Carbon residue for a fossil fuel can be defined as the tendency of that fuel to form carbon deposits at high temperature in an inert atmosphere. Carbon residue for a fuel is measured in weight percentage (wt %) or parts per million by weight (ppm wt). High carbon residue value is undesirable for a fuel.
Different carbon residue values can be defined for a fuel depending on the measurement method used. Ramsbottom Carbon Residue (RCR), Conradson Carbon Residue (CCR), Micro Carbon Residue (MCR) are the indexes used corresponding to three different measurement methods used.