Resuspension Calculator

Find the diluent volume that resuspends an oligo at a target concentration.

Mole, molar and volume scales, solved in either direction.

Updated October 19, 2026

CreatorFrank Zhao

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Introduction

A lyophilised oligo arrives as a dry pellet, and the first thing it needs is liquid. How much liquid decides everything that follows: the same pellet dissolved in half the volume gives a stock twice as concentrated, and every dilution made from that stock inherits the difference.

This page answers the question from the two numbers you already have. Enter the amount of oligo in the tube and the stock concentration your protocol asks for, and the diluent volume appears in the third box. The relationship behind it is the molar identity $n = c \times V$ , which also runs the other way: give the amount and the volume you actually used and you get the concentration of the stock you just made.

It is the standard bench step in molecular biology work, and it is why the page asks for exactly the two numbers an oligo order and a protocol supply: the molar amount that came in the tube, and the stock concentration the method wants. Resuspension is a one-time step, and getting it right the first time is easier than correcting every downstream dilution afterwards.

3 boxes, 1 identity

Any two values determine the third

Both directions

Volume, amount or concentration, whichever is missing

nmol with µM gives µL

The unit triple that makes the arithmetic convenient

Quick start

Enter the amount of oligo

The tube, the order confirmation or the spec sheet reports this in moles — a scale such as 25 nmol or 0.5 µmol. Type the number and read the box on the mole scale you were given; the value is converted internally, so nothing needs changing first.

Enter the stock concentration you want

This comes from your protocol rather than from the calculator: whatever concentration your method specifies goes in here, read on the same molar scale the protocol uses — a target written as 100 µM goes in as 100 with the micromolar scale selected.

Read the diluent volume

The third box fills itself in as soon as the other two are complete. Add that volume of TE buffer or nuclease-free water to the pellet.

Follow one calculation through

A tube labelled $50\ \mathrm{nmol}$ that should become a $100\ \mathrm{\mu M}$ stock:

V = \frac{n}{c}

=

\frac{50\ \mathrm{nmol}}{100\ \mathrm{\mu M}}

=

500\ \mathrm{\mu L}

So the pellet is brought up in 500 µL. Read the answer as the volume of liquid to add, not as a total to fill up to — the pellet itself contributes essentially nothing.

Check the scale if the answer surprises you

A volume that looks a thousand times too small or too large almost always means one box is on the wrong step of its scale, not that the arithmetic failed. The unit ladder below is the fastest way to see which one: nmol and µmol, µM and mM, µL and mL are each a factor of a thousand apart.

Calculation method

Everything on this page is one identity: the amount of substance equals its concentration multiplied by the volume it is dissolved in.

n = c \times V

Here $n$ is the amount of oligo in moles, $c$ is the concentration of the stock in moles per litre, and $V$ is the diluent volume in litres. The box you leave empty is the one that gets rearranged for:

V = \frac{n}{c}

\qquad

c = \frac{n}{V}

\qquad

n = c \times V

Because the three quantities sit in one statement rather than in a chain of steps, no box is a read-only readout: the volume box takes a value as readily as the other two. That is the point of it. If a tube already has liquid in it, the amount and the volume are the two values you know, and the concentration is what you want to read off.

Where the factor of a thousand comes from

Bench work almost never uses moles and litres directly. The comfortable triple is nanomoles for the pellet, micromolars for the target stock, and microlitres for the answer, and the single thousand in the familiar rule is simply what is left after those three unit scales are cancelled.

V_{\mu L} = \frac{n_{nmol} \times 10^{-9}\ \mathrm{mol}}{c_{\mu M} \times 10^{-6}\ \mathrm{mol/L}} \times \frac{10^{6}\ \mu L}{1\ \mathrm{L}}

=

\frac{n_{nmol} \times 10^{3}}{c_{\mu M}}

Nothing about the chemistry changes between the two forms — they are the same equation written twice — but the micromolar version is the one worth remembering, because it makes a unit slip readable. A volume that comes back a thousand times too small means the amount went in a thousand times too small or the concentration a thousand times too large, and only one of the two scale selections can be wrong.

Quantity	Scale steps	One step is
Oligo amount	µmol to nmol	1 µmol = 1,000 nmol
Oligo amount	nmol to pmol	1 nmol = 1,000 pmol
Concentration	mM to µM	1 mM = 1,000 µM
Concentration	µM to nM	1 µM = 1,000 nM
Volume	mL to µL	1 mL = 1,000 µL

What the page refuses and what it flags

A zero or negative value is refused in every box, and a refused box never drives the calculation — the remaining box stays empty rather than printing a result built on an impossible number. A zero concentration has no answer at all, so the volume box stays blank and says which division failed.

Two more results are computed but questioned. A diluent volume above 1 L and a stock concentration above 10 mM are both far outside a bench preparation, so the box is flagged for a second look while the number itself stays on screen. In practice those flags are scale checks: they are what a millimolar entry typed as micromolar, or a nanomole amount typed as picomoles, looks like once it reaches the answer.

The assumption the arithmetic cannot check

The identity treats the pellet as occupying no volume. For a normal pellet brought up in tens of microlitres that is far below the accuracy of the pipette, so adding the computed volume is exactly right. If the pellet is a large one and your protocol specifies a final volume, bring the tube up to the mark instead of adding the number blindly.

Worked examples

The two cases below use the other two directions, which is where this page earns its place next to the bench: one before the oligo is ordered, one after it has been dissolved.

Sizing an order before it is placed

Situation: A protocol needs a working stock of 100 µM and enough of it to fill a 200 µL tube. The oligo has not been ordered yet, so the question is the smallest synthesis scale that covers the job.

What goes in

Desired concentration100 µM

Diluent volume200 µL

What comes back

n = c \times V

=

\frac{100\ \mathrm{\mu M} \times 200\ \mathrm{\mu L}}{1000}

=

20\ \mathrm{nmol}

Twenty nanomoles is the exact requirement; the amount box reports it in nanomoles because that is the scale selected for it.

In practice: Order the next scale up rather than the exact amount. A 25 nmol scale leaves room to spare: the same 100 µM target is reached with 250 µL of diluent.

Checking a stock that is already in the freezer

Situation: A tube was resuspended a while ago with 500 µL of buffer and labelled 40 nmol, but the concentration was never written down. The amount and the volume are both known, so the concentration is the value to solve for.

What goes in

Oligo amount40 nmol

Diluent volume500 µL

What comes back

c = \frac{n}{V}

=

\frac{40\ \mathrm{nmol}}{500\ \mathrm{\mu L}}

=

80\ \mathrm{\mu M}

An 80 µM stock is a perfectly usable one. If the protocol calls for 10 µM working aliquots, the next step is a dilution series rather than a fresh resuspension — theserial dilution calculatorworks out the volumes for that ladder.

In practice: Label the tube with the concentration the page returns, then treat it as the starting point for whatever working dilution the experiment needs.

Questions

Should the diluent volume be the final volume in the tube?

For a typical pellet, yes — the dry oligo contributes essentially nothing to the volume, so the liquid you add is the volume you end up with. The distinction matters in one situation: if the pellet is a large one and the protocol specifies a final volume, adding the computed volume and then topping up to the mark is safer than trusting the pellet to be weightless.

What if the label gives the amount by mass instead of in nanomoles?

This page needs a molar amount, because the identity works in moles. A spec sheet that reports micrograms or an absorbance value has to be turned into moles first — a molar concentration from a weighed amount, a volume and a molecular weight is what themolarity calculatoris for. Once you have a concentration and an amount, the boxes here take over.

Why is a box flagged for a number that looks reasonable?

A diluent volume past 1 L or a stock past 10 mM is far beyond what a bench preparation looks like, so the page keeps the number but marks the box rather than quietly passing it on. Almost every real case is a unit that means something else: picomoles where nanomoles were intended, or a millimolar figure typed into a box reading micromolars.

How far to trust it

The arithmetic here is exact: the answer is the true consequence of the two numbers you enter. The uncertainty lives in those two numbers.

What the calculation cannot know

The amount on the label is nominal. A synthesis scale is what was ordered, not what came back in the tube. If a known concentration matters for the result, measure the resuspended stock rather than trusting the printed figure.
Small volumes carry large errors. A computed volume of a fraction of a microlitre cannot be delivered accurately by hand. Prepare at a volume you can pipette well and dilute from there instead of chasing the exact figure.
This covers the stock, not what follows. Working dilutions, mixes and reaction set-ups are separate calculations. This page fixes the concentration of the tube you are about to put in the freezer.

The two flags described above are scale checks, not statements about whether your oligo dissolves. How much buffer a particular sequence takes up, how long it needs, and how the stock should be stored all belong to your own protocol and to the supplier's handling notes.

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