# Is a real Grayscale image always 8 bit per pixel indexed?

I tested several Grayscale images downloaded from the Internet. Some of them are 8bpp indexed while others are 24bpp (true colored). Looks like different Grayscale images have different depths.

For instance, this online application converts a color image to Grayscale. But, when I check its bit depth, I see that bit depth is not changed.

I think, Grayscales with 24bpp are pseudo-grayscales. That is, they are the average of R, G and B components. I think, real Grayscales are always 8 bit per pixel indexed.

Is my notion correct?