Which length should I use?

The average fragment size of the final library including adapters, not the insert size alone. Read it off your fragment-analyzer trace.

Does this work for ssDNA or RNA?

No. The 650 g/mol constant is for double-stranded DNA. Single-stranded DNA and RNA have different per-base masses.

ng/µL ⇄ nM converter

Convert dsDNA concentration between mass units (ng/µL) and molarity (nM) for a fragment of a given length. Essential for library pooling and loading.

How it works

Formula

nM = (ng/µL × 1,000,000) ÷ (length in bp × 650). The 650 g/mol is the average molecular weight of one base pair of dsDNA.

Worked example

10 ng/µL of a 300 bp fragment: (10 × 1,000,000) ÷ (300 × 650) = 51.28 nM. Convert the other way and 51.28 nM of 300 bp dsDNA is back to 10 ng/µL.

When to use it

When pooling libraries to equal molarity, or diluting a library to the picomolar loading concentration your sequencer expects. Molarity — not mass — determines how many molecules cluster.

Sensible defaults

Defaults convert 10 ng/µL of an average 300 bp library. Use the mean fragment size from your Bioanalyzer/TapeStation trace, including adapters.

FAQ

Which length should I use?: The average fragment size of the final library including adapters, not the insert size alone. Read it off your fragment-analyzer trace.
Does this work for ssDNA or RNA?: No. The 650 g/mol constant is for double-stranded DNA. Single-stranded DNA and RNA have different per-base masses.