Calculate the insert mass needed for a chosen insert : vector molar ratio from vector mass and the lengths of both DNA fragments. The ratio counts insert molecules per vector molecule, not equal masses.

Enter the vector amount, both fragment lengths, and your chosen insert : vector molar ratio.

Assumes double-stranded DNA with the same average molar mass per base pair. Use the lengths of the actual vector and insert fragments weighed. The ratio counts molecules, not mass.

Enter the insert molecules per vector molecule, such as 3 for 3:1. Choose the ratio for your protocol; no universal ratio is assumed.

Related Calculators

Plasmid Molar Ratio Formula

Amount of molecules equals mass divided by molar mass. For vector and insert double-stranded DNA with the same average molar mass per base pair, the common factor cancels. If R is the desired number of insert molecules per vector molecule, the required insert mass is:

mแตข = mแตฅ ร— (Lแตข / Lแตฅ) ร— R

Variables:

  • m_i is the required insert mass (ng)
  • m_v is the vector mass being used (ng)
  • L_v is the length of the actual vector fragment represented by that mass (bp)
  • L_i is the length of the actual insert fragment (bp)

Multiply vector mass by insert length divided by vector length, then multiply by R. Both lengths must use the same normalized basis; 1 kb = 1000 bp. The calculator converts selected mass units to ng and reports required insert mass in ng.

What is a Plasmid Molar Ratio?

In cloning and ligation, a molar ratio counts insert molecules relative to vector molecules; 3:1 means three insert molecules per vector molecule. It does not imply a 3:1 mass ratio or an optimal reaction ratio. A common approximate dsDNA molar-mass convention is 660 g/mol per base pair, but that factor cancels in this same-chemistry length-ratio calculation. Separately, if insert and vector masses are equal, their insert : vector molecular ratio is L_v / L_i. A 5000 bp vector and 1000 bp insert at equal masses therefore have a 5:1 molecular ratio.

How to Calculate Plasmid Molar Ratio?

Use the actual vector and insert fragments represented by your masses. Supply all four numeric inputs; the calculator solves required insert mass rather than asking you to leave a copy-count variable blank.

  1. Enter the vector fragment length and select bp or kb.
  2. Enter the vector mass being used and select ng, ยตg or pg.
  3. Enter the insert fragment length and its unit.
  4. Enter the desired insert : vector molar ratio as R; for example, enter 3 for 3:1. Select the ratio for the applicable protocol.
  5. Select Calculate to find required insert mass using m_i = m_v ร— (L_i / L_v) ร— R.
  6. Check the reported molecular ratio, mass ratio and normalized fragment lengths. The amount calculation does not establish ligation or assembly success.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Vector fragment length L_v = 5000 bp

Vector mass m_v = 50 ng

Insert fragment length L_i = 1000 bp; desired insert : vector molar ratio R = 3:1

Required insert mass = 50 ng ร— (1000 bp / 5000 bp) ร— 3 = 30 ng. The insert : vector molecular ratio is 3:1 while the mass ratio is 30:50, or 0.6:1. Equal masses are not assumed.