DNA copy number calculator
Enter the amount of nucleic acid and its length to find how many molecules it contains, for example to set up a qPCR standard curve. Or enter the number of copies you need and get the mass to take. The calculator uses average masses of 650 g/mol per base pair of dsDNA, 330 per base of ssDNA and 340 per base of RNA.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose what to find: the number of copies or the mass.
- Pick dsDNA, ssDNA or RNA and enter the length in base pairs or bases.
- Enter the amount in ng, pg or µg, or the number of copies, and the volume it is in.
- Molar mass of the molecule: M = length × 650 (dsDNA), × 330 (ssDNA) or × 340 (RNA) g/mol.
- Copies: N = m × N_A ÷ M, with N_A = 6.02214076 × 10²³ per mol.
- Mass for N copies: m = N × M ÷ N_A. Femtomoles: m ÷ M × 10¹⁵.
Worked example
1 ng of a 5000 bp plasmid: M = 5000 × 650 = 3,250,000 g/mol, so N = 10⁻⁹ × 6.02214076 × 10²³ ÷ 3.25 × 10⁶ = 185,296,639 copies, or 0.307692 fmol.
For a standard with a million copies of the same plasmid you need 10⁶ × 3.25 × 10⁶ ÷ 6.02214076 × 10²³ g = 0.005397 ng, or about 5.4 pg.
Tips
- Linear and circular plasmids of the same length have the same mass per copy.
- Measure the concentration accurately, ideally with a dye-based assay; an error in ng is the same relative error in copies.
- Large copy numbers are printed in full, such as 18,248,911,394 for 10 ng of a 1000 nt single strand.
Frequently asked questions
How do you calculate DNA copy number?
Copies = mass in grams × 6.02214076 × 10²³ ÷ (length × average mass per base pair). For 1 ng of a 5000 bp plasmid, 1 × 10⁻⁹ × 6.022 × 10²³ ÷ (5000 × 650) ≈ 1.85 × 10⁸ copies.
Why 650 g/mol per base pair?
It is the usual rounded average for one base pair of double-stranded DNA. Some protocols use 660; the copy number then comes out about 1.5 % lower. Single strands are about half, 330 g/mol per base for DNA and 340 for RNA.
How do I make a qPCR standard curve?
Find the mass for your highest standard, for example 10⁸ copies in the volume you load, then make a tenfold serial dilution. The serial dilution calculator gives the volumes for each tube.
What is the volume field for?
If the amount is dissolved in some volume, enter it to see the copies per microlitre. With 1 µL the result equals the copy number; a concentration in ng/µL can be entered as the amount with a volume of 1 µL.