Synthetic DNA and RNA oligonucleotides – PCR primers, probes, adapters – are shipped as a dried pellet with the yield printed on the tube in nanomoles (and often micrograms). Before use, the pellet is dissolved in a measured volume of nuclease-free water or TE buffer to give a stock of known concentration, typically 100 µM. Getting that first step right matters, because every PCR, qPCR or sequencing reaction set up from the stock inherits its concentration.
The arithmetic is a simple mole balance: concentration equals amount divided by volume. A convenient shortcut follows from the units – nanomoles divided by micromolar gives millilitres, so multiplying by 1000 gives microlitres. For example, 25 nmol dissolved to 100 µM needs 250 µL. If only the mass is known, divide by the molar mass of the oligo (from the specification sheet, or roughly 330 g/mol per nucleotide for single-stranded DNA) to get moles first.
Most labs keep a concentrated stock in the freezer and use a 10 µM working dilution day to day, which limits freeze–thaw cycles and contamination of the master stock. The calculator also works out that dilution and the mass concentration (ng/µL) that a spectrophotometer would report.
How to use the Resuspension Calculator
- Calculate
- Oligo amount given as
- Oligo amount
- Oligo mass
- Molar mass of the oligo — From the synthesis report; ≈ 330 g/mol per nucleotide for ssDNA.
- Desired stock concentration
- Volume of diluent
- Working solution concentration
- Working solution volume
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Pick what to calculate. Enter the oligo amount (as moles, or as mass plus molar mass), the desired concentration or the volume. The calculator converts everything to moles and litres, applies C = n ÷ V, and then sizes the working dilution with C₁V₁ = C₂V₂.
Worked example
A primer tube reads 25 nmol. To make a 100 µM stock, add 25 × 1000 ÷ 100 = 250 µL of TE buffer. If only the mass were printed – say 150 µg with a molar mass of 6,100 g/mol – the amount is 150 ÷ 6,100 = 0.02459 µmol = 24.59 nmol, which would need 245.9 µL; the stock would then contain 100 µM × 6,100 g/mol = 0.61 g/L = 610 ng/µL. For 100 µL of a 10 µM working solution, take 10 × 100 ÷ 100 = 10 µL of stock and add 90 µL of water (a 10× dilution).
Quick reference
| Amount on tube | Volume for 100 µM | Volume for 10 µM |
|---|---|---|
| 10 nmol | 100 µL | 1,000 µL |
| 25 nmol | 250 µL | 2,500 µL |
| 50 nmol | 500 µL | 5,000 µL |
| 100 nmol | 1,000 µL | 10,000 µL |
Tips
- Keep enzyme and substrate concentrations in the same unit.
- Check that your data lie in the linear range of the standard curve.
- pH strongly affects enzyme activity and protein solubility.
- Everything is calculated in your browser.
Common mistakes to avoid
- Extrapolating a calibration curve far beyond the standards.
- Mixing units (U, kat, µmol/min).
- Ignoring dilution factors when reporting the original concentration.
Inputs explained
| Field | What to enter |
|---|---|
| Calculate | Choose one: Volume of diluent to add, Concentration from amount and volume, Amount from concentration and volume |
| Oligo amount given as | Choose one: Moles (nmol on the tube), Mass (µg) and molar mass |
| Oligo amount | Units: nmol, µmol, pmol, mmol, mol |
| Oligo mass | Units: µg, ng, mg, g |
| Molar mass of the oligo | Units: g/mol, kg/mol, Da (u), kDa. From the synthesis report; ≈ 330 g/mol per nucleotide for ssDNA. |
| Desired stock concentration | Units: µM, mM, M, nM |
| Volume of diluent | Units: µL, mL, L |
| Working solution concentration | Units: µM, mM, M, nM |
| Working solution volume | Units: µL, mL, L |
What each result means
- Diluent volume to add = [volume]
- Stock concentration = [concentration]
- Oligo amount = [moles]
- Oligo amount in moles = [moles]
- Oligo mass = [moles]×[Molar mass of the oligo]
- Mass concentration of the stock = [concentration]×[Molar mass of the oligo]
- Stock needed for the working solution = [Working solution concentration]×[Working solution volume]÷[concentration]
- Diluent to add to it = [Working solution volume] – [Working solution concentration]×[Working solution volume]÷[concentration]
- Dilution factor stock → working = [concentration]÷[Working solution concentration]
Formula
Volume (µL) = amount (nmol) × 1000 ÷ concentration (µM) · amount = mass ÷ molar mass · Working dilution: V_stock = C_work × V_work ÷ C_stockWorked example
Inputs — Calculate: Volume of diluent to add; Oligo amount given as: Moles (nmol on the tube); Oligo amount: 25 nmol; Desired stock concentration: 100 µM; Working solution concentration: 10 µM; Working solution volume: 100 µL.
Results — Diluent volume to add = 250 µL; Stock needed for the working solution = 10 µL; Diluent to add to it = 90 µL; Dilution factor stock → working = 10 ×.
Key terms
- Resuspension
- Dissolving a dried pellet (oligo, DNA, protein or cells) in a known volume of liquid.
- Oligonucleotide
- A short synthetic strand of DNA or RNA, typically 15–100 nucleotides.
- Stock solution
- A concentrated solution kept for storage and diluted before use.
- Working solution
- A diluted portion of the stock used directly in reactions.
Frequently asked questions
How much water do I add to make a 100 µM primer stock?
Multiply the nanomoles on the tube by 10: volume (µL) = nmol × 1000 ÷ 100 µM. 25 nmol needs 250 µL; 38.4 nmol needs 384 µL.
Water or TE buffer?
TE (10 mM Tris, 0.1 mM or 1 mM EDTA, pH 7.5–8) protects DNA from acid hydrolysis and nucleases during long storage; nuclease-free water is fine for short-term use and for downstream steps sensitive to EDTA.
Should I spin the tube first?
Yes – centrifuge briefly before opening so the dried pellet, which may sit on the cap or walls, is at the bottom.
How do I make a 10 µM working solution from a 100 µM stock?
Dilute 1:10 – for example 10 µL of stock plus 90 µL of water gives 100 µL at 10 µM.
How do I convert µg of oligo to nmol?
Divide the mass by the molar mass: 150 µg ÷ 6100 g/mol = 24.6 nmol. Without a specification sheet, estimate the molar mass as about 330 g/mol × number of bases.
Why is my measured concentration different?
Synthesis yields are measured by UV absorbance and pellets are not always completely dissolved; let the oligo hydrate for a few minutes, vortex and re-measure A260 if exact concentration matters.
How should I store the stock?
At −20 °C in aliquots; avoid repeated freeze–thaw cycles and keep fluorescent probes in the dark.
What formula does the Resuspension Calculator use?
Volume (µL) = amount (nmol) × 1000 ÷ concentration (µM) · amount = mass ÷ molar mass · Working dilution: V_stock = C_work × V_work ÷ C_stock.