JPG vs JPEG: they are the same format
JPG and JPEG are two spellings of one format. Only the file extension differs, the encoded image is identical, and renaming a file is not a conversion.
The short answer
JPG and JPEG are the same format. The encoding, the compression, the color data and the quality are identical. A file named photo.jpg and a file named photo.jpeg can be byte-for-byte the same file with a different name.
Any program that opens a .jpg opens a .jpeg. Any upload form that accepts image/jpeg accepts both extensions. There is no quality difference to choose between, and nothing is lost by using one spelling or the other.
Why two spellings exist
The format is named after its standards committee: the Joint Photographic Experts Group, which published it in 1992. The name is four letters long, so the natural file extension was .jpeg.
Older versions of Windows only allowed three-character file extensions. The extension was shortened to .jpg to fit, and the shorter form spread to websites, cameras and software everywhere. The four-letter form never disappeared: macOS, many professional applications and several camera makers still write .jpeg.
What the extra letter changes
Nothing inside the file. Both names point to the same internal structure: the same header, the same compression tables, the same pixel data. Renaming photo.jpeg to photo.jpg does not re-encode anything, does not change the file size, and does not move a single pixel.
This is why the question “which is better quality, JPG or JPEG?” has no answer. Choosing between them is a naming decision, not a format decision.
What .jfif is, and why some sites save it
Many JPEG files are stored inside a minimal container called JFIF, which records the pixel density and aspect ratio. A few browsers and web services therefore save images with a .jfif extension, and some people meet that name and assume it is a different format.
It is still JPEG data. If a tool refuses a .jfif file, renaming the extension to .jpg usually resolves it, because the bytes are unchanged.
Where the two names cause real problems
The name matters when software checks the extension instead of the content. An upload form that only lists .jpg can reject a perfectly valid .jpeg file. A script that filters on the last four characters can skip files it should accept. A colleague can receive a .jpeg, assume it is a different format, and convert it unnecessarily.
In those cases the fix is to rename the file, or to convert it if the destination also checks the declared media type. Both actions take seconds, and neither affects quality.
Renaming is not converting
Renaming a .jpeg to .jpg is safe because the content already matches the name. Renaming a PNG to .jpg is not: the file still contains PNG data, so a strict program will refuse to open it, and a lax one may show a broken or blank image.
The test is simple. If the source and the target are the same format, a rename is enough. If they are different formats, the file has to be decoded and re-encoded, which is what a converter does. Our PNG to JPG converter and JPG to PNG converter run that step inside the browser, so the file never leaves the device.
When you do need a real conversion
Change format when the destination needs a capability JPEG does not have. JPEG cannot store transparency, so a logo or a cut-out with a transparent background belongs in PNG. JPEG is also inefficient for screenshots, text and flat graphics, where PNG is both sharper and often smaller.
Go the other way when size matters more than perfect edges: a photograph saved as PNG is usually several times heavier than the same photo as JPEG. For the web, JPG to WEBP is often the smallest option that still looks correct. The format comparison guide explains the trade-offs in more detail.
A short checklist
Keep one extension per file and stay consistent inside a project, so nobody has to guess. Do not re-save an image just to change the name, because every save of a JPEG throws away a little more detail.
Before sending a file, check what the destination actually requires: some systems accept any JPEG data and only inspect the extension, while others verify the declared type. When in doubt, send .jpg, which is the extension almost every system recognises.