Convert RGB to Grayscale in ImageMagick command-line

Question

How do I convert a RGB image (3 channels) to a grayscale one, using the (r+g+b)/3 method? I look through an examples page: http://www.imagemagick.org/Usage/color_mods/#grays

Answer 1

convert -set colorspace Gray -separate -average gives the best result for any image for me.