DNA Molar Mass Calculator
Estimate the molar mass of a DNA fragment from its length in base pairs, then convert a sample mass into moles, picomoles and the number of copies.
How it works
molar mass = length × 660 g/mol per bp (dsDNA) or × 330 g/mol per nt (ssDNA) · moles = mass ÷ molar mass · copies = moles × 6.022×10²³
The average molar mass of a DNA base pair is about 660 g/mol (roughly 330 g/mol per single nucleotide), so multiplying the length by that constant gives a fragment’s molar mass. Dividing a measured mass by the molar mass converts nanograms or micrograms into moles, picomoles and — via Avogadro’s number — the actual number of molecules. This conversion is essential when a protocol calls for a specific molar amount of a fragment, such as setting up a ligation at a defined insert-to-vector ratio.
Worked example
A 1,000 bp double-stranded fragment has a molar mass of 1,000 × 660 = 660,000 g/mol (660 kDa). One microgram (1×10⁻⁶ g) is 1×10⁻⁶ ÷ 660,000 ≈ 1.52×10⁻¹² mol, i.e. 1.52 pmol, or about 9.1×10¹¹ copies.
Sources: Nucleic acid molar mass — Wikipedia
Frequently asked questions
Why 660 g/mol per base pair?
It is the average molar mass of the four possible base pairs including the sugar-phosphate backbone. The exact value depends on sequence, but 660 g/mol per bp (330 per nucleotide) is the standard approximation.
How do I get moles from a concentration?
Multiply the concentration by the volume to get the mass, then divide by the molar mass. This tool takes the mass directly; use the DNA concentration calculator first if you only have an A260 reading.
Does GC content matter?
Slightly — G-C and A-T pairs differ in mass — but the 660 g/mol average is close enough for cloning and quantification. Use exact per-base masses only when you need a precise oligo molar mass.
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Educational estimate, not medical or laboratory advice — verify formulas, units and assumptions for your specific protocol.