DNA & RNA Molecular Weight Table
Nucleic-acid molecular weight lets you convert between mass, moles, molarity and copy number. If you only know the length, use the average factors below. If you know the actual sequence, calculate the exact mass from base composition, especially for short primers and RNA oligos.
Average molecular weight by molecule type
| Molecule | Unit | Average mass | Typical use |
|---|---|---|---|
| Double-stranded DNA | base pair (bp) | 650 g/mol per bp | Plasmids, PCR products, genomic fragments |
| Single-stranded DNA | nucleotide (nt) | 330 g/mol per nt | Primers and DNA oligos when sequence is unknown |
| Single-stranded RNA | nucleotide (nt) | 340 g/mol per nt | RNA oligos, guides and transcripts when sequence is unknown |
Exact residue masses
Anhydrous monophosphate residue masses in g/mol. Add the appropriate end correction for a full oligo with terminal hydroxyl groups.
| DNA base | Mass |
|---|---|
| A Adenine | 313.21 |
| C Cytosine | 289.18 |
| G Guanine | 329.21 |
| T Thymine | 304.20 |
| RNA base | Mass |
|---|---|
| A Adenine | 329.21 |
| C Cytosine | 305.18 |
| G Guanine | 345.21 |
| U Uracil | 306.17 |
Common conversion formulas
| Conversion | Formula |
|---|---|
| pmol from ng | pmol = ng x 1000 / MW |
| ng from pmol | ng = pmol x MW / 1000 |
| µM from pmol and volume | µM = pmol / µL |
| nM from pmol and volume | nM = (pmol / µL) x 1000 |
| copy number | copies = moles x 6.022 x 10^23 |
MW is molecular weight in g/mol. Convert ng to grams before calculating moles; the formulas above include the 1000 factor for ng ↔ pmol.
Frequently asked questions
What is the molecular weight of double-stranded DNA?
A useful average is 650 g/mol per base pair. For example, a 1,000 bp dsDNA fragment is about 650,000 g/mol, or 650 kDa.
What is the molecular weight of an oligo?
For a rough estimate, use about 330 g/mol per nucleotide for ssDNA and 340 g/mol per nucleotide for ssRNA. For short oligos, exact base composition matters, so calculate from the sequence when possible.
How do I convert ng to pmol?
Use pmol = ng x 1000 / molecular weight, where molecular weight is in g/mol. The molecular weight can come from the length-based average or from the exact sequence.
Is pmol/µL the same as nM?
No. One pmol per microlitre equals one micromolar (µM), which is 1000 nM. To get nM from pmol and µL, divide pmol by volume in µL and multiply by 1000.
Learn more
Sources
- 1DNA and RNA Molecular Weights and ConversionsThermo Fisher Scientific (Ambion technical reference)Both 'Exact residue masses' tables. Fetched; the page gives 'Exact M.W. of ssDNA = (An x 313.2) + (Tn x 304.2) + (Cn x 289.2) + (Gn x 329.2) + 79.0' and 'Exact M.W. of ssRNA = (An x 329.2) + (Un x 306.2) + (Cn x 305.2) + (Gn x 345.2) + 159', which matches all eight SeqBench residue masses (A 313.21 / C 289.18 / G 329.21 / T 304.20; A 329.21 / C 305.18 / G 345.21 / U 306.17) to the rounding. Note the end corrections differ by intent: Thermo's +79.0 and +159 assume a 5'-monophosphate and a 5'-triphosphate respectively, whereas SeqBench's note ('add the appropriate end correction for a full oligo with terminal hydroxyl groups') matches the 5'-OH case — see the Eurogentec entry. This page does NOT contain the 650/330/340 averages; it uses 607.4/303.7/320.5 instead.
- 2Oligo Scale Determination & Quantitation (oligonucleotide molecular weight calculation)EurogentecThe exact two-decimal residue masses as SeqBench prints them, and the end-correction note under the table. Fetched: 'MW dA = 313.21; MW dC = 289.18; MW dG = 329.21; MW dT = 304.20', with RNA bases taken as the DNA counterpart + 16, which reproduces SeqBench's RNA column exactly (329.21, 305.18, 345.21, and rU from dU 290.17 + 16 = 306.17). It also publishes the formula 'Anhydrous MW = sum of base MW - 63.98 + 2.016', i.e. the -61.96 g/mol correction for a 5'-OH / 3'-OH oligo that SeqBench's caption refers to and that src/lib/bio/tm.ts hard-codes as END_CORRECTION. The same formula is published by IDT (www.idtdna.com oligo MW FAQ) but IDT's site defeats automated fetching with a geo-redirect loop, so Eurogentec is cited as the copy that actually resolves.
- 3RNA size, molecular weight, yield, distribution and conversion (QIAGEN Bench Guide, RNA)QIAGEN Bench GuideThe 'Single-stranded RNA / 340 g/mol per nt' row of the average-mass table only. Fetched; states verbatim 'The molecular weight (MW) of a single-stranded RNA molecule = (# of bases) x (340 daltons/base)'. This page carries no dsDNA or ssDNA average, so it does not back the 650 or 330 rows.
- 4CODATA/SI value of the Avogadro constant (N_A)NIST Reference on Constants, Units and Uncertainty (CODATA 2022)The 6.022 x 10^23 in the 'copy number' formula row (copies = moles x 6.022 x 10^23) and in the copy-number FAQ. Fetched: N_A = 6.022 140 76 x 10^23 mol^-1, exact by SI definition. src/lib/bio/concentration.ts uses the full 6.02214076e23, so the page's 6.022 is a display rounding of an exact constant, not an approximation in the calculation.