Resuspension Calculator (oligo nmol → buffer volume for a target µM)

Understanding the oligo resuspension calculator — concept and principle

This calculator tells you how much buffer to add to a dried (lyophilised) oligonucleotide to reach a target molar concentration. With the molecular weight from the spec sheet it also gives the oligo mass, the mass concentration and the number of copies per microlitre. Molecular biology labs use it for primers and probes.

nmol ÷ µM gives mL ÷ 1,000: 25.4 nmol at 100 µM needs 254 µL.

How it is calculated

Volume (µL) = amount (nmol) × 1,000 ÷ target concentration (µM)

Mass (µg) = amount (nmol) × MW ÷ 1,000

Mass concentration (ng/µL) = concentration (µM) × MW ÷ 1,000

Copies per µL = concentration × 10⁻¹² mol/µL × N_A

1 µM = 1 pmol/µL = 1 nmol/mL; N_A = 6.02214076 × 10²³ mol⁻¹.

The principle

Concentration is amount divided by volume, so the volume needed is the amount divided by the concentration you want. Because 1 µM is exactly 1 pmol per µL, nanomoles convert to microlitres with a factor of 1,000. The molecular weight links moles to grams, and Avogadro's number links moles to individual molecules.

Worked example

Default inputs: 25.4 nmol on the tube, target 100 µM, MW 6,100 g/mol.

Volume = 25.4 × 1,000 ÷ 100 = 254 µL = 0.254 mL.
Mass = 25.4 × 6,100 ÷ 1,000 = 154.9 µg.
Mass concentration = 100 × 6,100 ÷ 1,000 = 610 ng/µL.
Copies per µL = 100 × 10⁻¹² × 6.022 × 10²³ = 60,220,000,000,000 (6.02 × 10¹³).

Each point is the calculator re-run with only “Target concentration (µM = pmol/µL)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Target concentration (µM = pmol/µL)	Buffer to add (µL)
25	1,016
50	508
75	338.7
100	254
125	203.2
150	169.3
200	127

How to read the result

A 100 µM stock is a common choice for primers; working solutions are then diluted from it, often to 10 µM. Spin the tube before opening so the dried pellet is at the bottom, and mix well after adding buffer.

Common mistakes

Using the mass in µg instead of the amount in nmol.
Confusing µM with mM, a factor of 1,000.
Forgetting to centrifuge the tube before opening.

Keep in mind: Follow the supplier's instructions for buffer and storage. The nmol value on the tube is the supplier's measurement and has its own uncertainty.

Frequently asked questions

How much buffer for a 100 µM stock?

Multiply the nmol on the tube by 10 to get µL.

Why is 1 µM equal to 1 pmol/µL?

1 µmol per litre equals 1 pmol per microlitre, since a litre is a million microlitres.

Water or TE buffer?

Many labs use a buffered solution for long-term storage; follow your protocol.

How do I make a 10 µM working stock?

Dilute the 100 µM stock 1 in 10.

Sources: BIPM, SI Brochure, 9th edition (2019), Avogadro constant; standard definition of molar concentration c = n/V

Checked 9 Nov 2026. Standard definition of molar concentration (c = n/V) and the Avogadro constant N_A = 6.02214076 × 10²³ mol⁻¹ (exact, 2019 SI); standard laboratory practice for resuspending lyophilised oligonucleotides.

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