Why is the factor different for RNA and ssDNA?

Single-stranded molecules absorb more per unit mass than duplex DNA because base stacking (which reduces absorbance) is weaker, so a lower µg/mL corresponds to the same A260.

What about the A260/A280 purity ratio?

This tool converts A260 to concentration only. A pure DNA sample reads ~1.8 and RNA ~2.0 for A260/A280; a low ratio flags protein or phenol contamination that also inflates A260.

A260 to nucleic-acid concentration calculator

Convert a NanoDrop/spectrophotometer A260 reading to DNA or RNA concentration (ng/µL), using the standard absorbance factor for dsDNA, ssDNA or RNA, with a dilution factor.

How it works

Formula

concentration (ng/µL) = A260 × factor × dilution factor. The factor is the µg/mL per A260 unit for a 1 cm path: 50 for dsDNA, 33 for ssDNA, 40 for RNA. ng/µL and µg/mL are numerically equal.

Worked example

A260 = 0.5 for dsDNA diluted 10×: 0.5 × 50 × 10 = 250 ng/µL.

When to use it

To turn a spectrophotometer or NanoDrop reading into a concentration, remembering to multiply by any dilution you made before reading. Pick the factor that matches your molecule type.

Sensible defaults

Defaults read A260 = 0.5 of a 10× diluted dsDNA sample → 250 ng/µL. Set the dilution factor to 1 if you read the sample neat.

FAQ

Why is the factor different for RNA and ssDNA?: Single-stranded molecules absorb more per unit mass than duplex DNA because base stacking (which reduces absorbance) is weaker, so a lower µg/mL corresponds to the same A260.
What about the A260/A280 purity ratio?: This tool converts A260 to concentration only. A pure DNA sample reads ~1.8 and RNA ~2.0 for A260/A280; a low ratio flags protein or phenol contamination that also inflates A260.