DNA Ligation Molar Ratio Calculator
Setting up a cloning ligation means combining vector and insert in a chosen molar ratio, but you pipette by mass, not by molecule count. Because a DNA fragment's mass is proportional to its length, the insert mass needed is the vector mass scaled by the insert-to-vector length ratio and by the target molar ratio. This calculator returns the insert mass in nanograms for your chosen molar ratio. The relationship is a pure proportionality: the mass per base pair cancels, so no empirical constant is involved.
Ligation insert mass formula
insert mass = vector mass * (insert length / vector length) * molar ratio
length ratio = insert length / vector length
molar ratio = desired insert : vector (e.g. 3 for 3:1)
output in nanograms
For 50 ng of a 5,000 bp vector, a 1,000 bp insert at a 3-to-1 molar ratio needs 50 times (1000/5000) times 3, equal to 30 ng of insert.
Cloning facts
- DNA mass is proportional to fragment length in base pairs.
- A 3-to-1 insert-to-vector molar ratio is a common starting point.
- The mass per base pair cancels, so no density constant is needed.
- Use consistent length units; base pairs is standard.
- Optimal ratios vary with end type and fragment size, so test a range.
DNA ligation molar ratio: frequently asked questions
How is the insert mass for a ligation calculated?
Because the mass of a DNA fragment is proportional to its length, the insert mass needed equals the vector mass multiplied by the ratio of insert length to vector length, multiplied by the desired insert-to-vector molar ratio. This sets the molar amounts in the chosen proportion.
What molar ratio should I use?
A common starting point for cohesive-end ligations is a 3-to-1 insert-to-vector molar ratio, though ratios from 1-to-1 up to 5-to-1 or higher are used depending on the fragments. Enter the ratio your protocol calls for and the calculator returns the matching insert mass.
Why does fragment length matter?
Two DNA fragments of equal mass contain different numbers of molecules if their lengths differ, because a longer molecule weighs more. Scaling by the length ratio converts the molar ratio you want into the mass ratio you actually weigh out or pipette.
What units should I use?
Enter vector mass in nanograms and both lengths in base pairs. The insert mass output is in nanograms. Because the formula uses only ratios of length and a molar ratio, any consistent length unit works, but base pairs is the convention.
Does this assume a specific mass per base pair?
No. The result depends only on the ratio of lengths, so the average mass per base pair cancels out. That makes the calculation a pure proportionality with no empirical constant, valid for any double-stranded DNA.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.