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:

The R f value can be calculated by taking the distance travelled by the substance and dividing it by the distance travelled by the solvent front. On the right of the equation is a rectangular piece of paper or T L C plate. There is a reference line marked at the bottom of the plate with 3 ticks along the line for each sample. The solvent front is marked at the opposite end of the plate. The solvent front is 10 cm from the reference line. There are 3 spots on the plate, one for each sample. The first sample is 1.5 cm from the reference line, and has an R f of 0.15. The second spot is 3 cm from the reference line, and has an R f of 0.3. The third spot is 7 cm from the reference line, and has an R f of 0.7.

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.

NOTE: The Rf value is not to be confused with the retention factor k', also known as the capacity factor, used in HPLC.