How this instrument works
Nucleic acids absorb ultraviolet light strongly near 260 nanometers, which is why measuring absorbance at that wavelength (A260) is the standard quick method for estimating how much DNA is in a sample without running a gel or a fluorescent assay. The relationship between absorbance and concentration is linear within the instrument's reliable range, governed by the Beer-Lambert law, and for double-stranded DNA specifically, an A260 reading of exactly 1.0 corresponds by convention to a concentration of 50 ng/µL (equivalently 50 µg/mL) at a standard 1 cm path length.
This instrument applies that conversion directly: concentration = A260 reading × 50 ng/µL × dilution factor. The dilution factor matters because samples are frequently diluted before measurement — many spectrophotometers give their most reliable readings within a specific absorbance range, so a highly concentrated sample is often diluted first to bring the raw A260 into that range, and the dilution factor scales the result back up to the original, undiluted sample's true concentration.
This A260-based method estimates total nucleic acid content rather than confirming purity or integrity — RNA, free nucleotides and some contaminants also absorb near 260 nm, so a contaminated or RNA-mixed sample can read a higher DNA concentration than it actually contains. Labs commonly pair the A260 reading with an A260/A280 ratio (checking for protein contamination) and sometimes gel electrophoresis (checking for degradation) before trusting a concentration figure for a sensitive downstream application like sequencing or cloning.
- Enter the absorbance value your spectrophotometer measured at 260 nm into A260 absorbance reading.
- Enter the dilution factor used before measurement into Dilution factor — use 1 if the sample was read undiluted.
- Read DNA concentration (ng/µL) — the estimated double-stranded DNA concentration of your original, undiluted sample.
- This conversion assumes a 1 cm path length and applies the double-stranded DNA constant (50 ng/µL per A260 unit); single-stranded DNA and RNA use different constants and are not what this instrument computes.
Worked example — A260 of 0.5 at a 10x dilution
A sample was diluted 10-fold before measuring, and the spectrophotometer reads A260 = 0.5. Enter 0.5 into A260 absorbance reading and 10 into Dilution factor: DNA concentration (ng/µL) reads 250.0 — 0.5 × 50 × 10 = 250.
That figure describes the original, undiluted sample, not the diluted one that was actually measured — the diluted sample itself sits at only 0.5 × 50 = 25 ng/µL, and multiplying by the 10x dilution factor scales the estimate back up to what the concentration was before dilution, which is normally the number a researcher actually needs for downstream calculations like normalizing input mass for PCR or library preparation.
Questions
Where does the constant 50 ng/µL come from?
It is the standard, widely used conversion factor for double-stranded DNA at 260 nm and a 1 cm path length: an A260 reading of exactly 1.0 corresponds to a dsDNA concentration of 50 ng/µL (50 µg/mL). It is an empirically established constant specific to double-stranded DNA — single-stranded DNA (about 33 ng/µL per A260 unit) and RNA (about 40 ng/µL per A260 unit) use different constants because their UV absorbance per unit mass differs.
Why does the dilution factor matter so much to the result?
Because the reading a spectrophotometer actually measures is the diluted sample's absorbance, but the concentration a researcher usually needs is that of the original, undiluted stock. Multiplying by the dilution factor scales the measured value back up to represent the true concentration of the sample before it was diluted for measurement — omitting it, or using the wrong factor, produces a result that is off by exactly that multiple.
Does a high A260 reading guarantee pure, intact DNA?
No — A260 measures total UV absorbance near 260 nm, and RNA, free nucleotides and certain other contaminants also absorb in that range, so they can inflate the apparent DNA concentration without actually being DNA. A260 alone also says nothing about whether the DNA is degraded or fragmented; purity is more commonly assessed with the A260/A280 ratio, and integrity with gel electrophoresis, alongside this concentration estimate.
Why dilute a sample before measuring instead of reading it directly?
Spectrophotometers generally give their most linear, reliable readings within a specific absorbance range, and a highly concentrated sample can read outside that range, where the Beer-Lambert law's linear relationship between absorbance and concentration starts to break down. Diluting first brings the raw reading back into the instrument's reliable range, and the dilution factor in this formula corrects the result back to the true, undiluted concentration.
Can this formula be used for RNA instead of DNA?
Not with the constant used here — RNA uses a different conversion constant, approximately 40 ng/µL per A260 unit rather than DNA's 50, because RNA and DNA absorb UV light at 260 nm with slightly different efficiency per unit mass. Applying the dsDNA constant to an RNA sample would systematically overstate the RNA concentration by roughly 25%.