What it is
A WebP file can hold either of two entirely different things. The lossy side is built on the intra-frame coding of a video codec: it predicts each block from its neighbours before transforming what is left, which is why it holds a smooth gradient together at settings where a block-independent encoder starts to show its grid. The lossless side is a different algorithm again, closer in spirit to PNG but with a larger repertoire of transforms.
Both sides carry an alpha channel, which is the practical reason the format spread: it is the first widely supported option that is both smaller than JPEG on photographs and able to do what PNG does on transparency.
Where it opens
The rows below are about software a file will actually be handed to, not about a specification. A format is supported when the person you sent it to can open it.
| Where | What happens |
|---|---|
| Chrome, Edge, Firefox, Opera | Supported for years, lossy and lossless alike. |
| Safari and iOS | Supported since Safari 14 and iOS 14, which covers every device still receiving updates. |
| Windows 10 and 11 | The built-in photo viewer opens them. |
| macOS | Preview and Quick Look open them on Big Sur and later. |
| Older desktop software | Plenty of installed image editors, office suites and mail clients from before 2021 still do not know what it is. |
What it costs to write
Native in every browser this bench supports, and feature-detected rather than assumed: a browser that quietly answers a WebP request with a PNG is caught before anything is offered for download.
Where it goes wrong
- Lossy WebP with an alpha channel encodes the alpha losslessly and the colour lossily, so a semi-transparent gradient can look worse than the file size suggests.
- The lossy encoder is fond of smoothing. Fine film grain tends to go soft rather than go blocky, which some people prefer and some consider worse.
- An animated WebP handed to this bench is read as its first frame only. Every other frame is gone from whatever comes out.
- Somebody sending one to a colleague on a five-year-old laptop should expect a phone call.
When this is the right answer
A picture going onto a web page, where the browser is the only thing that has to open it and every current browser can. A photograph with transparency, which is the one combination JPEG cannot express and PNG stores expensively. Anywhere the twenty-odd per cent it typically saves over JPEG is worth having and the audience is a browser.
It is a worse choice for a file being emailed to someone, handed to a print shop, or opened in desktop software of unknown age. That is not a statement about the format; it is a statement about what is installed on other people’s computers.
Questions about WebP
- Is WebP always smaller than JPEG?
- No. It usually is on photographs at matched quality, and on some pictures — particularly ones with fine, high-contrast grain — it is level or worse. The bench exists precisely because the general answer and the answer for your file are different questions.
- Which of the two encoders does this site use?
- The lossy one, because that is what a browser's canvas exposes when a quality value is passed. A lossless WebP toggle is on the backlog rather than in the page, and until it is here the PNG card is the lossless candidate.
- Will a WebP open on an iPhone?
- Yes, since iOS 14. Every iPhone still receiving updates displays them in Safari, Messages and Photos.
- Why does my transparent WebP look worse than its size suggests?
- Because lossy WebP stores the alpha channel losslessly and the colour lossily. On a soft, semi-transparent edge the two halves degrade differently, and the seam between them is what you are seeing.
- What happens to an animated WebP on this bench?
- Only its first frame is read. Everything saved from this page is a single still, and the loss of the other frames is stated on the page rather than hidden in a footnote.
Encoding happens on this machine; the file stays where it is.