Convert WebP to Grayscale

Image decoding is asynchronous in the browser. Reading the dimensions too early would give a 0×0 canvas and a blank result, so the tool waits for the decode to finish, then sizes the canvas and applies the grayscale math to every pixel.

Different weights for R/G/B reflecting different colour primaries. Rec 601 (0.299R + 0.587G + 0.114B) was derived for NTSC TV phosphors in 1953 and lives on in JPEG / MPEG-2. Rec 709 (0.2126R + 0.7152G + 0.0722B) reflects modern sRGB / HDTV primaries – more accurate for content authored on modern displays. For photos shot on a modern camera and displayed on a modern monitor, Rec 709 is correct; for legacy SDR content, Rec 601 matches what colour-correction tooling expected.

Useful for non-photographic content (technical diagrams, vector art, screenshots of text) where matching human luminance perception doesn’t matter – you just want any single-channel projection. It’s also the simplest formula to explain in tutorials.

Grayscale conversion is a colour operation, not a transparency operation. Pixels that were 50% transparent stay 50% transparent; only their RGB values get collapsed to the equivalent gray.

Safari before iOS 14 and some older Firefox versions don’t support canvas.toDataURL('image/webp'). In that case, the browser falls back to PNG (the default for unrecognised types). The stats line reports the actual output format. Download will use the correct extension.

CSS filter is for display only – the source image stays full colour, the browser just renders it desaturated. This tool produces an actual grayscale file you can save and reuse. CSS filter also uses a hardcoded sRGB-style formula and doesn’t let you choose between algorithms.

No. All image processing happens through the Canvas API inside your browser – nothing is sent to a server, logged, or stored, and the tool keeps working offline once the page has loaded.

10 MB. Bigger files work in principle but get slow to read and decode in the main thread. For very large images, a Web Worker would be better – out of scope for this tool.

Grayscale images compress more efficiently than colour: the WebP encoder spends fewer bits on chrominance channels that are now constant. A 1 MB colour WebP often becomes a 400-700 KB grayscale WebP at the same quality setting.

The tool checks bytes 0-3 for RIFF (52 49 46 46) and bytes 8-11 for WEBP (57 45 42 50) – the standard WebP file signature per RFC 6386. Files that just have the .webp extension but contain something else are rejected.