DNA Molarity Calculator
Many molecular biology steps, from ligations to qPCR standards to sequencing library prep, depend on the number of DNA molecules present, not their total weight. A spectrophotometer or fluorometer gives you mass concentration, so you need to convert to molar concentration using the fragment length. This calculator multiplies length by the average molecular weight per base pair to get molar mass, then converts your mass concentration to nanomolar and reports total mass and femtomoles in your sample volume. The molecular weight per base pair is editable for sequence-specific work.
DNA molarity formula
Molar mass (g/mol) = length (bp) * MW per bp
Molar concentration (M) = (concentration g/L) / molar mass
nM = concentration (ng/µL) * 1e6 / molar mass
Total mass (ng) = concentration * volume
Total amount (fmol) = nM * volume (µL)
One ng/µL equals one g/L. Dividing by molar mass gives molarity; the 1e6 factor converts to nM. Femtomoles follow directly from nanomolar times microlitres.
Practical conversion notes
- Use 650 g/mol per base pair for double-stranded DNA, or about 325 g/mol per base for single-stranded.
- For sequence-specific accuracy, calculate exact molecular weight from base composition and override the default.
- Longer fragments contain fewer molecules per unit mass; always use the correct length.
- Library prep and ligation efficiencies depend on molar ratios, not mass ratios.
- Confirm mass concentration with a fluorometric method for dsDNA-specific quantitation.
DNA molarity: frequently asked questions
How do you convert DNA mass to moles?
The molar mass of a double-stranded DNA fragment is its length in base pairs times the average molecular weight per base pair, which is about 650 grams per mole per base pair. Moles equal mass divided by molar mass, and molar concentration equals moles divided by volume. This calculator applies that chain to convert nanograms per microlitre into nanomolar or femtomoles.
Why is 650 g/mol per base pair used?
650 grams per mole is the widely used average molecular weight of one base pair of double-stranded DNA, accounting for both strands and the sodium counter-ions. It is an average across the four bases; the exact value depends on sequence composition. For single-stranded DNA the figure is about 325 g/mol per base. The calculator exposes this as an editable input so you can use a sequence-specific value.
What is the difference between mass and molar concentration?
Mass concentration (such as ng per microlitre) measures how much DNA by weight is in a volume. Molar concentration (such as nM) measures how many molecules. For ligations, qPCR standards, and library prep you usually need molar concentration, because reactions depend on the number of fragment ends or molecules, not their total mass.
How does fragment length affect molarity?
For a fixed mass concentration, a longer fragment has fewer molecules because each molecule weighs more. Doubling the length roughly halves the molar concentration. That is why converting between mass and molarity always requires the fragment length in base pairs.
What units does this calculator use?
Enter concentration in nanograms per microlitre, length in base pairs, and volume in microlitres. Outputs are molar concentration in nanomolar, the molar mass of the fragment in grams per mole, total DNA mass in nanograms, and total moles in femtomoles. These are the units most common in molecular biology bench work.
Official sources
- National Center for Biotechnology Information: NCBI nucleic acid references.
- National Institute of Standards and Technology: NIST units and measurement.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.