Nucleic Acid Purity Ratio Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The A260/A280 purity ratio is calculated with absorbance at 260 nm ÷ absorbance at 280 nm. Pure DNA generally shows a ratio of 1.8 or higher, while pure RNA is generally around 2.0.
Explanation
The A260/A280 ratio is a standard quality-control measure in molecular biology, read directly from the same spectrophotometer (like a NanoDrop) used to measure a DNA or RNA sample's concentration — see our nucleic acid concentration calculator for that second measurement, usually taken at the same time. Absorbance at 260 nm mainly reflects nucleic acids (DNA or RNA), while absorbance at 280 nm mainly reflects proteins, particularly aromatic residues like tryptophan: a sample contaminated with proteins or phenolic residues (used in some extraction protocols) therefore absorbs more at 280 nm, which lowers the A260/A280 ratio. For DNA, a ratio of 1.8 or higher is generally considered a sign of good purity, while a ratio below 1.6 signals likely contamination; for RNA, richer in phosphate groups that absorb more at 260 nm, the expected ratio for a pure sample is slightly higher, around 2.0. This ratio nonetheless remains an indicative measure, not an absolute guarantee of purity: the solution's pH noticeably affects the result (an overly acidic solution underestimates the actual ratio by 0.2 to 0.3, an overly basic one overestimates it by similar amounts), and a ratio within the expected range doesn't rule out other forms of contamination detected by other measures, like the A260/A230 ratio (not covered by this calculator), which instead reveals contamination by salts or residual organic solvents. Before the absorbance measurement itself, an overly concentrated sample can also skew the reading: our serial dilution calculator helps prepare a dilution suited to the spectrophotometer's linear range.
Example: a DNA sample, A260 = 1.8, A280 = 1.0
Inputs
Type: DNA. Absorbance at 260 nm: 1.8. Absorbance at 280 nm: 1.0.
Calculation
Ratio = 1.8 ÷ 1.0 = 1.8, exactly at the expected purity threshold for DNA.
Result
This ratio of 1.8 indicates a pure DNA sample, with protein contamination unlikely.
Frequently asked questions
Why does the expected ratio differ between DNA and RNA?
Because the two molecules' chemical composition differs slightly (RNA contains ribose and uracil rather than deoxyribose and thymine), which slightly changes their UV absorbance profile. In practice, pure RNA absorbs a bit more strongly at 260 nm relative to 280 nm than pure DNA, hence a slightly higher reference ratio (about 2.0 versus 1.8).
Does a ratio within the expected range guarantee a completely pure sample?
No, this ratio mainly detects contamination by proteins or phenolic residues, but not other types of contamination like residual salts, carbohydrates, or certain organic solvents, better detected by the complementary A260/A230 ratio. An A260/A280 ratio within the expected range is a good sign, but doesn't replace other quality checks if in doubt (gel electrophoresis, for example).
Why does my ratio vary slightly from one measurement to another on the same sample?
The pH of the buffer solution used noticeably affects the measured value: a slightly acidic solution tends to underestimate the actual ratio by 0.2 to 0.3, while a slightly basic solution overestimates it by similar amounts. Using a neutral or slightly basic buffer (like Tris-EDTA, TE) to dilute the sample before measurement limits this variability.