What is the Rf value?
Enter how far the spot and the solvent front moved from the baseline. The calculator gives the retention factor Rf, or works out either distance from the Rf and the other distance.
- Answer
- Rf = 0.55
Rf = 0.55: the spot moved 55% as far as the solvent front.
- Rf (full)
- 0.55
- Share of the front’s distance (%)
- 55
- Spot distance (mm)
- 22
- Front distance (mm)
- 40
Answer: Rf = 0.55. Rf = 0.55: the spot moved 55% as far as the solvent front.
How to calculate
Finds the retention factor (Rf) of a spot in thin-layer or paper chromatography: the distance the spot moved divided by the distance the solvent front moved, or either distance from the other and the Rf.
Example with the default inputs (Find Rf, Distance the spot moved 22 mm, Distance the solvent front moved 40 mm): Rf = 0.55: the spot moved 55% as far as the solvent front.
Method: Rf = distance the spot moved ÷ distance the solvent front moved, both measured from the baseline; spot distance = Rf × front distance; front distance = spot distance ÷ Rf.
- Both distances are measured from the baseline (origin) on the same plate: to the middle of the spot and to the solvent front.
- The spot cannot move past the front, so the Rf is from 0 to 1. The Rf is shown to 2 decimals, rounded half up from its exact value.
Worked examples
Each example is checked against the calculator on every build.
- Find Rf, Distance the spot moved 22 mm, Distance the solvent front moved 40 mm gives Answer Rf = 0.55, Rf (full) 0.55, Share of the front’s distance (%) 55.Source: LibreTexts, Nichols, Organic Chemistry Lab Techniques, §2.3C The Retention Factor (Rf = distance traveled by the compound ÷ distance traveled by the solvent front), https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_Lab_Techniques_(Nichols)/02%3A_Chromatography/2.03%3A_Thin_Layer_Chromatography_(TLC)/2.3C%3A_The_Retention_Factor, Figure 2.14a: 22 mm ÷ 40 mm = 0.55
- Find Rf, Distance the spot moved 6.9 mm, Distance the solvent front moved 20 mm gives Answer Rf = 0.35, Rf (full) 0.345.
- Find Spot distance, Rf value 0.35, Distance the solvent front moved 50 mm gives Spot distance (mm) 17.5, Answer Spot distance = 17.5 mm (1.75 cm).Source: LibreTexts, Nichols, Organic Chemistry Lab Techniques, §2.3C The Retention Factor (Rf = distance traveled by the compound ÷ distance traveled by the solvent front), https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_Lab_Techniques_(Nichols)/02%3A_Chromatography/2.03%3A_Thin_Layer_Chromatography_(TLC)/2.3C%3A_The_Retention_Factor (Rf formula)
- Find Front distance, Rf value 0.55, Distance the spot moved 33 mm gives Front distance (mm) 60, Answer Solvent front distance = 60 mm (6 cm).Source: LibreTexts, Nichols, Organic Chemistry Lab Techniques, §2.3C The Retention Factor (Rf = distance traveled by the compound ÷ distance traveled by the solvent front), https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_Lab_Techniques_(Nichols)/02%3A_Chromatography/2.03%3A_Thin_Layer_Chromatography_(TLC)/2.3C%3A_The_Retention_Factor (Rf formula)
How it works
Formula.
Rf = spot distance ÷ front distance
- Spot distance: the distance from the baseline (where the sample was spotted) to the middle of the spot.
- Front distance: the distance from the baseline to the solvent front.
Pick what to find; the page rearranges the same formula:
- Rf from the two distances.
- Spot distance = Rf × front distance.
- Front distance = spot distance ÷ Rf.
Units. Distances may be typed in mm, cm or inches (1 in = 25.4 mm exactly); the page works in exact decimals and shows a distance it works out in mm and cm, to 6 significant figures with trailing zeros dropped (17.5 mm (1.75 cm)). A value below 0.000001 is written with a power of ten, in the sentence and in the number rows (5 × 10⁻⁷ mm).
Rounding. The headline Rf is rounded half up to 2 decimals from its exact value (6.9 mm ÷ 20 mm = 0.345 shows as 0.35). The full Rf is also listed to 6 significant figures, with the spot’s distance as a percent of the front’s (Rf × 100).
Rules the page checks.
- Both distances are from 0 to 1 m; the front’s distance is more than 0.
- The spot’s distance cannot be more than the front’s, so the Rf is from 0 to 1.
- To find the front’s distance, the Rf and the spot’s distance must both be more than 0 (a spot on the baseline says nothing about the front), and the result must be 1 m or less.
- A distance or Rf too small to hold as a number (its nearest number is 0: below about 2.5 × 10⁻³²⁴) gives no answer rather than 0.
Worked examples by hand
Spot at 22 mm, front at 40 mm (Nichols, Figure 2.14a). Rf = 22 ÷ 40 = 0.55: the spot moved 55% as far as the front.
Spot at 6.9 mm, front at 20 mm. Rf = 6.9 ÷ 20 = 0.345 exactly, which rounds half up to 0.35.
Rf 0.35, front at 5 cm. Spot distance = 0.35 × 50 mm = 17.5 mm (1.75 cm).
Rf 0.55, spot at 3.3 cm. Front distance = 33 mm ÷ 0.55 = 60 mm (6 cm).
Other questions people ask
How do I calculate an Rf value?
Measure two distances from the baseline (where you put the sample): to the middle of the spot, and to the solvent front. Divide the first by the second. A spot 22 mm from the baseline with the front at 40 mm has Rf = 22 ÷ 40 = 0.55.
What does an Rf of 0.55 mean?
The spot moved 55% as far as the solvent front. An Rf near 0 means the compound stayed near the baseline (it holds on to the plate); an Rf near 1 means it moved almost as far as the solvent.
Why is the Rf always between 0 and 1?
The compound is carried by the solvent, so its spot cannot move farther than the solvent front. The page refuses a spot distance larger than the front’s distance.
Does the Rf value have units?
No. It is a ratio of two lengths, so the units cancel. Measure both distances in the same unit (millimeters are usual); the page also takes centimeters and inches and converts them.
Where exactly do I measure to?
To the approximate middle of the spot, and to the solvent front. Mark the front in pencil as soon as you take the plate out of the chamber, because the solvent evaporates quickly and the front line disappears.
Why do I get a slightly different Rf each time?
Rf values depend on the plate, the solvent mixture, the temperature and how far the solvent runs, so they are approximate. Nichols reports acetophenone runs in one solvent at 0.33, 0.38, 0.35 and 0.37. Compare a sample with a known compound on the same plate.