DNA Concentration Calculator
A₂₆₀ to µg/mL, with purity ratio and melting temperature.
Work out DNA Concentration. A₂₆₀ to µg/mL, with purity ratio and melting temperature. Names the mistake before you make it.
For the purity ratio. Set to 0 to skip it.
Optional — gives molarity and a melting temperature estimate.
Nucleic acid concentration
50 µg/mL
260/280 = 1.818 — Within the usual clean range
An A₂₆₀ of 1.0 in a 1 cm path is 50 µg/mL of double-stranded DNA by definition, 40 for RNA and 33 for single-stranded. Those factors are conventions built on average base composition, not measurements of your specific sample. The 260/280 ratio should sit between 1.8 and 2.0 for clean nucleic acid. Below 1.7 usually means protein or residual phenol; above 2.1 can mean RNA contaminating a DNA prep. A 260/230 ratio is worth checking too — it catches guanidine and other buffer carryover that 260/280 misses entirely. The melting temperature uses the Wallace rule below 14 bases and the GC-content formula above it. Both ignore salt and sequence context, so treat the figure as a starting point for optimising an annealing temperature rather than a value to rely on.
How the DNA Concentration Calculator works
Enter your A₂₆₀ and A₂₈₀ readings to get concentration, molarity and the purity ratio, with a melting temperature estimate for oligonucleotides. The page states which Tm formula it used and refuses to give one where neither formula holds.
Also known as: a260 to ug/ml converter · nanodrop calculator · 260/280 ratio calculator · dna melting temperature calculator
Frequently asked questions
How do I calculate DNA concentration from A260?
Multiply by the conversion factor for the nucleic acid type: 50 µg/mL per absorbance unit for double-stranded DNA, 40 for RNA, 33 for single-stranded. Those factors assume a 1 cm path length.
What should the 260/280 ratio be?
Between 1.8 and 2.0 for clean nucleic acid. Below 1.7 usually means protein or leftover phenol; above 2.1 can mean RNA in a DNA prep. It is worth checking 260/230 as well, which catches guanidine carryover that 260/280 misses.
How is melting temperature calculated?
Below 14 bases by the Wallace rule — 2 °C per A or T and 4 °C per G or C. Above that by the GC-content formula. Both ignore salt concentration and sequence context, so they are starting points for optimisation rather than values to rely on.
Why does my Tm estimate not match my PCR results?
Because these formulas leave out salt, magnesium, and which bases sit next to which. Nearest-neighbour thermodynamics accounts for all three and is what primer design software uses.
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