When the plate is fully developed remove it from the tank. Immediately use a pencil to mark the solvent front.
Set the plate aside and allow the solvent to evaporate. When it is dry, measure the distance from the origin line to the solvent front (not shown). Also measure the distance from the origin line to each darkest portion of each spot (below). It is important to be consistent in how you measure. When all measurements are complete, use the formula given in the background to calculate each Rf.