Rf Value Calculator

What is the Rf value formula?

The Rf value (retention factor) is Rf = distance moved by the spot ÷ distance moved by the solvent front, both measured from the baseline where the sample was spotted. Because a spot can never travel further than the solvent carrying it, Rf always lies between 0 and 1 and has no units.

Measuring the two distances

Draw the baseline in pencil, because ink is itself a mixture of dyes and would run with the sample, and keep it above the level of the solvent in the tank so the spots do not dissolve into the pool below. When the solvent has climbed most of the way up, take the paper or plate out and mark the solvent front at once: the solvent evaporates, and the line it reached soon cannot be seen.

Measure to the centre of each spot, not to its top edge. A spot spreads as it travels, and its centre is where most of the substance sits, which is the point the IUPAC definition measures to. A ruler read to the nearest millimetre over a run of a few centimetres fixes Rf to about the second decimal place, which is why the first readout rounds to two: more figures than that claim a precision the measurement does not have.

What an Rf value tells you

Each substance divides its time between the moving solvent, the mobile phase, and the paper or the coating on the plate, the stationary phase. The more strongly the stationary phase holds it, the less of the run it keeps up with and the lower its Rf. On a silica plate, which is polar, polar compounds are held more strongly and run lower than nonpolar ones in the same solvent.

That makes an Rf a property of a substance under one set of conditions, not of the substance alone. A different solvent, a different paper or plate, or a different temperature gives a different value, so an unknown is identified by running it beside known references on the same sheet and comparing them, not by looking its Rf up in a table. Two substances can share an Rf in one solvent, so a match in one solvent is evidence rather than proof, and a second run in a different solvent that still matches is far stronger.

Column chromatography and HPLC use a retention factor too, but a different one: k, the time an analyte spends in the stationary phase relative to the mobile phase, which the chromatography simulator works with. The Rf on this page is the planar chromatography quantity IUPAC calls the retardation factor.

Spots that stay on the baseline or run with the front

An Rf of 0 means the substance did not move from where it was spotted, and an Rf of 1 means it travelled with the solvent front. Neither identifies anything, because every substance the solvent leaves behind shares the first value and every substance it carries freely shares the second. The calculator accepts both and warns about them. The cure is a different solvent: one that carries the substance more readily for a spot stuck at the baseline, and less readily for one at the front.

Common mistakes

• Measuring from the edge of the paper. Both distances start at the baseline. With the baseline 1.0 cm up, a spot whose centre is 5.1 cm from the edge and a front 8.0 cm from it give 4.1 ÷ 7.0 = 0.59, not 5.1 ÷ 8.0 = 0.64.
• Swapping the two distances. Solvent front over spot gives a number above 1, which no spot can have, so the calculator refuses it rather than reporting it.
• Mixing millimetres and centimetres. A spot at 18 mm with a front at 6.0 cm is 1.8 ÷ 6.0 = 0.30, not 18 ÷ 6.0 = 3. Choose the unit for each field and the calculator converts before dividing.
• Comparing Rf values from different runs. A change of solvent, paper, plate or temperature moves every Rf, so compare spots that ran side by side on the same sheet.

Converting units first? Use the length conversion table.