The retention factor or Rf value of a compound in TLC is a measure of how far a sample has moved up the plate in relation to the solvent front. It is a ratio and can be calculated as:
A TLC experiment should aim to have an Rf value in the range of 0.3 - 0.7. Rf values are specific to a solvent system and the sample, therefore if the experiment was replicated a database of Rf values can be used to identify the different compounds on the plate, for specific compounds and solvent systems. The value is most often used to distinguish between spots in a Lab report.