DNA and RNA molecular weight calculator
Paste a DNA or RNA sequence — FASTA headers, spaces and line numbers are skipped. Choose single-stranded DNA, double-stranded DNA (the complementary strand is added) or RNA, and whether the 5′ end carries a phosphate. You get the molecular weight, how many picomoles make one microgram, and the base counts.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Paste the sequence, 5′ to 3′. Plain text and FASTA both work.
- Choose the molecule: single-stranded DNA, double-stranded DNA or RNA.
- Choose the 5′ end: hydroxyl for most synthetic oligos, phosphate for enzymatic products or phosphorylated oligos.
- Read the molecular weight, the conversion between mass and moles, and the base counts.
MW (ssDNA) = 313.21 × A + 304.2 × T + 289.18 × C + 329.21 × G − 61.96
MW (RNA) = 329.21 × A + 306.17 × U + 305.18 × C + 345.21 × G − 61.96
- 5′ phosphate: + 79.98 per strand
- dsDNA: the sum of both strands
- 1 µg = 10⁶ ÷ MW pmol; 1 nmol = MW ÷ 1000 µg
Worked example
The oligo ATGC: 313.21 + 304.2 + 329.21 + 289.18 − 61.96 = 1,173.84 g/mol. As a duplex with its complement TACG (the same four bases), 2 × 1,173.84 = 2,347.68 g/mol. The RNA AUGC weighs 329.21 + 306.17 + 345.21 + 305.18 − 61.96 = 1,223.81 g/mol.
The default 66-base sequence has A 16, T 23, G 18 and C 9: 16 × 313.21 + 23 × 304.2 + 18 × 329.21 + 9 × 289.18 − 61.96 = 20,474.4 g/mol, so 1 µg is 48.841 pmol.
Tips
- Quick estimate. With these residue masses an average DNA nucleotide is about 309 g/mol and a base pair about 618 g/mol; use them to sanity-check a result.
- Ambiguous bases. N, R, Y and other codes have no defined mass, so the calculator asks for plain bases.
- Plasmids. For a circular plasmid there are no ends; the difference from the linear value (about 62 per strand) is negligible at thousands of base pairs.
Frequently asked questions
How is the molecular weight of DNA calculated?
By adding the mass of each nucleotide as it sits in the chain: A 313.21, T 304.2, C 289.18 and G 329.21 g/mol for DNA, then subtracting 61.96 for the ends of a strand with a 5′-OH. RNA uses A 329.21, U 306.17, C 305.18 and G 345.21. These are the values of the widely used Thermo Fisher and OligoCalc formula.
What does the −61.96 mean?
The residue masses each include one phosphate. A synthetic oligo has no phosphate at its 5′ end and a hydroxyl at the 3′ end, so one HPO₃ (79.98) is taken away and one water (18.02) added: −61.96. Choosing a 5′ phosphate adds the 79.98 back.
How is double-stranded DNA handled?
The complementary strand is built from your sequence (A pairs with T, G with C) and both strands are added, each with its own ends. The rows show the mass of each strand.
How do I convert micrograms to picomoles?
pmol = µg × 10⁶ ÷ molecular weight. The calculator shows how many picomoles are in 1 µg; multiply it by your amount in micrograms. For the 66-base default sequence, 1 µg is 48.841 pmol.
Why can my supplier’s number differ slightly?
Suppliers may use slightly different residue masses, count sodium salts, or include modifications (dyes, phosphorothioates). Differences of a few daltons in tens of thousands are normal.