DNA Concentration Calculator

One absorbance unit is 50 µg/mL for DNA, 40 for RNA, 33 for single-stranded.

Concentration from A260750 µg/mLat 50 µg/mL per absorbance unit, diluted 100-fold

Copies1.853 × 10¹⁰in 100 ng of a 5000 bp molecule

Molar mass of the molecule3.25 × 10⁶ g/molat 650 g/mol per base pair

Moles3.0769 × 10⁻¹⁴ mol

Copies per nanogram1.853 × 10⁸

Mass of a single copy5.3968 × 10⁻¹⁸ g

The same mass of a human genome2.8953 × 10⁴ copies3.2 billion base pairs — six orders of magnitude fewer

As RNA instead600 µg/mLRNA absorbs more per microgram

Copy number falls as the molecule gets longer, so the same mass of a long template is far fewer molecules. A standard curve must therefore be built from the same construct being measured — mass alone says almost nothing about how many templates are present.

The formula

copies = ng * 1e-9 * NA / (bp * 650)

From nanograms to molecules

A base pair of double-stranded DNA averages about 650 g/mol. Multiply by the length to get the molar mass of the whole molecule, divide the mass by that, and multiply by Avogadro's number. One nanogram of a 3,000 bp plasmid works out at about 3 × 10⁸ copies.

Length matters as much as mass

Copy number is inversely proportional to length, so the same nanogram of a human genome — 3.2 billion base pairs — is only about 300 copies. That six-order-of-magnitude difference is why a standard curve must be built from the same construct you are measuring, and why quantifying by mass alone tells you almost nothing about how many templates are in the tube.

The 260 nm shortcut

An absorbance of 1 at 260 nm corresponds to about 50 µg/mL of double-stranded DNA, 40 for RNA and 33 for single-stranded DNA. These are averages over base composition, and the 260/280 ratio is the accompanying purity check: around 1.8 for clean DNA, 2.0 for RNA, and lower when protein is carried over.

Ratios are expectations, not promises

Every genetic ratio on these pages is a probability distribution, not a guarantee. A 3:1 cross does not produce three dominant offspring for every recessive one in a litter of four — it produces each offspring independently with a three-quarters chance. Small families depart from the expected ratio routinely, and that is the reason Mendel needed thousands of pea plants rather than dozens.