A spectrophotometer does not measure DNA. It measures how much light a sample absorbs at 260 nanometres, and the conversion to a concentration rests on a convention about how strongly nucleic acid absorbs. That convention is good enough for most laboratory work and it fails in specific, predictable ways. The DNA concentration calculator above does the conversion and reports both purity ratios alongside, because a concentration quoted without them is only half a result.
Arb Digital publishes free science calculators that show what a number is built on. Here the important detail is that everything absorbing at 260 nm gets counted as your nucleic acid: free nucleotides, degraded RNA in a DNA prep, and some contaminants. The ratios are how you find out whether that has happened.
What This DNA Concentration Calculator Does
It converts an A260 reading into a concentration in nanograms per microlitre, applying the extinction convention for your chosen nucleic acid type, your dilution factor and your path length. It multiplies by your sample volume to give total yield in micrograms, computes the A260/A280 and A260/A230 purity ratios with a plain-language verdict on each, and converts the mass concentration into a molar concentration if you supply a fragment length.
Molarity matters more often than people expect. Ligation, sequencing library preparation and any calculation involving copy number all work in moles rather than mass, and the conversion depends entirely on fragment length: 42 nanograms per microlitre of a 500 base pair fragment is ten times the molar concentration of the same mass of a 5,000 base pair fragment. If you need to go further and count molecules, our DNA copy number calculator takes it to copies per microlitre.
The ratios are reported rather than folded into a single quality score on purpose. They fail for different reasons and a single score would hide which one has gone wrong.
How to Use It
- Enter the corrected A260. Blank against the same buffer you eluted into, not against water, because buffer components absorb too.
- Pick the right nucleic acid type. The conversion factor differs between double-stranded DNA, RNA and single strands, and choosing wrongly scales your answer by up to fifty percent.
- Set the dilution factor and path length to match how you read the sample. Microvolume instruments normally report already normalised to a 10 mm path.
- Add the sample volume for total yield, which is the number that tells you whether the prep is worth continuing with.
- Read both ratios before the concentration. A confident concentration from a contaminated sample is worse than no number at all.
The Formula and Where the Constants Come From
The calculation is concentration (µg/mL) = A260 × factor × dilution × (10 ÷ path length in mm), and because a microgram per millilitre is the same as a nanogram per microlitre, the answer is directly usable at the bench.
The factor is the long-standing convention that an absorbance of 1.0 at 260 nm over a 1 cm path corresponds to about 50 micrograms per millilitre of double-stranded DNA, about 40 for RNA, and about 33 for single-stranded DNA and oligonucleotides. These are averaged values that assume a typical base composition; they trace back to the absorption coefficients tabulated in Sambrook and Russell's Molecular Cloning, which is the source the OligoCalc oligonucleotide properties calculator, published in Nucleic Acids Research, cites for its own coefficients — while noting that such coefficients are strictly accurate only for single-stranded oligonucleotides.
A worked example matching the defaults. An A260 of 0.84 for double-stranded DNA, read neat on a 10 mm path, gives 0.84 × 50 = 42 micrograms per millilitre, which is 42 nanograms per microlitre. Across a 50 microlitre eluate that is 2,100 nanograms, or 2.1 micrograms of total yield. With an A280 of 0.45 the 260/280 ratio is 1.87, and with an A230 of 0.40 the 260/230 ratio is 2.10. For a 5,000 base pair fragment at roughly 617.96 grams per mole per base pair, the molar concentration works out at about 13.6 nanomolar.
Reading the Two Purity Ratios Properly
The Thermo Scientific technical bulletin T042 on nucleic acid purity ratios sets out the standard interpretation. A 260/280 ratio around 1.8 is generally accepted as pure for DNA and around 2.0 for RNA. Appreciably lower suggests protein, phenol or something else absorbing near 280 nm. The 260/230 ratio is a secondary measure, expected in the range 2.0 to 2.2, with low values pointing at EDTA, carbohydrates, guanidine salts or phenol.
Three things about these ratios are routinely misread. First, they are ratios, not concentrations, so a very dilute sample gives noisy ratios that swing wildly on tiny absolute differences. Below about 10 nanograms per microlitre the ratios are close to meaningless and should not be quoted.
Second, pH shifts them. The same bulletin notes that an acidic solution under-represents the 260/280 ratio by 0.2 to 0.3 and a basic solution over-represents it by the same amount. A sample eluted in water rather than a buffered eluent can read a full 0.2 lower for no reason connected to purity at all, which is one of the most common false alarms in a molecular biology lab.
Third, a high 260/280 is not better. A ratio well above 1.9 for a DNA prep usually means RNA contamination, because RNA reads higher on that measure. The scale is not a quality axis with more being better; it is a window with failure on both sides.
What Absorbance Cannot Tell You
This is the limitation that costs people the most time. Absorbance at 260 nm is not specific to intact DNA. Free nucleotides absorb there. Degraded RNA absorbs there. Single nucleotides released by nuclease damage absorb there just as strongly as the same bases would in a chromosome. So a sample of thoroughly sheared, useless DNA and a sample of high molecular weight DNA at the same mass concentration read identically.
That is why fluorescent dye-based quantification exists. Dyes that fluoresce only when bound to double-stranded DNA ignore free nucleotides and single-stranded material almost entirely, and routinely report lower concentrations than a spectrophotometer for the same sample. When the two disagree, the fluorometric figure is usually the one to work from for anything downstream that cares about intact template, and the difference between them is itself a useful signal about the state of the prep.
Neither method says anything about fragment length. Gel electrophoresis or a capillary sizing instrument answers that question, and for a prep destined for long-read sequencing or a large-insert clone it is the question that matters most. Concentration, purity and integrity are three separate measurements and only the first two appear on a spectrophotometer.
Small Details That Change the Answer
A few practical points that change results more than their obscurity suggests. Blank against the elution buffer rather than water, because Tris and EDTA both absorb and a water blank quietly adds their contribution to your sample. Take an A320 reading if your instrument offers one and subtract it from the others: absorbance at 320 nm should be zero for a clean sample, so anything there is turbidity or particulates scattering light across the whole spectrum, and it inflates every other reading.
Mix the sample before pipetting. Genomic DNA in particular is viscous and does not distribute evenly, so two microlitres from an unmixed tube can differ substantially from two microlitres from the same tube after mixing. Read at least twice and be suspicious of any disagreement. And keep the absorbance inside the linear range: on a 10 mm path most instruments are reliable up to around 1.0, above which the relationship between absorbance and concentration bends and the reading understates the truth. Diluting is not a compromise in that situation, it is the correct procedure — and the solution dilution calculator handles the arithmetic if you then need to work back.
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Browse All Free Tools Suggest a ToolCommon Mistakes to Avoid
- Using the dsDNA factor for RNA or an oligo — 50, 40 and 33 are different conventions, and picking the wrong one scales the answer by up to half.
- Blanking against water instead of elution buffer — Tris and EDTA absorb, and a water blank folds their contribution into your sample reading.
- Quoting ratios from a very dilute sample — below roughly 10 nanograms per microlitre both ratios are dominated by noise and mean very little.
- Treating a high 260/280 as extra pure — a DNA prep reading well above 1.9 usually contains RNA, because RNA reads higher on that ratio.
- Reading above the linear range — an A260 much over 1.0 on a 10 mm path understates the concentration, so dilute and read again rather than accepting the number.
Related Free Tools From Arb Digital
Take the result through to molecules with the DNA copy number calculator, and prepare working dilutions with the solution dilution calculator or the dilution ratio calculator. The molarity calculator handles solution concentration from mass, the molar mass calculator covers reagent formulas, and the cell dilution calculator deals with the culture side of the same workflow. For coursework, the DNA to mRNA converter handles transcription. The full free online tools hub lists everything else.
Frequently Asked Questions
Multiply the A260 reading by 50 for double-stranded DNA, then by your dilution factor and by ten divided by the path length in millimetres. The result is in micrograms per millilitre, which is numerically the same as nanograms per microlitre.
They are conventional averaged absorption coefficients for typical base composition, tabulated in standard molecular cloning references. Double-stranded DNA absorbs slightly less per unit mass than RNA or single strands because base stacking reduces absorbance.
Around 1.8 is generally accepted as pure for DNA and around 2.0 for RNA. Values appreciably lower suggest protein or phenol, and values well above the expected figure for a DNA prep usually indicate RNA contamination.
It points at contaminants absorbing near 230 nm, most often guanidine salts carried over from a column, EDTA, carbohydrates or phenol. The expected range is roughly 2.0 to 2.2, and a low value often predicts trouble in downstream enzymatic reactions.
Because the absorbance spectrum of nucleic acid shifts with pH. An acidic solution reads the 260/280 ratio 0.2 to 0.3 lower and a basic solution the same amount higher, so eluting in water rather than a buffered eluent can produce an alarming ratio for no real reason.
Because absorbance counts everything that absorbs at 260 nm, including free nucleotides and degraded RNA, while dye-based fluorescence responds mainly to intact double-stranded DNA. For downstream work needing real template, the fluorometric figure is usually the more useful one.
Only for molar concentration. Mass concentration and total yield do not depend on it, but molarity does, because the same mass of a short fragment contains many more molecules than the same mass of a long one.
No. Sheared DNA and intact DNA at the same mass concentration give the same A260. Fragment integrity has to be assessed by gel electrophoresis or a capillary sizing instrument.
This calculator is provided for education and general laboratory reference. It applies conventional averaged extinction values that assume typical base composition, and it is not a substitute for the validated protocols, instrument calibration and quality controls used in your own laboratory.
