Quick answer: void volume is the mobile phase inside the column. Predict it with V₀ = π × (ID ÷ 2)² × L × εT, or measure it with V₀ = t₀ × F using an unretained marker. A 4.6 × 150 mm fully porous column comes out at about 1.7 mL either way. The measured figure is the one to trust.
Void volume, dead volume, hold-up volume — what is actually different
Three names, and depending on who you ask, two or three meanings. It is worth being precise, because the ambiguity causes real errors:
| Term | What it is | Typical size | You want it… |
|---|---|---|---|
| Void volume (V₀, VM) = dead volume, hold-up volume | Mobile phase inside the packed bed | 1.7 mL (4.6 × 150 mm) | Measured accurately — it is a property, not a fault |
| Extra-column volume | Tubing, injector, frits, detector cell — everything outside the bed | 10–100 µL | As small as possible; it broadens peaks |
| Dwell volume (VD) | Mixer to column head — delays a gradient reaching the column | 100 µL–1 mL | Known, so methods transfer between instruments |
| Interstitial volume (Ve) | Only the gaps between particles, excluding pores | 1.0 mL (same column) | Known for size-exclusion work |
The trap is the second row borrowing the first row's name. When a vendor sells you "low dead volume fittings", they mean extra-column volume. When a method says "corrected for dead volume", it almost certainly means void volume. If you need the third one, the dwell volume calculator handles it separately.
The two formulas
Measured: V₀ = t₀ × F
Porosity from measurement: εT = (t₀ × F) ÷ Vcol
Take a 4.6 × 150 mm column, an unretained uracil peak at 1.65 min, and a 1.0 mL/min flow:
- Measured void volume: 1.65 × 1.0 = 1.65 mL
- Geometric column volume: π × 2.3² × 150 = 2,493 mm³ = 2.49 mL
- Implied total porosity: 1.65 ÷ 2.49 = 0.66
0.66 against a textbook 0.68 — the column is packed as expected. That third step is the useful one and almost nobody does it. It turns a routine void-time injection into a column health check: a porosity that drifts upward over a column's life is a bed settling and voiding; one far below 0.6 on a fully porous phase suggests the marker is being excluded or the phase is not what the label says.
Void volume chart for HPLC columns
Predicted void volumes at three porosities, so you can read across to whichever packing you actually have. Bold column is the fully porous default.
| ID × length | Column volume | Core–shell ε=0.55 | Fully porous ε=0.68 | Monolith ε=0.80 |
|---|---|---|---|---|
| 2.1 × 50 mm | 0.173 mL | 0.095 mL | 0.118 mL | 0.139 mL |
| 2.1 × 100 mm | 0.346 mL | 0.190 mL | 0.236 mL | 0.277 mL |
| 2.1 × 150 mm | 0.520 mL | 0.286 mL | 0.353 mL | 0.416 mL |
| 3.0 × 50 mm | 0.353 mL | 0.194 mL | 0.240 mL | 0.283 mL |
| 3.0 × 100 mm | 0.707 mL | 0.389 mL | 0.481 mL | 0.565 mL |
| 3.0 × 150 mm | 1.060 mL | 0.583 mL | 0.721 mL | 0.848 mL |
| 4.6 × 50 mm | 0.831 mL | 0.457 mL | 0.565 mL | 0.665 mL |
| 4.6 × 100 mm | 1.662 mL | 0.914 mL | 1.130 mL | 1.330 mL |
| 4.6 × 150 mm | 2.493 mL | 1.371 mL | 1.695 mL | 1.994 mL |
| 4.6 × 250 mm | 4.155 mL | 2.285 mL | 2.826 mL | 3.324 mL |
| 10 × 250 mm | 19.63 mL | 10.80 mL | 13.35 mL | 15.71 mL |
Read the spread across a row rather than any single figure. On a 4.6 × 150 mm column the plausible range is 1.37 to 1.99 mL — a 45% window. That is how much you can be wrong by if you assume porosity instead of measuring it, and it is why a single unretained injection is worth more than this whole table.
Choosing an unretained marker
The marker has one job: travel through every part of the mobile phase and interact with nothing. Failing either half gives a wrong answer in a predictable direction.
| Marker | Mode | Detection | Watch out for |
|---|---|---|---|
| Uracil | Reversed phase | UV ~254 nm | Slight retention on very hydrophilic phases at high aqueous |
| Thiourea | Reversed phase | UV ~250 nm | Can retain slightly on bare or polar-embedded silica |
| Potassium / sodium nitrate | Reversed phase | UV ~210 nm | Ionic exclusion on residual silanols; avoid at low ionic strength |
| Deuterated mobile phase | Any | MS or RI | Excellent, but needs the right detector |
| Toluene | Normal phase | UV ~254 nm | Retains on silica — use only as a rough marker |
| Blue dextran | Size exclusion | UV ~280 nm | Reports interstitial volume, not total void — that is the point in SEC |
Blue dextran deserves the footnote. It is deliberately too large to enter the pores, so in size-exclusion work it measures the column's void in the SEC sense — the interstitial volume, around 0.40 porosity. Using it on a reversed-phase column and calling the answer void volume will under-read by about 40 percent.
Common mistakes & pro tips
- Measuring t₀ before the column has equilibrated. Retention drifts for the first several column volumes after a mobile phase change. Wait for a stable baseline and stable back pressure.
- Not subtracting extra-column volume. On a 2.1 mm column with a 0.12 mL void, a 40 µL system contributes a third of what you measure. Run the same injection with a union in place of the column and subtract.
- Taking the peak front instead of the apex. Consistently under-reads t₀ and inflates every retention factor derived from it.
- Using a size-excluded marker on a porous phase. Reports interstitial volume, roughly 40% of the column instead of 68%.
- Pro tip — track it over the column's life. Log the measured porosity each time you change mobile phase. A steady rise means the bed is settling; a step change means something has gone wrong at the inlet frit.
- Pro tip — retention factors need one number, not two. If two analysts on the same method disagree about k, the first thing to check is that they used the same t₀ and got it the same way.
How to use this calculator
- Choose a mode. "From dimensions" predicts void volume before you have run anything. "From measured t₀" converts an actual chromatogram into a void volume and a real porosity.
- Enter the column ID and length in millimetres. Both modes need these — the measured mode uses them to back-calculate porosity.
- Add the flow rate. Required in measured mode; in predicted mode it gives you t₀.
- Optionally add a retained peak's retention time. The calculator returns its retention factor k against your void time.
Frequently asked questions
What is void volume?
Void volume is the volume of mobile phase held inside a packed chromatography column — the space between and inside the particles, everything that is liquid rather than silica. It is what an unretained compound travels through, so it sets the earliest possible elution time. For a typical 4.6 × 150 mm fully porous column it is about 1.7 mL, roughly two-thirds of the 2.49 mL empty tube.
What is the difference between void volume and dead volume?
In everyday lab usage they are the same thing: the mobile phase volume inside the column. The confusion arises because instrument vendors also use "dead volume" for extra-column volume — tubing, fittings, the detector flow cell — which is a completely different quantity and a problem to be minimised rather than a property to be measured. If someone says they are reducing dead volume, they mean plumbing. If they say they are measuring it, they mean the column.
How do you calculate void volume?
Two ways. Predicted: V₀ = π × (ID ÷ 2)² × L × εT, using a total porosity of about 0.68 for fully porous silica. Measured: V₀ = t₀ × F, where t₀ is the retention time of an unretained marker and F is the flow rate. The measured value is the one to trust — it accounts for the actual packing rather than a textbook average.
What is void time in HPLC?
Void time, t₀, is how long an unretained compound takes to pass through the column. It equals void volume divided by flow rate. Every retention factor is measured against it: k = (tR − t₀) / t₀. Get t₀ wrong and every k value in the method is wrong with it, which is why regulated methods specify how it was determined.
Which marker should I use to measure void volume?
In reversed phase, uracil and thiourea are the standard UV-detectable choices; potassium nitrate and sodium nitrate also work but can be slightly retained or excluded on some phases. Deuterated solvent works well with mass spectrometric or refractive index detection. Avoid anything with charge on an ion-exchange phase, and avoid large molecules on any phase where size exclusion could keep them out of the pores — an excluded marker reports the interstitial volume only, which is about 40 percent rather than 68 percent of the column.
Why is my measured void volume lower than the calculated one?
The usual cause is a marker that does not reach all the pore space — either partially excluded by size, or repelled electrostatically. Core–shell columns also genuinely run lower, near 0.55 porosity rather than 0.68, so a calculation that assumed fully porous silica will read high by about 20 percent. A void volume far below prediction on a column that used to match is a warning sign of a collapsed or voided bed.
References & further reading
- IUPAC Compendium of Chemical Terminology (the Gold Book) — definitions of hold-up volume and retention factor.
- Gritti, F. & Guiochon, G., on the measurement of column porosity and the phase-volume problem, Journal of Chromatography A.
- USP General Chapter <621> Chromatography — retention factor and system suitability requirements.
- Phenomenex and Thermo Fisher technical notes distinguishing column dead volume from extra-column and dwell volume.
Predicted values use published typical porosities and will not match your column exactly. For method validation, measure t₀ directly. See our accuracy policy.