How to use
- Choose the nucleic acid. The default per-unit molecular weight follows from it.
- Enter the length. dsDNA is measured in base pairs (bp), single-stranded species in bases (nt).
- Fill in whichever of mass (ng) and amount (pmol) you know — one box only — and press Calculate. The other one is filled in for you.
- The result comes with the total molecular weight and the copy number.
- The per-unit molecular weight can be edited. The default is shown on screen, and if your convention uses another value you can enter it. Changing the nucleic acid restores the default.
Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula and practical notes
Mass and amount of substance are linked by molecular weight. Amount is mass divided by molecular weight, and once the unit scales are lined up it comes out as this:
- Amount
- pmol = mass(ng) × 1000 ÷ MW(g/mol)
The 1000 is there because of the units. A ng is 10⁻⁹ g and a pmol is 10⁻¹² mol, and the ratio between them is 10³.
The molecular weight of a nucleic acid is proportional to its length, which gives this approximation:
- Molecular weight
- MW ≈ length × per-unit MW
| Nucleic acid | Per-unit MW (default) |
|---|---|
| dsDNA | 650 g/mol per bp |
| ssDNA · oligo | 330 g/mol per nt |
| RNA | 340 g/mol per nt |
Copy number comes from Avogadro's number. One mol is 6.022 × 10²³ molecules, so one pmol is 6.022 × 10¹¹ of them.
- Copy number
- copies = pmol × 6.022 × 10¹¹
Practical notes
- Conventions differ on the per-unit molecular weight. Some places take dsDNA as 650 and others as 660, and the answer moves by about 1.5%. That is why this calculator lets the value be edited — record which one you used.
- For oligos, a molecular weight calculated from the actual base composition beats the length approximation. The shorter the oligo, the bigger the gap. If your synthesis supplier gives a molecular weight, use theirs.
- Do not confuse dsDNA with ssDNA. At the same length the per-unit molecular weight differs by nearly a factor of two.
- The length is the length of the actual product. For a plasmid that is the whole length; for a PCR product it is the length of the amplicon.
- Copy number assumes every molecule is intact and full-length. With fragmentation there are more molecules than this and they are shorter.