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DNA and RNA Molarity Calculator

Convert a DNA or RNA concentration between mass per volume and molarity from the molecule's length, with pmol per µg, molecules per µL and the number of DNA ends.

Formula

MW=wˉbase×L\mathrm{MW} = \bar{w}_{\text{base}} \times L
cmol=cmassMWc_{\mathrm{mol}} = \dfrac{c_{\mathrm{mass}}}{\mathrm{MW}}
cmass=cmol×MWc_{\mathrm{mass}} = c_{\mathrm{mol}} \times \mathrm{MW}
pmol/μg=106MW\mathrm{pmol}/\mu\mathrm{g} = \dfrac{10^{6}}{\mathrm{MW}}
wˉbase\bar{w}_{\text{base}}
650 g/mol per bp of double-stranded DNA, 330 g/mol per nt of single-stranded DNA, 340 g/mol per nt of RNA
LL
base pairs (double-stranded) or nucleotides (single-stranded)
MW\mathrm{MW}
molecular weight of the whole molecule, in g/mol

How it works

Library preparation, ligation, transfection and standard curves are all planned in moles, but nucleic acids are measured in mass per volume. The bridge is the molecular weight of the molecule. For a long molecule of mixed sequence the average weight per base, multiplied by the length, is a good estimate.

If you know the exact weight (for example from the sequence), choose that option and enter it instead. The double-stranded DNA result also gives the number of ends per microlitre, which is the number that matters when planning a ligation with linear fragments.

Worked example

A 3,000 bp linear PCR product at 50 ng/µL.

  1. MW = 650 g/mol per bp × 3,000 bp = 1,950,000 g/mol.
  2. 50 ng/µL = 0.05 g/L, so molarity = 0.05 / 1,950,000 = 2.564 × 10⁻⁸ mol/L.

25.64 nM, which is 0.5128 pmol per µg, or 1.54 × 10¹⁰ molecules per µL.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The average weight per base is an approximation. It is within a percent or two for double-stranded DNA of ordinary composition, and less exact for short or very GC-rich molecules.
  • Some references use 660 g/mol per bp instead of 650. The two differ by 1.5%, well inside the error of most concentration measurements.
  • The concentration refers only to the molecule whose length you entered, with no other nucleic acid present.

Common mistakes

  • Entering the length in kilobases. The tool expects bases or base pairs (3000, not 3).
  • Using the double-stranded average for a single-stranded molecule, which overstates its weight by nearly a factor of two.
  • Forgetting that a spectrophotometer reports total nucleic acid, so contamination with RNA or primers inflates the number.

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.