Calculate the number of DNA molecules (copy number) from the mass and length of a fragment, then convert to copies per microliter and build a qPCR standard dilution series.
Fragment
Enter the mass, length, and strandedness of your DNA fragment to calculate its molecule copy number.
Strandedness sets the average per-base(-pair) mass used in the formula.
Base pairs (dsDNA) or bases (ssDNA).
Total mass of DNA in nanograms, e.g. from a spectrophotometer or Qubit reading.
Result
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Enter a mass and length above to calculate copy number.
Sample volume
Uses the same mass, length, and type as the Copy Number tab, divided across the volume your sample is dissolved in.
Total volume the fragment's mass (from the Copy Number tab) is dissolved in.
Result
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Enter a volume above to compute copies per µL.
Dilution series
Builds a serial dilution series for a qPCR standard curve, starting from the neat copies/µL concentration on the Copies per µL tab.
Fold-dilution applied at each step (10 is standard for qPCR standard curves).
Number of standard-curve points, including the neat (undiluted) stock.
Result
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Enter a volume on the Copies per µL tab to build the dilution series.
Absolute quantification by qPCR depends on comparing an unknown sample against a standard curve of known molecule counts, not just known mass.
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Walk-through
How to Use This Calculator
4 steps▸
1
Choose type, length, and mass
On the Copy Number tab, select dsDNA or ssDNA, enter the fragment length in base pairs (or bases for ssDNA), and enter the mass in nanograms — for example, from a Qubit or spectrophotometer reading.
2
Read the copy number
The result card updates instantly, showing the total number of DNA molecules in scientific notation, calculated from the mass and the average mass per base pair.
3
Convert to copies per µL
Switch to the Copies per µL tab and enter the volume your sample is dissolved in. The calculator divides the total copy number from step 2 by that volume.
4
Build a qPCR dilution series
On the qPCR Standard tab, set a dilution factor (10 is standard) and a number of steps to generate a serial dilution table for a qPCR absolute-quantification standard curve.
The mass in nanograms is converted to grams, multiplied by Avogadro's number (molecules per mole), and divided by the total molecular weight of the fragment (length × the average mass per base pair). dsDNA uses 660 g/mol per base pair; ssDNA uses 330 g/mol per base, since a single strand has roughly half the mass of a base pair.
Copies per microliter
copies/µL = total copies ÷ volume (µL)
Dividing the total copy number by the volume the sample is dissolved in gives the concentration in copies per microliter — the unit qPCR standard curves are typically expressed in.
qPCR serial dilution series
Cₙ = C₁ / factor^(n-1)
Starting from the neat (undiluted) copies/µL concentration (C₁), each successive standard-curve point is diluted by the same fold-factor — 10-fold is standard, giving each point one log lower in concentration than the last.
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Glossary
Key Terms Explained
7 terms▸
Copy number ↗The total number of individual DNA molecules present in a sample, calculated from its mass and the molecular weight of a single molecule using Avogadro's number.
dsDNA ↗Double-stranded DNA. Length is measured in base pairs (bp); this calculator uses an average mass of 660 g/mol per base pair.
Base pair (bp) ↗Two complementary nucleotides (A-T or G-C) joined across the two strands of double-stranded DNA — the standard unit for measuring dsDNA fragment length.
Avogadro's number ↗The number of molecules in one mole of a substance, 6.022 × 10²³. Multiplying moles of DNA by this constant gives the total molecule count.
Average bp mass ↗The mean molecular weight contributed by one base pair (or, for single-stranded DNA, one base) — approximately 660 g/mol for dsDNA and 330 g/mol for ssDNA. This sequence-independent approximation is standard across molecular biology, though the exact value varies slightly with base composition.
qPCR ↗Quantitative (real-time) PCR — a technique that measures DNA amplification as it happens, often used to determine the absolute copy number of a target sequence in an unknown sample by comparing it to a standard curve of known-concentration dilutions.
Plasmid ↗A small, circular, double-stranded DNA molecule, separate from chromosomal DNA, commonly used to carry a gene of interest and often used to prepare qPCR copy-number standards because its exact size and sequence are known.
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Scenarios
Real-World Examples
3 worked examples▸
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Quantifying a plasmid prep
100 ng of a 1,000 bp dsDNA fragment
Type dsDNALength 1,000 bpMass 100 ng
copies = (100 × 10⁻⁹ × 6.022 × 10²³) / (1,000 × 660) ≈ 9.12 × 10¹⁰ copies. This is the total molecule count in the whole 100 ng sample, before accounting for what volume it's dissolved in.
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Converting to a working concentration
Same 100 ng sample, dissolved in 50 µL
Total copies ≈ 9.12 × 10¹⁰Volume 50 µL
copies/µL = 9.12 × 10¹⁰ ÷ 50 ≈ 1.82 × 10⁹ copies/µL. This concentration is what you'd load into a qPCR reaction, or use as the neat point of a standard curve.
Each step divides the previous concentration by 10, ending at ≈ 1.82 × 10³ copies/µL on point 7 — a six-log spread that's typical for establishing a reliable qPCR standard curve.
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Reference
Cite This Calculator
APA & MLA▸
Use either format to cite this calculator in a paper, report, or resource list.
APA
MLA
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Deep Dive
From Mass to Molecules: DNA Copy Number and qPCR Standards
Absolute quantification by qPCR depends on comparing an unknown sample against a standard curve of known molecule counts, not just known mass. Converting a DNA fragment's mass into its actual copy number — and then into a dilution series — is the calculation that bridges a spectrophotometer or Qubit reading to a usable standard curve.
Why copy number matters more than mass alone
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Two DNA samples with the same mass can contain very different numbers of molecules if their fragment lengths differ — a short 200 bp fragment has far more molecules per nanogram than a 10,000 bp fragment of the same mass, since each individual molecule weighs less. For qPCR standard curves, what matters is the actual molecule count (copy number), because Ct values track how many target copies are present, not how many nanograms. This calculator converts mass and length into copy number using Avogadro's number, the same constant used for any mole-to-molecule conversion in chemistry.
Why dsDNA and ssDNA use different average masses
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Double-stranded DNA length is measured in base pairs, where each unit already accounts for both complementary strands, so the standard approximation of 660 g/mol per base pair reflects the combined mass of both strands. Single-stranded DNA length is measured per individual base, so its average mass (330 g/mol per base) is roughly half that of a base pair — it's the mass of just one strand's worth of nucleotide. Using the wrong strandedness setting for your sample type will produce a copy number that's off by roughly a factor of two.
Building a serial dilution series for a standard curve
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A qPCR standard curve is built by amplifying several known-concentration dilutions of a reference template — often a linearized plasmid or PCR product — and plotting their Ct values against log(copy number). A 10-fold serial dilution across 6–8 points is the conventional choice, since it spans several orders of magnitude while keeping the number of reactions manageable. Once you know the neat concentration in copies/µL, each subsequent standard is simply the previous one diluted by the same factor, which is exactly what the qPCR Standard tab calculates.
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Questions
Frequently Asked Questions
6 questions▸
What formula does the DNA Copy Number Calculator use?+
copies = (mass in ng × 10⁻⁹ × Avogadro's number) / (length × average mass per bp or base). Avogadro's number is 6.022 × 10²³ molecules per mole. dsDNA uses 660 g/mol per base pair; ssDNA uses 330 g/mol per base.
What average mass per base pair does this calculator use, and why?+
660 g/mol per base pair for dsDNA and 330 g/mol per base for ssDNA. These are standard sequence-independent approximations used throughout molecular biology; the exact value for a real sequence varies slightly (roughly 650–660 g/mol/bp) depending on its precise A/T/G/C composition.
Why would I need a qPCR standard curve at all?+
Real-time qPCR measures amplification cycles (Ct values), not molecule counts directly. To determine the absolute number of target copies in an unknown sample, you compare its Ct value against a standard curve built from serial dilutions of a reference template with known copy numbers — which is exactly what the qPCR Standard tab helps you build.
What units does the result use?+
The Copy Number tab reports the total number of DNA molecules (a plain count, shown in scientific notation since the numbers are very large). The Copies per µL tab reports that same count divided by your sample volume, in copies per microliter — the standard unit for qPCR standard-curve concentrations.
How is ssDNA different from dsDNA in this calculator?+
Select ssDNA if your sample is single-stranded (e.g. a synthetic oligo or ssDNA virus genome) and enter its length in bases rather than base pairs. The calculator then uses 330 g/mol per base instead of 660 g/mol per base pair, since a single strand carries roughly half the mass of a base pair.
How do I convert my total copy number into copies per µL?+
Enter the volume your DNA is dissolved in (in microliters) on the Copies per µL tab. The calculator divides the total copy number from the Copy Number tab by that volume to give the concentration in copies/µL, which you can then carry into the qPCR Standard tab.
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