# OD260/280 Nucleic Acid Purity Ratio Calculator Judge DNA/RNA purity from A260/A280 (pure DNA ~1.8, pure RNA ~2.0; low = protein/phenol contamination, DNA > 2.0 = RNA contamination) plus the optional A260/A230 secondary check. Derived from Manchester 1995, Sambrook & Russell, Thermo Fisher T042. Educational use only. > Canonical page: https://elysiatools.com/en/tools/od260-280-ratio - **Category:** Education - **Keywords:** od260/280 ratio, a260/a280 ratio, nucleic acid purity, dna purity check, rna purity check, protein contamination nucleic acid, phenol contamination, a260/a230 ratio, nanodrop purity, spectrophotometer purity, molecular biology, lab calculator ## Overview The OD260/280 Nucleic Acid Purity Ratio Calculator evaluates DNA or RNA purity from A260 and A280 absorbance readings, with an optional A260/A230 secondary check for organic or salt carryover. It compares the calculated ratios with expected ranges for DNA and RNA and is intended for educational use. ## Inputs - **Nucleic Acid Type** (select): Expected ratio: DNA ~1.8, RNA ~2.0 — the acceptable window shifts with type. - **A260 Absorbance** (number): Absorbance at 260 nm (nucleic acid peak), same diluted sample as the A280 reading. - **A280 Absorbance** (number): Absorbance at 280 nm (protein/phenol peak) of the same diluted sample. - **A230 Absorbance (optional)** (number): Absorbance at 230 nm; enter 0 to skip the secondary A260/A230 ratio. - **Decimal Places** (number) ## When to use - Check whether a DNA preparation is near the expected A260/A280 ratio of 1.8. - Assess whether an RNA preparation is near the expected A260/A280 ratio of 2.0. - Use A260/A230 as a secondary check for guanidine, phenol, organic, or salt carryover. ## How it works - Select DNA or RNA as the nucleic acid type. - Enter A260 and A280 readings from the same diluted sample. - The calculator divides A260 by A280 and compares the result with the selected nucleic acid range. - Optionally enter A230 to calculate and interpret the A260/A230 ratio, then choose the desired decimal places. ## Use cases - Screen plasmid DNA preparations before restriction digestion or sequencing. - Review RNA extraction quality before RT-qPCR. - Teach spectrophotometric purity assessment using realistic absorbance readings. ## Frequently asked questions ### What does the A260/A280 ratio measure? It compares absorbance at 260 nm and 280 nm to help assess nucleic acid purity and identify possible protein or phenol contamination. ### What is the expected A260/A280 ratio for pure DNA? Pure DNA is expected to be approximately 1.8, with an acceptable window of about 1.7–2.0. ### What is the expected A260/A280 ratio for pure RNA? Pure RNA is expected to be approximately 2.0, with an acceptable window of about 1.8–2.2. ### What does a low A260/A280 ratio indicate? A low ratio can indicate protein or phenol contamination; interpretation depends on whether the sample is DNA or RNA. ### What is the optional A260/A230 check used for? A low A260/A230 ratio can indicate carryover of guanidine, phenol, organic compounds, or salts. A pure sample is commonly near 2.0–2.2. ## Related tools - [DNA Concentration Calculator (A260 Absorbance)](https://elysiatools.com/en/tools/dna-concentration-a260): Beer–Lambert DNA quantitation: c = A260 × dilution factor × K / path length with 1 OD = 50 µg/mL dsDNA or 33 µg/mL ssDNA, plus ng/µL, total yield, and a 0.1–1.5 linear-range check. Derived from Marmur & Doty, Sambrook & Russell, Thermo Fisher NanoDrop notes. Educational use only. - [OD260/230 Nucleic Acid Purity Ratio Calculator](https://elysiatools.com/en/tools/od260-230-ratio): Check salt/organic-solvent contamination with A260/A230 (pure 2.0–2.2; below 2.0 = guanidinium/phenol/EDTA/salt carryover from TRIzol or columns), with an optional A260/280 cross-check. Derived from Thermo Fisher T042, Sambrook & Russell, Manchester 1995. 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