DNA Molarity Calculator

Molarity (nM) = [DNA concentration (ng/µL) × 10⁶] ÷ [length (bp or nt) × average molecular weight per base]. This works because ng/µL and g/L are proportional, and dividing a mass concentration by a molecular weight gives a molar concentration.

For double-stranded DNA (dsDNA), 650 g/mol per base pair is the standard approximation widely used in molecular biology. For single-stranded DNA or oligonucleotides, 330 g/mol per nucleotide is the standard approximation. Both are averages across the four bases — for a very precise oligo molarity, use the oligo's exact calculated molecular weight (from its actual sequence) instead.

Many protocols specify reaction components by molar amount (e.g., pmol of primer or insert), not by mass — converting your stock's ng/µL concentration (from a spectrophotometer or Qubit reading) to nM lets you pipette the correct molar amount.

No — it's a commonly used average; the true average molecular weight per base pair varies slightly with GC content. For most cloning and PCR applications, the 650 g/mol/bp approximation is standard and sufficiently accurate.

Concentration (ng/µL) = A260 reading × dilution factor × extinction coefficient constant. The standard constant is 50 ng·cm/µL for dsDNA, 33 for single-stranded DNA/oligos, and 40 for RNA — this is the standard Beer-Lambert-based convention used by spectrophotometers like the NanoDrop.