Calculate molarity in M, mM, µM, nM or pM from a ng/µL reading plus molecular weight or DNA/RNA length, and reverse it to solve for ng/µL or MW.
ng/uL to Molar Formula
The core relationship between mass concentration and molarity is:
Where M is molarity in mol/L, C is mass concentration in ng/uL, and MW is molar mass in g/mol (often called molecular weight in laboratory usage). The factor of 1,000 accounts for the unit conversion from nanograms per microliter to grams per liter (since 1 ng/uL = 0.001 g/L).
This formula works because 1 ng/uL is equivalent to 1 ug/mL and 1 mg/L. These three concentration units are interchangeable, which means any value expressed in ug/mL or mg/L can be plugged directly into the calculator above without conversion.
Dimensional Analysis
Breaking the conversion into explicit unit steps makes the math transparent. Start with ng/uL, convert to g/L, then divide by molecular weight in g/mol to arrive at mol/L (molarity):
(ng/uL) x (1 ug / 1000 ng) x (1 mg / 1000 ug) x (1 g / 1000 mg) x (10^6 uL / 1 L) = g/L
The net conversion factor is 10^6 / 10^9 = 10^-3, confirming that 1 ng/uL = 0.001 g/L. From there: Molarity = (g/L) / (g/mol) = mol/L.
Molecular Weight Reference Values for Nucleic Acids
Nucleic acid molar mass depends on sequence, length, end groups and modifications. The following length factors are approximate conventions; use a sequence-specific or supplier-provided molar mass when available, especially for short or modified oligonucleotides:
| Nucleic Acid Type | Average MW per Unit | Unit | Notes |
|---|---|---|---|
| dsDNA | 660 | g/mol per bp | Most common for genomic DNA, PCR products, plasmids |
| ssDNA | 330 | g/mol per nt | Oligonucleotides, primers, ssDNA viral genomes |
| ssRNA | 340 | g/mol per nt | mRNA, total RNA, ssRNA viral genomes |
| dsRNA | 680 | g/mol per bp | siRNA duplexes, dsRNA viral genomes |
| Total MW = (average MW per unit) x (number of bases or base pairs). For a 300 bp dsDNA fragment: MW = 660 x 300 = 198,000 g/mol. | |||
For the length-based estimate, nM = (ng/uL x 10^6) / (average molar mass per unit x length in bp or nt). Some next-generation sequencing workflows use this estimate for library normalization or pooling. Use the library-size definition, quantification method and concentration specified by the applicable instrument and library-preparation protocol.
Common Protein Molecular Weights
For protein work, mass depends on species, isoform, processing, glycosylation and oligomeric state. The table gives approximate examples, not universal values for every preparation. Molecular mass in kDa and molar mass in g/mol are different quantities: 1 kDa = 1,000 Da exactly, corresponding approximately to 1,000 g/mol under the usual laboratory numerical convention.
| Protein | MW (kDa) | MW (g/mol) | 1 ng/uL in nM |
|---|---|---|---|
| Insulin | 5.8 | 5,800 | 172.4 |
| Lysozyme | 14.3 | 14,300 | 69.9 |
| GFP | 26.9 | 26,900 | 37.2 |
| BSA (bovine serum albumin; approximate) | 66.5 | 66,500 | 15.0 |
| Transferrin | 79.6 | 79,600 | 12.6 |
| IgG Antibody | 150 | 150,000 | 6.7 |
| Fibrinogen | 340 | 340,000 | 2.9 |
| IgM Antibody | 900 | 900,000 | 1.1 |
| nM values calculated as: nM = (1 ng/uL x 10^6) / MW in g/mol. A smaller protein yields a higher molar concentration at the same mass concentration. | |||
Why This Conversion Matters in the Lab
Quantification instruments like the Qubit fluorometer and NanoDrop spectrophotometer report concentration in ng/uL. However, most downstream molecular biology protocols require molar concentrations. The mismatch between how samples are measured and how they need to be prepared makes this conversion one of the most frequent calculations in any life sciences lab.
NGS Library Preparation: Library normalization, pooling and final loading concentrations are protocol-specific and are not interchangeable. For example, some Illumina workflows start from libraries in nM and then denature and dilute them to a final loading concentration in pM. Use the current instructions for the exact instrument, reagent kit and library type; there is no universal 2 to 4 nM loading target.
qPCR Quantification: When using qPCR to quantify sequencing libraries, results are often reported in copies/uL or ng/uL. Converting to nM allows accurate pooling of multiple libraries at equimolar ratios, which is critical for balanced read distribution across samples in multiplexed runs.
Protein Assays and Drug Dosing: Enzyme kinetics experiments require substrate and inhibitor concentrations in molar units (uM or nM) to calculate Km, Vmax, IC50, and Ki values. Mass spectrometry calibration curves, ELISA standard curves, and Western blot loading controls all benefit from converting mass concentrations into molar terms for accurate comparison across molecules of different sizes.
ng/uL to Molarity Conversion Table
| Concentration (ng/uL) | Molarity (M) | Molarity (mM) | Molarity (uM) |
|---|---|---|---|
| 0.5 | 0.000001 | 0.001 | 1.000 |
| 1 | 0.000002 | 0.002 | 2.000 |
| 2 | 0.000004 | 0.004 | 4.000 |
| 5 | 0.000010 | 0.010 | 10.000 |
| 10 | 0.000020 | 0.020 | 20.000 |
| 25 | 0.000050 | 0.050 | 50.000 |
| 50 | 0.000100 | 0.100 | 100.000 |
| 75 | 0.000150 | 0.150 | 150.000 |
| 100 | 0.000200 | 0.200 | 200.000 |
| 150 | 0.000300 | 0.300 | 300.000 |
| 200 | 0.000400 | 0.400 | 400.000 |
| 250 | 0.000500 | 0.500 | 500.000 |
| Assumes MW = 500 g/mol. For a different MW, molarity scales inversely: doubling MW halves the molarity at the same ng/uL. | |||
Unit Equivalence Quick Reference
The metric mass-concentration and molar-concentration equalities below do not require an aqueous solvent. Relating mg/L to mass-fraction ppm additionally depends on solution density; the familiar approximate equality assumes a density near 1 kg/L.
| Equivalence | Relationship |
|---|---|
| 1 ng/uL | = 1 ug/mL = 1 mg/L; approximately 1 ppm by mass only when density is about 1 kg/L |
| 1 uM | = 1 umol/L = 1 pmol/uL = 0.001 mM |
| 1 nM | = 1 nmol/L = 1 fmol/uL = 0.001 uM |
| 1 kDa | = 1,000 Da exactly; corresponds approximately to 1,000 g/mol molar mass |
| uM from ug/mL | uM = (ug/mL) / (MW in kDa) |
| nM from ng/mL | nM = (ng/mL) / (MW in kDa) |
Common Pitfalls
The most frequent error in this conversion is using the wrong molecular weight. For nucleic acids, the total MW depends on fragment length, not just the per-base value. A 20-mer ssDNA primer (MW = 330 x 20 = 6,600 g/mol) and a 10,000 bp plasmid (MW = 660 x 10,000 = 6,600,000 g/mol) differ by three orders of magnitude. Using a generic MW for both will produce wildly incorrect molar concentrations.
Another common mistake is confusing ng/uL with ng/mL. These differ by a factor of 1,000 (since 1 uL = 0.001 mL). A Qubit reading of 25 ng/uL is 25,000 ng/mL, and mixing up the two will throw off downstream calculations by 1,000x.
For proteins measured by Bradford or BCA assays, the reported concentration is typically in ug/mL. Since 1 ug/mL = 1 ng/uL, the value can be entered directly into the calculator. However, if the assay reports in mg/mL, multiply by 1,000 to get ng/uL before using the formula.
