sRGB channel values 0–255—quick luma estimate (not WCAG contrast full pipeline).
The page uses the common BT.601 luma weights Y = 0.299R + 0.587G + 0.114B with RGB channels clamped to 0–255. Gamma and alpha are not handled.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.