DNA Concentration from A260 Calculator

A spectrophotometer reads how much light your nucleic acid absorbs at 260 nm, and that absorbance converts to concentration through a fixed factor: an A260 of 1.0 in a 1 cm path is 50 µg/mL for double-stranded DNA, 33 µg/mL for single-stranded DNA and 40 µg/mL for RNA. This calculator applies conc = A260 × factor × dilution factor ÷ path length in one step, warns when the reading sits outside the linear range where it can be trusted, and — if you give it a molecular weight — turns the mass concentration into a molar one in nM.

Almost every nucleic-acid workflow begins with "how concentrated is my sample", and the default answer comes from an A260 reading. The arithmetic is trivial but the pitfalls are not: people forget the dilution they made before reading, use the wrong factor for their sample type, assume a 1 cm path on an instrument that uses a much shorter one, and trust a reading that is far outside the linear range of the detector. This page is part of the DNA & RNA calculator set and it keeps every one of those terms visible in a single equation so nothing is silently dropped.

The conversion rests on the Beer–Lambert law, A = ε·c·l: absorbance is proportional to concentration and to the path length the light travels. Nucleic-acid work compresses ε into a conventional factor — the concentration that gives A260 = 1 in a 1 cm cuvette — because that is easier to remember than a molar extinction coefficient for a heterogeneous polymer. Those factors are 50 µg/mL for double-stranded DNA, 33 for single-stranded DNA and 40 for RNA. Pick the wrong one and a plasmid prep reads as 50% more concentrated than a denatured version of the same sample.

Two corrections turn a raw reading into a real concentration. If you diluted the sample before measuring — the usual case, because neat preps read far above the linear range — you must multiply by the dilution factor. If your instrument does not use a 1 cm path, you must divide by the actual path length in cm. Microvolume spectrophotometers often use a path of a fraction of a millimetre and report a path-corrected A260 already; if yours does not, entering the true path length is what makes the number right. Forgetting the dilution factor is the single most common error and it always makes the reported concentration too low.

A mass concentration in µg/mL is what you need for most cloning and labelling work, but reactions care about molecules, not grams. Supply a molecular weight — from the DNA molecular weight calculator — and this tool also reports the molar concentration in nM, so a defined oligo can be dosed by moles without opening a second calculator. And because absorbance cannot tell a clean sample from a contaminated one, the notes below cover the A260/A280 purity ratio and when a fluorescence assay beats absorbance entirely.

A260 absorbance

Absorbance at 260 nm. Most instruments are linear between ~0.1 and ~1.5.

Sample type

Dilution factor

Fold the measured sample was diluted, e.g. 10 for 1 µL in 9 µL buffer.

Path length (cm)

1 cm for a standard cuvette; enter your instrument's path if shorter and not auto-corrected.

Molecular weight (optional) (g/mol)

Supply to also report molar concentration (nM). Get it from the DNA Molecular Weight calculator.

A280 absorbance (optional)

Measured at 280 nm on the same diluted sample. Enables the A260/A280 protein-contamination check.

A230 absorbance (optional)

Measured at 230 nm on the same diluted sample. Enables the A260/A230 carryover check.

Concentration40 µg/mL

Concentration0.04 mg/mL

Factor50 µg/mL per A260

Dilution factor1 ×

Path length1 cm

Molar concentration—

A260/A280—

A260/A230—

Purity ratios at a glance

Absorbance ratios flag contamination without any extra work: measure A260, A280 and A230 on the same diluted sample and compare. A pure nucleic-acid prep sits close to the expected ratio; a value that is too low or too high points at a specific contaminant.

Sample	A260/A280	A260/A230	If the ratio is low
Double-stranded DNA	~1.8	~2.0–2.2	Protein or phenol carryover (A260/A280); salts, carbohydrate, phenol or guanidine (A260/A230)
Single-stranded DNA	~1.8	~2.0–2.2	Protein or phenol carryover (A260/A280); salts, carbohydrate, phenol or guanidine (A260/A230)
RNA	~2.0–2.1	~2.0–2.2	Protein or phenol carryover (A260/A280); salts, carbohydrate, phenol or guanidine (A260/A230)

A high A260/A280 (above ~2.2) is not "cleaner" — for RNA it usually means the blank, solvent pH or ionic strength is off; for DNA it can signal residual RNA. Ratios are only meaningful when the sample is measured in a neutral, low-ionic-strength buffer, because pH and salt shift the reading.

What this calculator gives you that others don’t: The top converters either do A260-to-concentration or mass-to-moles but not both, none of them warn you when the reading is outside the linear range, and almost none turn the same reading into a purity verdict. This calculator puts the whole relation — conc = A260 × factor × dilution factor ÷ path length — in one visible equation with the dsDNA/ssDNA/RNA factors side by side, flags readings below 0.1 or above 1.5 A260 as unreliable, links the molar (nM) output to a molecular weight from the DNA MW calculator so mass and moles live in the same view, and — uniquely for a single-input tool — accepts optional A280 and A230 to compute the A260/A280 and A260/A230 ratios live, interpret them against the ~1.8 (DNA) / ~2.0 (RNA) and ~2.0–2.2 expectations for the chosen sample type, and name the likely contaminant when a ratio falls outside the band. A static threshold table beside the widget makes the same cutoffs citable without entering anything, four hand-worked examples cover the classic forgotten-dilution-factor error, and every constant carries tiered sources and a review date.

How to use the dna concentration calculator

Enter the A260 your instrument reported. If it flagged the reading as out of range, dilute and re-measure rather than trusting it.

Choose the Sample type: double-stranded DNA (×50), single-stranded DNA (×33) or RNA (×40). This sets the conversion factor.

Enter the Dilution factor if you diluted before reading — for example 10 if you put 1 µL of sample in 9 µL of buffer. Leave it at 1 for a neat measurement.

Enter the Path length in cm. Use 1 for a standard cuvette; enter your instrument’s actual path if it is shorter and it does not already correct for it.

Optionally enter a Molecular weight in g/mol to also get the molar concentration in nM. Pull it from the DNA molecular weight calculator for a defined oligo.

Optionally enter A280 and A230 (measured on the same diluted sample) to turn on the purity ratios. The calculator reports A260/A280 and A260/A230 and flags a value that sits outside the expected band for your sample type.

Press Calculate. The primary card is the concentration in µg/mL; the cards below give mg/mL, the factor used, the dilution factor and path length applied, nM when a molecular weight was supplied, and the purity ratios when A280/A230 were entered.

Use the result to plan a dilution with the dilution calculator or a molar working concentration with the molarity calculator.

DNA Concentration from A260 formula

Mass concentration: conc (µg/mL) = A260 × factor × dilution factor / path length (cm)

Factors: dsDNA 50, ssDNA 33, RNA 40 (µg/mL per A260 unit at 1 cm)

mg/mL: conc (mg/mL) = conc (µg/mL) / 1000

Molar (if MW given): conc (nM) = conc (µg/mL) × 1e6 / MW (g/mol)

Purity (optional): A260/A280 and A260/A230, each from the same diluted reading

This is the Beer–Lambert law A = ε·c·l with ε folded into the conventional per-type factor for a 1 cm path. The dilution factor multiplies (you diluted, so the original was stronger); the path length divides (a longer path absorbs more for the same concentration). Molar concentration needs a molecular weight and so applies to defined oligos, not heterogeneous genomic DNA.

Variables and units

Symbol	Meaning	Unit
A260	Absorbance at 260 nm	—
factor	Conventional conversion factor by sample type	µg/mL per A260
DF	Dilution factor applied before measuring	×
l	Path length	cm
MW	Molecular weight (optional, for molar output)	g/mol
A280	Absorbance at 280 nm (optional, for the purity ratio)	—
A230	Absorbance at 230 nm (optional, for the purity ratio)	—
conc	Concentration	µg/mL / nM

How dna concentration is calculated

Read the A260 and confirm it is inside the linear range (~0.1–1.5); the tool warns if it is not.
Select the factor for the sample type: 50 (dsDNA), 33 (ssDNA) or 40 (RNA).
Multiply A260 by the factor and by the dilution factor.
Divide by the path length in cm to get the concentration in µg/mL.
Divide by 1000 for mg/mL if you prefer it.
If a molecular weight was supplied, compute nM = µg/mL × 1e6 / MW.

Worked laboratory examples

Example 1 — A neat dsDNA reading

Given: A260 = 0.8 · dsDNA · dilution factor 1 · path length 1 cm

conc = 0.8 × 50 × 1 / 1 = 40 µg/mL.
In mg/mL that is 0.040.
A260 0.8 sits comfortably inside the linear range, so the reading is trustworthy.

Result: 40 µg/mL (0.040 mg/mL)

Example 2 — The dilution factor people forget

Given: A260 = 0.5 measured after a 1:10 dilution · dsDNA · path length 1 cm

Without the dilution factor you would report 0.5 × 50 = 25 µg/mL — wrong.
With it: 0.5 × 50 × 10 = 250 µg/mL.
The original sample is ten times more concentrated than the cuvette contained.

Result: 250 µg/mL — not 25 µg/mL

Example 3 — RNA on a short-path instrument

Given: A260 = 1.2 · RNA (factor 40) · dilution factor 1 · path length 0.5 cm

conc = 1.2 × 40 × 1 / 0.5 = 48 / 0.5 = 96 µg/mL.
Halving the path length doubles the reported concentration for the same absorbance.
If the instrument had already path-corrected the A260 you would enter 1 cm instead and get 48 µg/mL — check which your machine reports.

Result: 96 µg/mL at a 0.5 cm path

Example 4 — Mass to molar for a defined oligo

Given: A260 = 1.0 · ssDNA (factor 33) · MW = 5524.6 g/mol (an 18-mer)

conc = 1.0 × 33 = 33 µg/mL.
nM = 33 × 1e6 / 5524.6 = 5973 nM ≈ 5.97 µM.
The molecular weight came from the DNA molecular weight calculator for the same sequence.

Result: 33 µg/mL ≈ 5.97 µM

Want the reasoning behind the numbers? Read How to Calculate DNA Concentration from A260.

How to interpret the result

The primary µg/mL figure is a mass concentration and is only as good as the reading behind it. Absorbance at 260 nm is not specific to nucleic acid: free nucleotides, residual phenol, guanidine from a column prep, and to a lesser extent protein all absorb nearby and inflate the number. A "concentration" from a dirty sample is really a concentration of everything that absorbs at 260 nm.

The linear-range warning is the most actionable output. Below ~0.1 A260 the signal is close to the noise floor and the percentage error is large; above ~1.5 the detector response curves off and the reading under-reports the true concentration. Either way the fix is the same — bring the sample into range by concentrating or diluting and re-measure, rather than extrapolate.

The molar output in nM applies only when you supplied a molecular weight, which means a defined sequence. For genomic DNA or a mixed RNA population there is no single MW, so a molar figure would be meaningless and the tool leaves it out rather than inventing one.

A260 alone never tells you purity. Enter A280 and A230 alongside it and this calculator computes both ratios and flags anything outside the expected band: A260/A280 should sit near 1.8 for clean DNA and 2.0 for clean RNA, with a low value pointing at protein or phenol carry-over, while a low A260/A230 flags salts, residual phenol, guanidine or carbohydrate. A high A260/A280 is not "cleaner" — for RNA it usually means the blank or solvent pH/ionic strength is off, and for DNA it can signal residual RNA. The ratios are only meaningful in a near-neutral, low-ionic-strength buffer, since pH and salt shift them; treat the flags as advisory, not verdicts. When purity is in doubt or the sample is dilute, a fluorescence assay that binds nucleic acid specifically is more accurate than absorbance.

Assumptions

The sample is in a buffer that does not itself absorb significantly at 260 nm.
The reported A260 is the raw absorbance at the path length entered; if the instrument already path-corrects, use 1 cm.
The conventional factors (50/33/40) are used rather than a sequence-specific extinction coefficient.
A molar concentration is meaningful only for a defined-length molecule with a known molecular weight.
The reading lies inside the linear range; outside it the result is reported with a warning, not silently.

Limitations

Absorbance cannot distinguish nucleic acid from other 260 nm absorbers, so contaminants overstate concentration.
The purity ratios are advisory, not diagnostic: they assume a near-neutral, low-ionic-strength buffer, and pH or salt can shift A260/A280 and A260/A230 by enough to trigger a false flag.
The per-type factors are conventions; a defined oligo has its own extinction coefficient that can differ slightly.
For dilute or impure samples a fluorescence-based assay is more accurate than A260.

Common mistakes

Forgetting to multiply by the dilution factor, which reports the original sample as far too weak.
Using the dsDNA factor of 50 for RNA or single-stranded DNA and getting a concentration out by 20–50%.
Assuming a 1 cm path on a microvolume instrument that uses a much shorter one and does not auto-correct.
Trusting a reading above the linear range instead of diluting and re-measuring.
Treating a high A260 as proof of a pure sample and skipping the A260/A280 check.

FAQ

What A260 equals 1 µg/mL of DNA?

For double-stranded DNA an A260 of 1.0 in a 1 cm path is 50 µg/mL, so 1 µg/mL corresponds to A260 = 0.02. Single-stranded DNA uses 33 µg/mL and RNA 40 µg/mL per A260 unit.

Why do dsDNA, ssDNA and RNA have different factors?

Because base stacking changes how strongly the bases absorb. In a double helix the stacked bases are partly shielded (hypochromicity), so dsDNA absorbs less per unit mass than the same bases single-stranded — hence 50 for dsDNA versus 33 for ssDNA. RNA sits in between at 40.

Do I multiply or divide by the dilution factor?

Multiply. You diluted the sample to measure it, so the original was more concentrated: conc = A260 × factor × dilution factor ÷ path length. A 1:10 dilution means multiply the cuvette concentration by 10.

What is a good A260/A280 ratio?

About 1.8 for pure DNA and 2.0 for pure RNA. A ratio well below those suggests protein or phenol carry-over, which also means the A260 concentration is inflated. Enter A280 (and A230) here and the calculator reports both ratios and flags a value outside the expected band for your sample type.

What does a low A260/A230 ratio mean?

A260/A230 should sit near 2.0–2.2 for a clean prep. A low value points at carry-over of contaminants that absorb around 230 nm — salts, residual phenol, guanidine from a column, carbohydrate or glycogen. It is a preparation-quality flag, not a concentration correction.

How do I get a molar concentration (nM) from A260?

You need a molecular weight. First get the mass concentration in µg/mL, then nM = µg/mL × 1e6 ÷ MW. Enter the MW from the DNA molecular weight calculator and this tool reports nM automatically. It only makes sense for a defined-length molecule.

When should I use a fluorescence assay instead of A260?

When the sample is dilute (below ~0.1 A260), when purity is questionable, or when you need to measure DNA in the presence of RNA or free nucleotides. Fluorescent dyes bind nucleic acid specifically and are far more sensitive and selective than absorbance.

Scientific Sources

Wikipedia — Nucleic acid quantitation (dsDNA 50, ssDNA 33, RNA 40 µg/mL per A260) · Tier 3 source
Wikipedia — Beer–Lambert law (A = ε·c·l) · Tier 3 source
Wikipedia — Base pair (mass basis for nucleic-acid quantitation) · Tier 3 source

External links open in a new tab and carry rel="nofollow". We cite primary references (NIST, NIH/NCBI, peer-reviewed papers) and established manufacturer technical documentation (NEB, Thermo Fisher, Promega, IDT). Calculated results are estimates; your protocol and manufacturer guidance take precedence.

Last reviewed: 2026-09 · Method version: 1.0

DNA Concentration from A260 Calculator