DNA µg to pmol calculator (dsDNA mass ↔ moles)
Maintained by the Talindrew team · Last updated
To convert double-stranded DNA from micrograms to picomoles, divide the mass by the fragment length times 660 g/mol per base pair: pmol = µg × 10⁶ / (N × 660). One microgram of a 1,000 bp fragment is 1.52 pmol. The reverse conversion multiplies instead of dividing.
Result
Assumes an average of 660 g/mol per base pair (Promega BioMath convention).
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What it looks like
Formula
pmol = µg × 10⁶ / (N bp × 660)
µg = pmol × N bp × 660 / 10⁶
Source: Inverse of the same relation
Worked example
Given: 1 µg of a 1,000 bp PCR product
- 1.Molar mass of the fragment: 1,000 bp × 660 g/mol/bp = 660,000 g/mol.
- 2.Convert µg to pmol: 1 µg × 10⁶ / 660,000 = 1.515 pmol.
1 µg of 1,000 bp dsDNA ≈ 1.52 pmol
How the calculation flows
Units & constants
| Average bp weight | 660 g/mol (Promega BioMath convention) |
|---|---|
| Exact alternative | NEB uses 617.96 × N + 36.04 g/mol; difference < 0.2% for typical fragments |
| Inputs | Mass (µg) or amount (pmol), fragment length (bp) |
| Outputs | pmol or µg |
| Applies to | Double-stranded DNA: plasmids, PCR products, gene fragments |
| Price | Free |
How many picomoles are in 1 µg of a 1 kb fragment?
1.52 pmol. The arithmetic is mass divided by molar mass: a 1,000 bp double-stranded fragment weighs about 660,000 g/mol (1,000 × 660), and 1 µg ÷ 660,000 g/mol = 1.52 × 10⁻¹² mol. Because the per-molecule weight scales linearly with length, a 10 kb plasmid gives ten times fewer picomoles for the same mass: 0.152 pmol/µg.
When do you need moles instead of mass?
Any reaction that depends on the number of molecules rather than their weight is set up in moles.
- Ligations: the insert:vector ratio is a molar ratio, so both must be in pmol.
- Gibson and Golden Gate assemblies: equimolar fragment mixes.
- qPCR standards: copy number is moles × Avogadro's number.
- In vitro transcription and translation: template molarity sets yield.
Why 660 g/mol per base pair?
An average DNA base pair, one purine plus one pyrimidine nucleotide with the phosphate backbone, weighs close to 660 Da. Vendors round differently: Promega uses 660, some tools use 650, and NEBioCalculator computes the exact 617.96 × N + 36.04 formula that accounts for end groups. For bench-scale ligation and assembly math the spread is well under 2% and irrelevant next to pipetting error; this calculator states its convention so you can reproduce the number elsewhere.
Frequently asked questions
Yes. A plasmid is double-stranded DNA, so the same 660 g/mol per bp applies. Enter the full plasmid size in bp. A 3,000 bp plasmid at 100 ng is 0.0505 pmol: the number you need on the vector side of a ligation calculation.
Use the ssDNA converter instead, which uses 330 g/mol per nucleotide, half the double-stranded value because only one strand is present. Using the dsDNA form on an oligo doubles the assumed weight and halves the computed picomoles.
NEBioCalculator uses the exact molecular-weight formula 617.96 × N + 36.04 g/mol while this calculator uses the Promega 660 g/mol per bp convention. The results differ by less than about 0.2% for fragments over a few hundred bp: far below pipetting precision.
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