Rf refers to reference front and is used in paper chromatography. It is one of the things that is asked of students to calc in ap bio lab on photosynthesis. For ex in that lab, you use a solvent to move the various pigments of photosynthesis up chromatography paper, each pigment moving with the solvent at a dif rate. The Rf value is calculated using the number of mm the solvent moved divided by the distance the pigment migrated.
Rf is referred to the distance travelled by a certain molecule (or group of molecules) in relation to the distance travelled by the solvent front.
In planar chromatography (thin layer chromatography, paper chromatography) Rf means retardation factor, and it is described as the following ratio:
Rf= (migration distance of substance)/(migration distance of solvent front)
The distance unit can be mm or cm. So you should you measure the distance from the start to the solvent front and the distance from the start to the spot of interest, then you can calculate Rf.
Also, Rf represents the relative mobility of a protein in an electrophoretic separation. It is calculated in a similar way.
Rf= (migration distance of a protein band)/(migration distance of the dye front).
Rf is related to the molecular mass of the protein, and this parameter can be used for the estimation of the molecular weight of an unknown protein, by calibration with molecular weight standards.