Calculate the missing plasmid molar ratio value from plasmid and insert copy numbers and molecular weights using a 1:1 mass basis.

Plasmid Molar Ratio Calculator

Enter any 3 values to calculate the missing variable (this uses a 1:1 mass basis: total plasmid mass = total insert mass)

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Plasmid Molar Ratio Formula

The following relationship comes from the mass–mole connection (mass = moles × molar mass). On a 1:1 mass basis (total plasmid mass equals total insert mass), the copy (mole) counts satisfy: P · Wp = I · Wi. Rearranging gives the missing variable formula below.

P = (I * Wi) / Wp

Variables:

  • P is the number of copies of plasmid
  • I is the number of copies of insert
  • Wp is the molecular weight (molar mass) of plasmid
  • Wi is the molecular weight (molar mass) of insert

To calculate the number of copies of plasmid (on a 1:1 mass basis), multiply the number of copies of insert by the molecular weight of the insert, then divide by the molecular weight of the plasmid.

What is a Plasmid Molar Ratio?

In cloning and ligation workflows, a “molar ratio” typically refers to the ratio of insert molecules to vector (plasmid) molecules (for example, 3:1 insert:vector). This page’s calculator uses molecular weights (molar masses) to relate insert and plasmid copy counts on a 1:1 mass basis, which can help you understand what molar ratio you get when you start from equal masses. For double-stranded DNA, a common approximation is ~660 g/mol per base pair to estimate molecular weight from length.

How to Calculate Plasmid Molar Ratio?

The following steps outline how to use the relationship above (1:1 mass basis) to calculate the missing value.

  1. First, determine the number of copies of plasmid (P) or leave it blank to solve for it.
  2. Next, determine the molecular weight (molar mass) of the insert (Wi).
  3. Next, determine the number of copies of insert (I).
  4. Next, determine the molecular weight (molar mass) of the plasmid (Wp).
  5. Finally, calculate the missing variable using the formula provided above (derived from P · Wp = I · Wi).
  6. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

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

Number of copies of insert (I) = 10

Molecular weight of plasmid (Wp) = 1,980,000 g/mol

Molecular weight of insert (Wi) = 330,000 g/mol

Using P = (I · Wi) / Wp: P = (10 × 330,000) / 1,980,000 ≈ 1.67 (i.e., on an equal-mass basis, this corresponds to an insert:plasmid molar ratio of about 6:1).