEG has limits worth knowing. It has no transparency, its common baseline form stores 8 bits per colour channel (T.81 also defines a 12-bit extended mode), and its largest picture is 65,535 × 65,535 pixels. For a logo or a screenshot, the PNG vs JPG comparison shows when PNG wins.

Why are there two extensions, .jpg and .jpeg?

Because the standard never named one. T.81 describes the compressed data, and the file format most JPEGs use, JFIF 1.02 by Eric Hamilton of C-Cube Microsystems, dated 1 September 1992, describes the header. Neither mentions a file extension. The extension was left to the computers.

In 1992 a PC running MS-DOS or Windows 3.1 used the FAT file system, which allowed 8 characters for the name and 3 for the extension. A four-letter .jpeg did not fit, so .jpg did. Windows 95, which shipped on 24 August 1995, brought names of up to 255 characters, and .jpeg worked from then on. Both stuck.

Why do cameras and phones save .JPG in capitals?

Because the camera standards say so. The Exif standard, first published in October 1995, sets file names of 8 characters or fewer and the extension “.JPG” in its version 2.3 text. DCF, the camera file system rule from 1998, puts it plainly: “The extension is ‘JPG’.” It also makes the first four characters upper case, then a number from 0001 to 9999.

That is where names like DSC00001.JPG and the DCIM folder come from: DCF says DCIM stands for Digital Camera Images. A name that starts with an underscore, such as _DSC0001.JPG, marks a photo in the optional colour space DCF allows. Both rules were built on the FAT 8.3 names above, which is why camera files end in .JPG.

Which extension does your browser save?

It depends on the browser, even for the same file. We served our JPEG from a local address with no extension, labelled image/jpeg, and downloaded it in the three browser engines on a Mac:

Firefox’s own table lists jpg first; Chromium and WebKit follow the Mac, which prefers jpeg. On Windows, Chromium follows the system’s mapping, which on recent versions can name the file .jfif. That is the same format again; the JFIF to JPG page explains the Windows setting behind it.

What is inside a .jpg file: JFIF, Exif or neither

Two JPEGs can differ inside while sharing a name. After the FF D8 start marker, a JFIF file carries a JFIF header, a camera file carries an Exif header with the shot’s details, and some files carry neither. We read three files segment by segment: our camera photo, a 400 × 267 JPEG we wrote with a camera-style Exif header, and a JPEG our engine wrote.

Our engine writes no JFIF or Exif header at all, and every browser opens its files, because browsers check the first bytes. What a name cannot do is turn one format into another. We saved a PNG as photo.jpg: it still starts with 50 4E 47, the letters PNG, and both macOS and sharp still call it a PNG.

Should you rename .jpeg to .jpg, or use one over the other?

Rename when a form or an app insists on one spelling: the file inside does not change. A converter re-encodes the picture, which changes the bytes and can lose detail, so use one only when you want a smaller file or a real change of format. The JPEG to JPG page has the rename steps for Windows 11 and Mac and converts a batch when a rename is not enough.

For your own files, pick .jpg in lower case and keep to it. The Library of Congress notes that .jpg is the extension used most often, and cameras write it. On a website, a file named photo.JPG and a link to photo.jpg can be two different addresses, so one consistent spelling saves broken images.