A true one-channel JPEG: only brightness stored, the grey mix your choice, the size against colour measured.
Grey pixels are not a grey file
Most converters make a black-and-white picture by painting every pixel grey and then saving an ordinary colour JPEG. The file still carries two colour channels, they just happen to be flat. This page uses its own encoder to write a single-channel JPEG instead: brightness only, which is what “grayscale” means to the format.
The Quality tab opens with the sampling set to Greyscale and the similarity measured. Under Colour sampling it also writes a colour copy at the same quality and prints both sizes, so the saving you see is two real files, not a rule of thumb.
Three steps
- 1Open or paste the PNG. The JPG side of the stage shows the grey file itself, decoded back.
- 2Pick the grey mix: Rec. 709 for today's screens, Rec. 601 to match older software.
- 3Save. The button shows the measured size of the one-channel file.
When not to use it
- A form that asks for a colour photo: a one-channel file fails that check, and the forms page says so.
- Pictures you may want in colour later: the JPG holds no colour to bring back. Keep the PNG.
- Line art or text that must stay crisp: grey or not, JPEG softens hard edges — the verdict will say when the PNG is better.
Questions about grayscale JPGs
What is a one-channel JPG?
A JPEG that stores brightness only. A colour JPEG stores three channels — brightness and two colour-difference channels — even when every pixel is grey. A one-channel file has one, so there is nothing left for a viewer to tint, and the file is usually smaller. The page reads the channel count back out of the written file and prints it.
How much smaller is it than a colour JPG?
It depends on the picture, so the page measures it: it writes a colour copy at the same quality and prints both sizes. Colourful pictures save more than ones that were nearly grey already.
What is the difference between Rec. 709 and Rec. 601?
They are two recipes for mixing red, green and blue into grey. Rec. 709 (0.2126 red, 0.7152 green, 0.0722 blue) matches sRGB screens and is the default here. Rec. 601 (0.299, 0.587, 0.114) is the older video mix: reds come out lighter and blues darker. Use it to match older software.
What happens to the transparent parts?
The matte goes under them first, as on every page here, and then the whole picture is turned grey. A coloured matte therefore becomes a grey too.
Can I go back to colour from the JPG?
No. The colour is not in the file at all. Keep the PNG if you may need colour later.
