# DNA and RNA concentration, copy number and qPCR standards

> Free A260 calculator for DNA, RNA and oligos (and A280 protein), with purity ratios, ng ↔ nM ↔ copies per µL, and a qPCR standard dilution series.

URL: https://www.labcanvas.com/tools/dna-concentration-calculator

Enter an A260 and the calculator gives the concentration (dsDNA 50, RNA 40 or ssDNA 38 ng/µL per absorbance unit at 1 cm) with the A260/A280 and A260/A230 ratios; then converts ng/µL to nM and copies per µL from the length or sequence of your plasmid, PCR product, oligo or RNA, and lays out a serial dilution series for a qPCR standard curve.

## Quick answer

- **What it does:** Converts A260 to DNA or RNA concentration with purity ratios, converts ng/µL to nM and copies per µL, and plans a qPCR standard dilution series.
- **Formula:** c (ng/µL) = A260 × factor (dsDNA 50, RNA 40, ssDNA 38) × dilution ÷ path (cm); copies/µL = c (g/µL) ÷ M (g/mol) × 6.02214076 × 10²³.
- **Example:** A260 0.75 at 1 cm: 0.75 × 50 = 37.5 ng/µL dsDNA; for a 3,000 bp plasmid (1.98559 × 10⁶ g/mol) that is 1.137 × 10¹⁰ copies/µL.

Free, in the browser, no account needed. Updated 2026-11-08.

## Features

- A260 to concentration for double-stranded DNA, single-stranded DNA and oligos, RNA, or your own factor, with path length and dilution
- A260/A280 and A260/A230 purity ratios, read against published ranges
- Protein concentration from A280 with its extinction coefficient
- ng/µL ⇄ nM ⇄ copies per µL for linear or circular dsDNA, ssDNA and RNA
- Exact molar mass from a pasted sequence, or from the length with equal base composition; sodium salt or free acid; 5′-OH or 5′ phosphate
- qPCR standard series: transfer and diluent volumes for each tube, and copies per reaction
- Copy the result or download a CSV; nothing is uploaded

## How to use it

1. **Measure the absorbance**: Enter the A260 (and A280 and A230 if you have them), the path length in cm (1 for a standard cuvette) and any dilution factor.
2. **Pick the sample type**: Double-stranded DNA, single-stranded DNA or oligo, RNA, a custom factor, or protein at 280 nm.
3. **Convert to copies**: Click “Use in copy number”, choose the molecule (plasmid, PCR product, oligo or RNA) and enter its length or paste its sequence.
4. **Plan the standard curve**: Click “Make a standard series”, set the top standard, fold dilution, number of points and tube volume.
5. **Save it**: Copy the result or download the dilution series as a CSV.

## At a glance

| | |
| --- | --- |
| Concentration | \(A_{260} \times \text{factor}\) \({}\times \text{dilution} / \text{path}\); dsDNA 50, RNA 40, ssDNA oligo 38 ng/µL per unit |
| Purity ranges | \(A_{260}/A_{280} \approx 1.8\) (DNA), \(\approx 2\) (RNA); \(\le 1.6\) suggests contamination; \(A_{260}/A_{230}\) 2.0–2.2 |
| Copy number | \(c \div \text{molar mass}\) \({}\times 6.02214076\times10^{23}\ \mathrm{mol^{-1}}\) |
| Molar mass | From nucleotide elemental formulas and CIAAW 2021 atomic weights; linear or circular, sodium salt or free acid |
| Standard series | Transfer and diluent per tube, copies per reaction; CSV |

## How it works

$$c\ (\mathrm{ng}/\upmu\mathrm{L}) = \frac{A_{260} \times \text{factor} \times \text{dilution}}{\text{path (cm)}}$$

The factors are the concentration that gives an absorbance of 1 at 260 nm through 1 cm: 50 ng/µL for pure double-stranded DNA ([Kline & Duewer 2020](https://doi.org/10.1007/s00216-020-02733-2), who give 37–40 for single-stranded DNA), 38 ng/µL for single-stranded DNA oligos of non-repetitive sequence and 40–80 % GC ([Cavaluzzi & Borer 2004](https://doi.org/10.1093/nar/gnh015), who found that predictions from nearest-neighbour or base-composition extinction coefficients give concentrations 10–30 % too low for typical PCR primers), and 40 ng/µL for single-stranded RNA, a widely used estimate with a ±10–20 % error ([Mol Ther Nucleic Acids 2024](https://doi.org/10.1016/j.omtn.2024.102171)). For a protein, \(c = A_{280} \times \text{dilution} / (\varepsilon \times \text{path})\), using the protein’s own molar extinction coefficient.

Purity: an A260/A280 of about 1.8 is generally accepted as pure nucleic acid and 1.6 or below suggests protein, phenol or other contaminants absorbing near 280 nm; A260/A230 is commonly 2.0–2.2 for pure nucleic acid ([Diagnostics 2024](https://doi.org/10.3390/diagnostics14171934)). Pure RNA gives ratios close to or above 2 ([Sci Rep 2020](https://doi.org/10.1038/s41598-020-73958-5)).

$$\text{copies}/\upmu\mathrm{L} = \frac{c\ (\mathrm{g}/\upmu\mathrm{L})}{M\ (\mathrm{g/mol})} \times N_\mathrm{A}$$

\(M\) is the molar mass of the whole molecule, added up from the elemental formulas of the nucleotides (dAMP C₁₀H₁₄N₅O₆P, dCMP C₉H₁₄N₃O₇P, dGMP C₁₀H₁₄N₅O₇P, dTMP C₁₀H₁₅N₂O₈P and the RNA monophosphates) with the CIAAW 2021 atomic weights: each residue is a monophosphate less one water, a linear strand with 5′-OH and 3′-OH ends has one phosphate fewer than residues, and a circular strand has one per residue. A double-stranded molecule adds the complementary strand. With no sequence, equal amounts of each base are assumed. The sodium salt replaces the acidic hydrogen of each phosphate with sodium. \(N_\mathrm{A} = 6.02214076\times10^{23}\ \mathrm{mol^{-1}}\), exact since the 2019 redefinition of the SI ([Natl Sci Rev 2020](https://doi.org/10.1093/nsr/nwz211)).

## Worked example

**A plasmid miniprep reads A260 0.75, A280 0.40, A230 0.36 in a 1 cm cuvette.** Concentration \(= 0.75 \times 50 = \mathbf{37.5\ ng/\upmu L}\); A260/A280 = 1.88 and A260/A230 = 2.08, both within the ranges given for pure nucleic acid.

**Copies of a 3,000 bp plasmid.** As the sodium salt with equal base composition, the molar mass is \(1.98559\times10^{6}\ \mathrm{g/mol}\) (661.86 g/mol per base pair).

$$\begin{aligned} 37.5\ \mathrm{ng}/\upmu\mathrm{L} &= \frac{37.5\times10^{-9}\ \mathrm{g}/\upmu\mathrm{L}}{1.98559\times10^{6}\ \mathrm{g/mol}} \\ &= 18.89\ \mathrm{fmol}/\upmu\mathrm{L}\ \ (18.89\ \mathrm{nM}) \\ &{}\times 6.02214076\times10^{23} \\ &= \mathbf{1.137\times10^{10}\ copies/\upmu L} \end{aligned}$$

**Standard series.** For seven 10-fold standards from \(10^{7}\) copies/µL in 100 µL tubes, the first tube needs only 0.088 µL of this stock, so make an intermediate dilution first; each later tube is 10 µL of the one before plus 90 µL diluent, down to 10 copies/µL (20 copies in a 2 µL template).

## Questions

### How do you calculate DNA concentration from A260?

Multiply the A260 by 50 ng/µL for double-stranded DNA (40 for RNA, about 38 for single-stranded oligos) and by any dilution factor, and divide by the path length in cm. An A260 of 0.75 in a 1 cm cuvette is 37.5 ng/µL of dsDNA.

### What A260/A280 ratio means my DNA is pure?

A ratio of about 1.8 is generally accepted as pure for DNA; 1.6 or below suggests protein, phenol or another contaminant absorbing near 280 nm. Pure RNA gives about 2.

### What does the A260/A230 ratio tell me?

It is a second purity check: values of 2.0–2.2 are commonly expected for pure nucleic acid, so a lower value suggests a less pure sample.

### Which factor should I use for oligonucleotides?

Cavaluzzi and Borer (2004) recommend 38 µg/mL per absorbance unit for single-stranded DNA of non-repetitive sequence with 40–80 % GC; they found that predictions from nearest-neighbour or base-composition extinction coefficients give concentrations 10–30 % too low for typical PCR primers. Kline and Duewer (2020) give 37–40 for single-stranded DNA.

### How do I calculate copy number from ng/µL?

Divide the concentration in g/µL by the molar mass of the molecule in g/mol and multiply by Avogadro’s constant, \(6.02214076\times10^{23}\ \mathrm{mol^{-1}}\). For 37.5 ng/µL of a 3,000 bp plasmid (sodium salt), that is \(1.137\times10^{10}\) copies/µL.

### Why does the counter-ion change the copy number?

The molar mass includes the counter-ion on each phosphate. With equal base composition a base pair is 661.86 g/mol as the sodium salt and 617.90 g/mol as the free acid, a 7 % difference in copies per ng.

### How do I make a qPCR standard curve dilution series?

Dilute the stock to the top standard, then make each further point by taking a fixed volume of the previous tube into diluent (10 µL into 90 µL for 10-fold steps). The series tab lists the transfer and diluent for every tube.