Resuspension Calculator
Enter any two of the three values, amount, concentration, and volume, and this calculator solves for the third. Switch between Oligo mode (for DNA/RNA primers delivered in nmol) and General Reagent mode (for powders with a known molecular weight). A built-in dilution section then lets you work out C1V1 = C2V2 working-stock volumes in one step.
What is resuspension and why does it matter?
When you order a synthetic oligonucleotide (DNA or RNA primer, probe, siRNA, or CRISPR guide), it arrives lyophilized (freeze-dried) as a white pellet at the bottom of a tube. The vendor lists the amount in nanomoles (nmol) or micromoles (umol) on the tube or the datasheet. Resuspension is the process of adding a precise volume of liquid, typically nuclease-free water or TE buffer (10 mM Tris-HCl, 0.1-1 mM EDTA, pH 8.0), to dissolve the pellet and reach a known, reproducible concentration. Getting this right is important because every downstream calculation, from PCR primer concentrations to CRISPR complex molarity, depends on knowing the exact stock concentration. A careless resuspension that produces a 90 uM stock instead of 100 uM will silently under-dose every reaction that follows.
Oligo mode: the resuspension formula
For oligonucleotides, the relationship between the three quantities is: Volume (uL) = Amount (nmol) x 1000 / Concentration (uM). The factor of 1000 converts nmol to pmol and uM to nM, which makes the units cancel cleanly. For example, a 25 nmol oligo that you want at 100 uM: V = 25 x 1000 / 100 = 250 uL. This means you add 250 uL of buffer to the tube. Enter any two of the three quantities and the calculator solves for the third, so you can also work backward from a fixed volume to find out what concentration you will get, or find out how many nmol you need to order to make a given volume at a given concentration.
General reagent mode: molarity from mass
For any chemical powder or lyophilized reagent where you know the molecular weight (MW), the molar concentration formula is: C (mol/L) = m (g) / (MW (g/mol) x V (L)). Rearranging gives the volume: V (L) = m (g) / (MW (g/mol) x C (mol/L)), or the mass: m (g) = C (mol/L) x V (L) x MW (g/mol). The calculator accepts mass in ug, mg, or g; volume in uL, mL, or L; and concentration in uM, mM, or M. For example, to make 1 mL of a 100 mM sucrose solution (MW = 342.3 g/mol): m = 0.1 mol/L x 0.001 L x 342.3 g/mol = 0.03423 g = 34.23 mg.
Dilution mode: C1V1 = C2V2
Once you have a concentrated stock, you will usually need working aliquots at a lower concentration. The dilution equation C1V1 = C2V2 states that the number of moles of solute before and after dilution is the same: stock concentration times stock volume equals working concentration times working volume. Solving for the volume of stock to transfer: V1 = C2 x V2 / C1. Toggle on "Calculate working-stock dilution," enter your stock concentration, the working concentration you want, and the total volume you need, and the calculator tells you exactly how many uL to pipette from the stock tube and how many uL of buffer to add. The dilution factor (C1 / C2) is also shown so you can label your tubes clearly.
Common resuspension concentrations for oligos
| Use case | Recommended stock (uM) | Typical working dilution (uM) | Notes |
|---|---|---|---|
| PCR primer stock | 100 | 10 | Dilute 1:10 from stock for reactions |
| PCR primer working aliquot | 10 | 0.2-0.5 | Use 0.2-0.5 uM per 20 uL reaction |
| qPCR primer | 100 | 2-10 | 200-500 nM final in 20 uL reaction |
| siRNA / RNAi | 20 | 1-5 | Transfection at nM range |
| CRISPR gRNA | 10-100 | 1-10 | RNP complex or plasmid delivery |
| Sequencing primer | 100 | 3.2 | 3.2 pmol/uL per Sanger reaction |
| Hybridization probe | 10 | 0.1-1 | Application-dependent |
Standard conventions used by most molecular biology labs. The 100 uM stock is the most common starting point.
Frequently asked questions
What diluent should I use to resuspend my oligos?
Nuclease-free water is the simplest choice and works for most applications. TE buffer (10 mM Tris-HCl pH 8.0, 0.1 mM EDTA) is preferred for long-term storage because the mild alkaline pH prevents hydrolysis and the EDTA chelates divalent cations that could activate nucleases. Avoid regular tap water or buffers that contain magnesium or calcium, as these activate DNases. For RNA oligos, use DEPC-treated nuclease-free water and keep everything on ice.
Why does the formula use 1000 as a factor?
The standard resuspension formula is V (uL) = nmol x 1000 / C (uM). The 1000 comes from unit conversion: 1 nmol = 1000 pmol, and 1 uM = 1 pmol/uL, so the 1000 converts nanomoles to picomoles. Alternatively, think of it as: uM is umol/L = nmol/mL = pmol/uL, so to get uL from nmol at a given uM concentration, multiply nmol by 1000 (converting to pmol) and divide by the uM value (pmol/uL).
What concentration should I resuspend my PCR primers to?
The standard is 100 uM for the stock and 10 uM for working aliquots. The 100 uM stock is stable at -20 C for years. Working aliquots at 10 uM are used directly in PCR by adding 1-2 uL per 20-50 uL reaction to give a final primer concentration of about 0.2-0.5 uM, which is the typical range for standard PCR. Keep working aliquots at 4 C or -20 C and make multiple small aliquots to avoid repeated freeze-thaw cycles on your stock.
How do I resuspend if I do not know the molecular weight of my compound?
Look up the compound in PubChem (pubchem.ncbi.nlm.nih.gov) by name, CAS number, or SMILES. The "Molecular Weight" is listed on the compound summary page. For commercial reagents, the molecular weight is printed on the vial label or in the Certificate of Analysis. If you are working with a salt form (e.g., hydrochloride salt), use the MW of the salt form as listed on your specific reagent, since the extra mass of the counterion contributes to what you weigh out.
What is the dilution factor and why does it matter?
The dilution factor is C1 / C2, the ratio of stock concentration to working concentration. A 10x dilution means you are making something 10 times less concentrated than the starting material. Knowing the dilution factor matters for labeling tubes, planning serial dilutions, and troubleshooting: if you find that PCR amplification is weak, doubling the primer concentration means starting from a 5x dilution instead of 10x, and you need to know your dilution factor to plan that change quickly.
Can I use this calculator for proteins?
Yes, in General Reagent mode. Enter the protein mass in mg or ug, enter the molecular weight (usually in kDa, but remember to convert to g/mol: 1 kDa = 1000 g/mol), choose your target concentration in uM or mM, and the calculator gives the volume of buffer to add. Keep in mind that protein solubility varies widely and many proteins require specific buffer conditions, detergents, or cofactors to stay folded. The math is the same, but always verify solubility for your specific protein.
My oligo tube says it contains 4.2 OD units, not nmol. What do I enter?
OD (optical density) units are a different measure of quantity. To convert OD to nmol you need the extinction coefficient, which depends on the specific sequence. IDT and most suppliers provide a "nmol" figure alongside the OD value on the datasheet or tube label. Use the nmol figure directly in this calculator. If you only have OD units, use the supplier's analysis page or the nearest sequence-specific extinction coefficient calculator to convert first.