DNA Molarity Calculator

How do you convert DNA ng/µL to nM?

Estimate DNA molecular weight from sequence length, convert ng/µL to a molar amount, and report the result in nM or pM.

For routine dsDNA calculations, this page uses an average molecular weight of 660 g/mol per base pair. For ssDNA it uses 330 g/mol per nucleotide. The estimated molecular weight is therefore length × 660 for dsDNA or length × 330 for ssDNA.

Because 1 ng/µL equals 1 mg/L, the conversion can be condensed to nM = ng/µL × 1,000,000 ÷ molecular weight in g/mol. The tool performs that unit conversion automatically.

Why DNA fragment length changes molarity

At the same mass concentration, shorter DNA fragments produce a higher molar concentration because each molecule has less mass.

A 100 bp dsDNA fragment and a 1,000 bp dsDNA fragment differ by roughly tenfold in molecular weight. If both samples are 10 ng/µL, the 100 bp sample contains about ten times as many DNA molecules per unit volume and therefore has about ten times the molarity.

This distinction is important in cloning, ligation, amplicon pooling, sequencing-library normalization, and any experiment that depends on molecule counts or molar ratios rather than only total DNA mass.

DNA molarity for NGS libraries and amplicon pools

Library molarity calculations use mass concentration together with an appropriate average library fragment length, but the value is still an estimate unless concentration and size measurements accurately represent the library population.

For an NGS library, the relevant length may include adapters as well as the insert depending on the platform and normalization method. Use the fragment-size definition specified by the library kit or sequencing workflow.

A broad fragment-size distribution makes a single average length less exact. Likewise, UV-based DNA concentration can overestimate amplifiable library molecules if contaminants or non-library nucleic acids contribute to the mass measurement. Fluorescence and qPCR-based library quantification can provide more application-specific information.

• Use an average or modal library fragment length that matches your validated workflow.
• Confirm whether adapter length is already included in the reported fragment size.
• Use an appropriate DNA concentration measurement for low-concentration libraries.
• Treat calculated molarity as a planning value when the library is heterogeneous or contains non-amplifiable material.

Why some references use 650 instead of 660 g/mol per bp

Average dsDNA molecular-weight conventions vary slightly by reference, so 650 and 660 g/mol per base pair are both seen in laboratory calculations.

Promega commonly uses 660 as an average molecular weight for a nucleotide pair in practical conversion formulas. NEB lists an average dsDNA molecular-weight approximation of 650 daltons per base pair in its nucleic-acid data guidance while also listing more exact base-pair mass information.

For long DNA fragments, the difference between 650 and 660 is usually small relative to other experimental uncertainties. For short oligos or high-precision work, exact sequence composition, terminal groups, and chemical modifications matter more, so sequence-specific molecular weight is preferable.

Common DNA molarity calculation mistakes

The most common errors are using the wrong DNA type, confusing bp with nt, using insert length instead of full library length, or assuming a mass concentration method counts only functional DNA molecules.

• Use bp for dsDNA length and nt for ssDNA length.
• Do not use 330 g/mol per nucleotide for a dsDNA fragment unless you explicitly account for both strands.
• Do not confuse nM with pM; 1 nM equals 1,000 pM.
• For NGS libraries, follow the kit's fragment-length convention and normalization method.
• For modified oligos, use the supplier's exact molecular weight when available.
• Remember that the output can only be as accurate as the input concentration and length estimate.