RNA Copy Number Calculator
Enter RNA mass, nucleotide length, and average molar mass per nucleotide to compute RNA molecule copy number, for qRT-PCR standards and IVT quantification.
Input Data
Results
At a glance:RNA copy number is the number of molecules in a given RNA mass, the core of building qRT-PCR absolute-quantification standard curves. The molecule count is moles (mass ÷ molar mass) times Avogadro's number. Single-stranded RNA averages about 320.5 g/mol per nucleotide, so an RNA molecule's molar mass ≈ length (nt) × 320.5, and copy number = mass × 6.022×10²³ ÷ (length × 10⁹ × 320.5) with mass in ng.
Formula
RNA molar mass: MW ≈ length (nt) × molar mass per nt (≈ 320.5 g/mol).
Moles: n = mass (g) ÷ MW (mass_ng × 10⁻⁹ to g).
Copy number = n × 6.022×10²³.
$$N = \dfrac{m_{ng} \times 6.022\times10^{23}}{L_{nt} \times 10^{9} \times g_{nt}}$$$$g_{nt} \approx 320.5 \text{ g/mol}$$How to Use
- Enter RNA mass (ng) and nucleotide length (nt).
- Confirm the average molar mass per nt (commonly 320.5 g/mol).
- The right panel shows the RNA molecule copy number instantly.
RNA mass vs copy number at common lengths (320.5 g/nt)
| RNA mass (ng) | Length (nt) | Copy number |
|---|---|---|
| 100 | 1,000 | 1.88 × 10¹¹ |
| 100 | 1,500 | 1.25 × 10¹¹ |
| 50 | 1,000 | 9.40 × 10¹⁰ |
| 10 | 2,000 | 9.40 × 10⁹ |
| 1 | 1,000 | 1.88 × 10⁹ |
At fixed mass, longer RNA has fewer molecules (larger molar mass); when preparing qRT-PCR standards, convert to copies per reaction then do 10-fold serial dilutions.
Case Studies
qRT-PCR standard copy number
An IVT 1,500 nt RNA standard at 100 ng/µL.
Copies per µL = 100 × 6.022×10²³ ÷ (1500 × 10⁹ × 320.5) ≈ 1.25 × 10¹¹.
Use this as the start for 10-fold serial dilutions (10¹⁰, 10⁹…) to build an absolute-quantification standard curve across orders of magnitude.
Molecule count of a small sample
A 1,000 nt mRNA yields only 50 ng after purification.
Copy number = 50 × 6.022×10²³ ÷ (1000 × 10⁹ × 320.5) ≈ 9.4 × 10¹⁰.
After conversion, assess whether the starting molecule amount for downstream reactions (reverse transcription, library prep) is sufficient.
FAQ
Why does RNA use 320.5 while DNA uses 650 g/mol?
320.5 g/mol is the average molar mass of one single-stranded RNA nucleotide; dsDNA per base pair contains two strands totaling about 650 g/mol. So RNA is computed by nucleotide (nt) length × 320.5 per nt, and DNA by base-pair (bp) length × 650 per bp — the two must not be mixed.
Should mass be entered in ng or other units?
This calculator takes nanograms (ng); the formula already includes 10⁹ to convert to grams. If your data is µg, multiply by 1000 to ng; if pg, divide by 1000. Enter length as nucleotide count (nt).
How to determine IVT product length?
Use the actual transcript length dictated by the template (include poly-A tail and 5' cap-related sequences if present). If confirmed by agarose gel or bioanalyzer band size, use the measured nucleotide count for a more accurate copy number.
How does copy-number conversion relate to A260 quantification?
A260 gives RNA mass concentration (ng/µL); this calculator further converts mass to molecule count. The combined flow: A260 → mass concentration → copy number by length. If the 260/280 ratio deviates from about 2.0, protein or phenol contamination biases both mass and copy-number estimates.
Why use copy number rather than mass for standard curves?
Absolute quantification asks 'how many molecules in the sample'; copy number is the comparable physical quantity. Different-length transcripts at the same mass have very different molecule counts, so ng alone cannot compare across genes. Convert to copies, then do 10-fold serial dilutions to build the curve from which Ct values back-calculate unknown molecule counts.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.