DNA Copy Number Calculator
Convert between DNA mass and copy number.
Copies to mass
copies = mass(g) / (length(bp) x 660) x 6.022e23
Number of copies
9.124e+8copies
copies / µL
9.124e+7
Mass per copy
1.096e-9 ng
| Template | Length (bp) | Copies per ng |
|---|---|---|
| qPCR amplicon | 150 | 6.083e+9 |
| PCR product | 500 | 1.825e+9 |
| 3 kb plasmid | 3,000 | 3.041e+8 |
| 10 kb plasmid | 10,000 | 9.124e+7 |
| E. coli genome | 4,600,000 | 1.984e+5 |
| Human genome | 3,100,000,000 | 294.3 |
Calculations use 6.0221e+23 as Avogadro's number and 660 g/mol per base pair as the average molecular weight of double-stranded DNA.
Learn more
What it does
The DNA copy number calculator tells you how many molecules are in a given mass of DNA, or, the other way round, how much mass you need to reach a target number of copies. Bench scientists use it whenever a protocol calls for a specific copy number rather than a mass — qPCR standards, transfection DNA, cloned libraries and dilution series all fall into this category. It works for any double-stranded DNA template as long as you know its length in base pairs.
How it works
The calculator uses the average molecular weight of double-stranded DNA, about 660 g/mol per base pair (sodium salt, average base composition), together with Avogadro's number. The core relation is copies = mass(g) / (length(bp) x 660) x 6.02214076e23. To go the other way, mass(ng) = copies x length(bp) x 660 x 1e9 / 6.02214076e23. Using 650 instead of 660 underestimates the copy number by about 1.5 percent, which is usually negligible but worth noting for very precise work.
Worked example
Take 1 ng of a 1000 bp template. The calculator returns 9.12 x 10^8 copies, and if that DNA is dissolved in 10 µL the concentration is 9.12 x 10^7 copies per µL. A single copy of this template weighs 1.096 x 10^-9 ng. Running the reverse calculation, to obtain 1 x 10^9 copies of a 1000 bp fragment you need 1.096 ng of DNA. The quick-reference table below lists copies per nanogram for common templates from a 150 bp qPCR amplicon up to the human genome.
When to use it
Use this tool when a protocol or a reagent datasheet specifies an amount in copies rather than mass, which is common for qPCR standards, spike-in controls and transfection-grade plasmids. It is also handy for designing dilution series, for checking whether a sequencing library carries enough molecules per microliter, and for converting between the mass and copy language that different methods use. Whenever you need to compare two experiments that differ only in template length, converting both to copy number puts them on the same footing.
FAQ
- Why do 1 ng samples have wildly different copy numbers depending on template length?
- Copy number scales inversely with length, because a fixed mass is divided among more base pairs. One nanogram of a 150 bp amplicon holds about 6.08 x 10^9 copies, while 1 ng of the 3.1 Gb human genome holds only about 294 copies — a difference of roughly seven orders of magnitude. That is why two equally weighed samples can behave nothing alike in a reaction.
- Where does the 660 factor come from, and when should I use a different value?
- The value 660 g/mol per base pair is the average molecular weight of double-stranded DNA as the sodium salt with average base composition. Real genomes differ slightly in GC content, so a more exact value can be computed from the specific sequence, but the difference is small. Using 650 instead of 660 lowers the estimate by about 1.5 percent; for most bench work 660 is the right default.
- Why must qPCR work in copies rather than mass?
- qPCR measures the number of template molecules amplified in each cycle, so the relevant variable is how many copies are in each reaction, not their total mass. Because 1 ng of a short amplicon contains far more molecules than 1 ng of a long plasmid, expressing standards by mass alone hides that difference. Reporting standards as copies per microliter makes curves comparable across different template lengths.
- How do I convert between µg and ng?
- The conversion is a factor of 1000: 1 µg equals 1000 ng, and 1 ng equals 0.001 µg. When entering mass, keep the unit consistent with the field label — if the field expects ng, multiply a µg value by 1000 first. Mixing the two up by a factor of 1000 is one of the most common copy-number mistakes.