Is this for single- or double-stranded DNA?

Single-stranded — it is for oligos and primers. For duplex dsDNA fragments use the ng/µL ⇄ nM tool, which uses the 650 g/mol-per-base-pair average.

Does it include a 5′ phosphate or modifications?

No. It assumes a standard 5′-OH oligo with no modifications. A 5′ phosphate adds ~80 g/mol, and dyes or other modifications add their own mass.

Oligo molecular weight calculator

Calculate the molecular weight (g/mol) of a single-stranded DNA oligo or primer from its sequence, using standard anhydrous per-base masses.

How it works

Formula

MW = nA·313.21 + nT·304.20 + nC·289.18 + nG·329.21 − 61.96 g/mol. The masses are anhydrous per-base values for a 5′-OH oligo; the −61.96 offset removes one phosphate the sum overcounts.

Worked example

ACGTACGT has 2 A, 2 C, 2 G, 2 T: 2(313.21 + 289.18 + 329.21 + 304.20) − 61.96 = 2,409.64 g/mol.

When to use it

To convert between mass and moles of an oligo — e.g. working out how many µg are in a nmol, or turning an OD reading into a concentration. One nmol weighs MW ÷ 1000 µg.

Sensible defaults

The default ACGTACGT is a balanced 8-mer at 2,409.64 g/mol. Paste your primer to get its mass.

FAQ

Is this for single- or double-stranded DNA?: Single-stranded — it is for oligos and primers. For duplex dsDNA fragments use the ng/µL ⇄ nM tool, which uses the 650 g/mol-per-base-pair average.
Does it include a 5′ phosphate or modifications?: No. It assumes a standard 5′-OH oligo with no modifications. A 5′ phosphate adds ~80 g/mol, and dyes or other modifications add their own mass.