A260/A230 Ratio Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 10/10/2026
The A260/A230 ratio is calculated with absorbance at 260 nm ÷ absorbance at 230 nm. A clean DNA or RNA sample generally shows a ratio between 2.0 and 2.2; a notably lower value signals likely contamination by salts, residual phenol, or carbohydrates.
Explanation
The A260/A230 ratio is a second quality-control measure in spectrophotometry, complementing the already widely used A260/A280 ratio: while the latter mainly detects protein contamination, the A260/A230 ratio instead reveals contamination by chaotropic salts like guanidinium (widely used in many column-based extraction kits), residual phenol (from certain solvent-based extraction protocols), or carbohydrates (common in extractions from plant tissue). A clean sample generally shows a ratio between 2.0 and 2.2; below that range, contamination by one of these compounds becomes increasingly likely. Unlike the A260/A280 ratio, whose purity thresholds are fairly widely agreed upon, there is no strict, universally recognized lower limit for the A260/A230 ratio: some labs tolerate lower values depending on the sample's intended use (routine PCR is generally more tolerant than next-generation sequencing, for example). It's therefore presented here as an indicative benchmark rather than an absolute rule. As with the A260/A280 ratio, an overly concentrated sample can skew the reading even before the measurement itself: our serial dilution calculator helps prepare a dilution suited to the spectrophotometer's linear range.
Example: a sample with A260 = 1.0 and A230 = 0.5
Inputs
Absorbance at 260 nm: 1.0. Absorbance at 230 nm: 0.5.
Calculation
Ratio = 1.0 ÷ 0.5 = 2.0, within the range usually considered clean.
Result
This ratio of 2.0 is a good sign, indicating unlikely contamination by residual salts or solvents.
Frequently asked questions
Why are two ratios (A260/A280 and A260/A230) needed?
Because they detect different types of contamination: the A260/A280 ratio mainly reveals protein contamination, while the A260/A230 ratio instead reveals contamination by chaotropic salts, residual phenol, or carbohydrates. A sample can have a good A260/A280 ratio while having a poor A260/A230 ratio, or the reverse: the two measures are complementary, not interchangeable.
Why isn't there a strict threshold for this ratio, unlike A260/A280?
Because the scientific literature and instrument manufacturers don't agree on a single, universal lower limit for this ratio, unlike the A260/A280 ratio, whose purity thresholds (1.8 for DNA, about 2.0 for RNA) are more widely agreed upon. The acceptable tolerance also depends on the sample's intended use: a less sensitive downstream application (routine PCR) generally tolerates a lower ratio than a highly sensitive one (sequencing, library preparation).
What should I do if my A260/A230 ratio is low despite a good A260/A280 ratio?
This suggests specific contamination by residual salts (often guanidinium if a column-based extraction was used) or carbohydrates, rather than proteins. An additional purification step (an ethanol wash, for example) or re-precipitating the sample generally improves this ratio before a downstream application sensitive to this type of contamination.