⚖️ DNA / RNA Molecular Weight & ng↔pmol
Get the molecular weight of a DNA or RNA sequence, convert between ng, pmol and molecule copies, and read concentration from an A260 measurement.
14,706.7 g/mol
dsDNA molecular weight · 24 nt
Moles
68 pmol
Molecule copies
4.095e+13
pmol = ng × 1000 ÷ MW · copies = mol × 6.022×10²³. 🔒 Computed in your browser.
How the dna / rna molecular weight & ng↔pmol works
Molecular weight is summed from the anhydrous monophosphate masses of each base (dsDNA counts both strands). From the MW, moles = mass ÷ MW, so pmol = ng × 1000 ÷ MW, and copies = moles × Avogadro’s number (6.022×10²³). The A260 tab converts absorbance to concentration using the standard factors — 1 A260 = 50 ng/µL for dsDNA, 33 for ssDNA and 40 for RNA — times any dilution.
Every constant here (nucleotide masses, Avogadro’s number, the A260 factors) is a fixed physical value, so the results are exact and never go out of date. The sequence stays in your browser.
Frequently asked questions
How do you calculate the molecular weight of DNA?
Sum the anhydrous molecular weights of each nucleotide monophosphate (A 313.21, T 304.2, C 289.18, G 329.21 g/mol for DNA) and subtract 61.96 for the terminal phosphate. Double-stranded DNA adds the complementary strand. This tool does it from your pasted sequence.
How do I convert ng to pmol?
pmol = nanograms × 1000 ÷ molecular weight (g/mol). The tool computes the MW from your sequence, so you just enter the amount in ng to get pmol and the number of molecule copies.
How do I get DNA concentration from A260?
Multiply the A260 reading by the nucleic-acid factor and any dilution: 1 A260 unit = 50 ng/µL for double-stranded DNA, 33 for single-stranded DNA and 40 for RNA. So an A260 of 1.0 for dsDNA at 1× dilution is 50 ng/µL.
What does the A260/A280 ratio mean?
It is a purity check: about 1.8 indicates pure DNA and about 2.0 pure RNA. Lower ratios suggest protein or phenol contamination. This tool focuses on the concentration; the ratio is your quality gauge.
What is the rule of thumb for oligo amount?
Roughly 1 pmol of an oligo ≈ 0.33 ng per nucleotide of length. For exact work, use the molecular weight from the sequence, which this tool provides.
Does dsDNA weigh twice as much as ssDNA?
Approximately — dsDNA molecular weight is the sum of both complementary strands, so it is close to (but not exactly) double the single strand, depending on base composition.