TLC Rf Value Calculator – Measure, Compare and Interpret Your Plate
Thin-layer chromatography is the quickest check in an organic lab: a spot of sample, a few minutes in a chamber, and you know whether a reaction has moved, whether two compounds match, or whether an eluent is worth running on a column. The number that makes a plate comparable is the Rf value. This TLC Rf value calculator works it out for every spot on every lane, attaches the reading uncertainty, groups matching spots, reads a reaction-monitoring plate for you, and can measure straight off a photo of the plate.
What Rf measures
As the eluent climbs the plate, each compound is continually partitioned between the stationary phase (the silica or alumina layer) and the mobile phase (the solvent). A compound that spends most of its time dissolved in the eluent travels nearly as far as the solvent; one that clings to the layer barely moves. The ratio of the two distances captures that balance:
Rf = d_spot / d_frontBoth distances are measured from the origin line, where the sample was applied, and the spot distance is taken to the centre of the spot. Rf is dimensionless and lies between 0 and 1, so it does not matter whether you measure in millimetres, centimetres, inches or pixels.
Measuring it properly
The most common mistake is measuring from the bottom edge of the plate: it adds the same extra length to both distances and inflates every Rf. If you have already done it, switch the reading mode to bottom edge and enter the origin height; the tool subtracts it for you. A ruler read to the nearest millimetre carries an uncertainty of about ±0.05 cm, and the calculator propagates it as σ_Rf = (σ / d_front) · √(1 + Rf²). On a 6 cm run that is about ±0.01, which is why a third decimal is usually fake precision. For smeared spots, enter the leading and trailing edges as well: the tool reports the Rf range and flags tailing.
Why Rf is not a constant
The same compound can give Rf 0.33 on Monday and 0.38 on Tuesday. Eluent composition, chamber saturation, temperature, layer thickness and how much water the silica has absorbed all shift it. Treat Rf as a comparison on one plate, not a physical property to look up.
Co-spotting and reaction monitoring
A classic monitoring plate has three lanes: starting material, a co-spot (both samples applied on top of each other) and the reaction mixture. Give each lane its role and the calculator reports whether the starting material is still visible, which reaction spots are new, and whether the co-spot shows one spot or resolves two. Matching Rf values are consistent with the same compound but never prove it; a second eluent is the usual next test.
Choosing an eluent for a flash column
For a flash column, aim for the compound you want at roughly Rf 0.2–0.4, with about ΔRf ≥ 0.2 to its nearest neighbour for an easy separation. A useful rule of thumb is that a compound elutes after about 1/Rf column volumes. Changing the solvent ratio moves all spots up or down together; when two spots stay stuck together, change the polar solvent itself (EtOAc to acetone, ether or DCM) to change selectivity.
Normal phase versus reversed phase
On silica and alumina, a more polar eluent raises Rf. On reversed-phase (C18) plates the stationary phase is non-polar, so the advice flips: to raise Rf, make the eluent less polar by adding more organic solvent and less water. Pick the stationary phase in the Record section and the guidance follows.
Measuring on a photo
Because Rf is a ratio, a photo needs no scale calibration. Load an image, tap two points on the origin line, tap along the solvent front, then tap each spot. The measurement is made perpendicular to the origin line, so a slightly rotated photo gives the same answer, and an uneven front is followed point to point. Photograph the plate straight on, and circle UV spots in pencil first. The photo stays in your browser, and the annotated PNG you download carries no EXIF data.