SeqrunOligo molecular weight, explained
The molecular weight of a single-stranded oligo is just the sum of its base masses, with one small correction. It is the number that lets you convert between a mass of oligo (µg) and a number of molecules (nmol).
Adding up the bases
Each nucleotide contributes a fixed anhydrous mass to a DNA oligo:
MW = nA·313.21 + nT·304.20 + nC·289.18 + nG·329.21 − 61.96 g/mol
The −61.96 offset removes one phosphate the running sum overcounts, giving the mass of a standard 5′-OH oligo. For ACGTACGT (2 each of A, C, G, T): 2(313.21 + 289.18 + 329.21 + 304.20) − 61.96 =2,409.64 g/mol.
Why not 650 per base?
The familiar 650 g/mol figure is per base pair of double-stranded DNA — it averages both strands plus the counter-ions. An oligo is single-stranded, so its per-base mass is roughly half that, around 300–330 g/mol depending on the base. Using the dsDNA rule on an oligo would nearly double the answer.
Mass to moles
Once you have MW, the conversions fall out. One nmol of oligo weighs MW ÷ 1000 micrograms — so the 2,409.64 g/mol oligo above is about 2.41 µg per nmol. This is how a vendor’s µg or OD figure becomes the nmol you need for resuspension, and how you turn a weighed mass into a molar concentration.
What it assumes
The formula is for an unmodified oligo with a free 5′-OH. A 5′ phosphate adds about 80 g/mol; fluorescent dyes, biotin, or other modifications add their own mass and shift the total. For those, use the exact figure on your synthesis spec sheet.