Quick answer: the empty column volume is π × (diameter ÷ 2)² × length. The void volume — mobile phase actually inside — is that times the porosity (≈ 0.65–0.70 for packed HPLC). Dead time is void volume ÷ flow rate.
The three quantities
Void volume: V0 = Vcol × porosity
Dead time: t0 = V0 ÷ flow rate
Worked example — a 4.6 mm ID column, 150 mm long, porosity 0.68, flow 1.0 mL/min:
- Geometric volume: π × (4.6/2)² × 150 = π × 5.29 × 150 = 2,493 mm³ = 2.49 mL
- Void volume: 2.49 × 0.68 = 1.69 mL
- Dead time: 1.69 ÷ 1.0 = 1.69 min
The geometric volume is pure cylinder geometry — the same cylinder formula used everywhere else on this site. What makes a chromatography column different is that most of that volume is packing material; only the porosity fraction is fluid the mobile phase can occupy.
Common column volumes
| ID × length | Geometric | Void (0.68) | Dead time at 1 mL/min |
|---|---|---|---|
| 2.1 × 50 mm | 0.173 mL | 0.118 mL | 0.12 min |
| 2.1 × 100 mm | 0.346 mL | 0.236 mL | 0.24 min |
| 3.0 × 100 mm | 0.707 mL | 0.481 mL | 0.48 min |
| 4.6 × 100 mm | 1.662 mL | 1.130 mL | 1.13 min |
| 4.6 × 150 mm | 2.493 mL | 1.695 mL | 1.70 min |
| 4.6 × 250 mm | 4.155 mL | 2.826 mL | 2.83 min |
| 10 × 250 mm (prep) | 19.63 mL | 13.35 mL | 13.35 min |
Void volumes assume a total porosity of 0.68; your actual value depends on the packing and is best confirmed with an unretained marker. Dead times scale inversely with flow rate — double the flow and the dead time halves.
Why column volume matters for method transfer
When you move a method between column sizes — a common step from method development to production — gradient volumes and injection amounts should scale with the column volume, not arbitrarily. Expressing a gradient in column volumes rather than minutes makes it transferable: a gradient run over 10 column volumes behaves the same on a small analytical column and a large preparative one, even though the times and absolute volumes differ enormously. This calculator gives you the column-volume figure that scaling depends on.
Frequently asked questions
How do I calculate column volume?
The empty (geometric) column volume is V = π × (diameter ÷ 2)² × length. A 4.6 mm internal diameter column 150 mm long has a volume of π × 2.3² × 150 ≈ 2,493 mm³, or about 2.49 mL. The void volume — the mobile phase actually inside — is this multiplied by the column porosity.
What is void volume in HPLC?
Void volume, or dead volume, is the volume of mobile phase inside the column — the empty column volume times the porosity. For a typical packed HPLC column the total porosity is around 0.65 to 0.70, so the void volume is roughly two-thirds of the geometric column volume. An unretained compound elutes at this volume.
How do I calculate dead time?
Dead time (t₀) is the void volume divided by the flow rate: t₀ = void volume ÷ flow rate. For a 1.66 mL void volume at 1.0 mL/min, dead time is about 1.66 minutes. It is the time an unretained compound takes to pass through the column, and it anchors retention factor calculations.
What porosity should I use?
For a totally porous silica HPLC column, total porosity is commonly around 0.65 to 0.70. Core–shell columns run a little lower and monoliths a little higher. If you need an exact figure, measure the void time experimentally with an unretained marker rather than assuming a porosity.
Why does column volume matter?
It sets how much sample and solvent a method needs, how long gradients should be scaled when transferring between column sizes, and the void time that underpins retention factors. Getting it right is central to method development and to scaling methods between analytical and preparative columns.