If the ratio of DNA OD260/OD280 is between 1.8 and 2.0, the DNA purity (free from protein contaminants) is ~90% or better. If the ratio of RNA OD260/OD280 is ~2.0, the RNA purity is ~90% or better.
This ratio is used as a secondary measure of nucleic acid purity. The 260/230 values for “pure” nucleic acid are often higher than the respective 260/280 values. Expected 260/230 values are commonly in the range of 2.0-2.2. If the ratio is appreciably lower than expected, it may indicate the presence of contaminants which absorb at 230 nm.
The most common purity calculation is the ratio of the absorbance at 260nm divided by the reading at 280nm. Good-quality DNA will have an A260/A280 ratio of 1.8–2.0.