---
title: "Column Chromatography Calculator: Rf to CV | MolDraw"
url: https://www.moldraw.com/tools/free-chem-tools/column-chromatography-calculator.html
description: "Flash column calculator: TLC Rf to column volumes (CV = 1/Rf), ΔCV, silica mass and column size. Example: Rf 0.25 elutes at 4 CV; 1 g at 50:1 = 50 g."
lang: en
publisher: Scidart Academy
generated: 2026-10-31
---
[MolDraw](https://www.moldraw.com/) › [Chemistry tools](https://www.moldraw.com/tools/) › Column Chromatography Calculator

# Column Chromatography Calculator (Flash, from TLC)

**In short:** Enter the TLC Rf values of your spots and the crude mass. You get the column volumes to elute each spot (CV = 1/Rf), the ΔCV separation check, silica mass, column size, eluent volume and fraction size. A helper suggests the next hexane/EtOAc ratio to try. Free, in your browser.

TLC spots (same eluent and silica as the column). Mark the compound you want.

Crude mass (g) Silica : sample

Elute every spot (solvent to the slowest spot)

**Assumptions (edit if you know your silica)**

Packed density (g/mL) Bed height (cm) CV per g silica (mL)

Results appear here.

### Eluent helper: move Rf toward a target

Empirical estimate for two-solvent mixtures such as hexane/EtOAc or DCM/MeOH. Use it to choose the next TLC plate.

Polar solvent now (%) Measured Rf Target Rf Slope n

I have a second TLC plate at another ratio (fit n)

## How to use it

1. Run a TLC plate in the solvent you plan to use for the column. You can draw it with the [TLC diagram tool](https://www.moldraw.com/tools/free-chem-tools/tlc-diagram-tool.html).
2. Enter each spot's Rf and mark your product as the target.
3. Enter the crude mass. The silica ratio is chosen from the ΔCV of the closest spot, or pick your own.
4. If the target Rf is outside 0.2–0.35, use the eluent helper to pick the next ratio, run a new TLC and enter the new Rf values.
5. Pack the column, load the sample in a narrow band and collect fractions of about ¼ CV. Check them by TLC.

## Worked examples

- **Rf to CV:** Rf 0.25 → 1/0.25 = **4 CV**. Rf 0.35 → 2.9 CV. Rf 0.5 → 2 CV.
- **ΔCV:** spots at Rf 0.5 and 0.3 → 1/0.3 − 1/0.5 = 3.33 − 2.00 = **1.33 CV**. That is above 1, so the separation is routine.
- **Same ΔRf, different ΔCV:** Rf 0.7 vs 0.5 (ΔRf 0.2) gives ΔCV 0.57; Rf 0.3 vs 0.1 (also ΔRf 0.2) gives ΔCV 6.7. Lower Rf spreads spots further apart on the column.
- **Silica:** 1 g crude at 50:1 → **50 g silica**. At 0.5 g/mL that is a 100 mL bed: a 3 cm column filled about 14 cm high, with one CV of roughly 65 mL.

## The rules behind the numbers

### Column volumes from Rf

`CV = 1 / Rf ΔCV = 1/Rf(slower) − 1/Rf(faster) k = CV − 1 = (1 − Rf) / Rf`

A compound with retention factor k elutes after (1 + k) column volumes. On TLC, Rf = 1/(1 + k), so CV ≈ 1/Rf when the column uses the same silica and the same solvent as the plate. This is the basis of the CV method used by flash-system makers such as Biotage and Teledyne ISCO. Real columns elute a little differently because the TLC plate is not in equilibrium with the vapour and the binder changes the silica slightly.

### Target Rf and separation

Still, Kahn and Mitra (J. Org. Chem. 1978, 43, 2923), who introduced flash chromatography, recommended a solvent that gives the product Rf ≈ 0.35, about 15 cm (6 in) of 40–63 µm silica and a flow of about 5 cm/min. Today 0.2–0.35 is the usual window. The verdicts used here are:

| Closest ΔCV | Verdict | Default silica:sample |
| --- | --- | --- |
| ≥ 2 | Easy | 30:1 |
| 1 – 2 | Good (routine) | 50:1 |
| 0.5 – 1 | Difficult | 100:1 |
| < 0.5 | Very difficult: change the solvent first | 150:1 or more |

The 30–100:1 range is the common lab rule for silica:crude (w/w). The tiers are a rule of thumb, not a published standard. As a check, around Rf 0.35 a ΔRf of 0.2 corresponds to ΔCV ≈ 1, which is the "easy" row of the Still table below.

### Still, Kahn & Mitra (1978) column table

| Column diameter (mm) | Eluent (mL) | Load, ΔRf ≥ 0.2 (mg) | Load, ΔRf ≥ 0.1 (mg) | Fraction (mL) |
| --- | --- | --- | --- | --- |
| 10 | 100 | 100 | 40 | 5 |
| 20 | 200 | 400 | 160 | 10 |
| 30 | 400 | 900 | 360 | 20 |
| 40 | 600 | 1600 | 600 | 30 |
| 50 | 1000 | 2500 | 1000 | 50 |

### Column size, solvent and fractions

- Bed volume = silica mass ÷ packed density (default 0.5 g/mL). Diameter is chosen for a 15 cm bed and rounded to a common glass-column size.
- One CV ≈ 1.3 mL per g of silica (typical of packed 40–63 µm silica and prepacked cartridges, which list about 1.2–1.5 mL/g).
- Eluent ≈ 2 CV to equilibrate + 1/Rf CV to elute the target (or the slowest spot) + 2 CV for the tail of the band. Fractions ≈ ¼ CV.

### Eluent helper

For a strong solvent B in a weak solvent, retention on silica roughly follows the Snyder–Soczewinski relation `log k = log k₀ − n·log φB`. From one plate and an assumed n (default 1), the tool solves for the fraction φB that gives the target Rf. With two plates at different ratios it fits n. The relation is approximate, fails near 0% or 100% polar solvent, and does not handle changes in selectivity. Always confirm with a new TLC.

## Limitations

- CV = 1/Rf assumes the same silica, the same solvent and isocratic elution. Column Rf values are often a little lower than on the plate.
- The silica ratio, density and CV-per-gram values are typical numbers, not measurements of your silica. Edit them under Assumptions.
- The tool does not model gradients, band broadening, dry loading versus wet loading, or reversed-phase columns.
- The eluent helper is an empirical guide for the next TLC plate. It cannot predict changes in spot order.
- Very polar or acid/base-sensitive compounds may streak or decompose on silica; no calculation predicts that.

## Frequently asked questions

### How do I convert TLC Rf to column volumes?

CV = 1/Rf. A compound with Rf 0.25 on TLC elutes after about 4 column volumes of the same eluent on a silica column; Rf 0.5 elutes after about 2 CV. This holds only when the column and the plate use the same silica and the same solvent.

### What Rf should I aim for in flash chromatography?

About 0.2 to 0.35 for the compound you want. Still, Kahn and Mitra (1978) recommended Rf 0.35. Higher Rf values squeeze the spots together in CV; lower values waste solvent and broaden the band.

### What is ΔCV and why is it better than ΔRf?

ΔCV is the difference in column volumes between two spots: 1/Rf1 − 1/Rf2. Rf 0.5 and 0.3 give ΔCV = 2 − 3.33 = 1.33. The same ΔRf of 0.2 gives a much larger ΔCV at low Rf, so ΔCV describes the column separation better. ΔCV of 1 or more is usually a routine separation.

### How much silica do I need for a column?

Multiply the crude mass by a silica:sample ratio. Common ratios are 30:1 for easy separations, about 50:1 for routine ones and 100:1 or more for difficult ones. 1 g of crude at 50:1 needs 50 g of silica.

### How do I change the hexane/ethyl acetate ratio to get a lower Rf?

Lower the ethyl acetate fraction. The helper uses the empirical relation log k = log k0 − n log φ (k = 1/Rf − 1). With n = 1, a spot at Rf 0.6 in 30% EtOAc moves to about Rf 0.3 in 9% EtOAc. Treat this as the next ratio to test on TLC, not as a final answer.

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Last updated: October 30, 2026