Ligation Calculator
Calculate the insert DNA mass needed for a chosen insert-to-vector molar ratio.
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CalculatorHow to useFormula & methodWorked exampleMolar ratio is a molecule ratioFrom mass to a reaction planChoosing a ratioFrequently asked questionsRelated calculatorsSourcesHow to use this calculator
- Enter insert and vector lengths, choosing the unit beside each field.
- Enter the amount of vector DNA planned for the reaction.
- Specify the desired number of insert molecules per vector molecule.
Formula and method
For the assumptions behind these results, read the explanation below. You can also explore DNA Concentration and DNA Copy Number.
A worked example
A 1,000 bp insert paired with a 3,000 bp vector at 50 ng of vector and a 3:1 molar ratio requires 50 ng of insert. Equal mass is not generally equal molecule count.
Molar ratio is a molecule ratio
Ligation joins DNA ends. Planning the reaction therefore involves the relative number of insert and vector molecules, rather than their masses alone. A long DNA fragment contains more mass per molecule than a short fragment. When both are the same form of DNA, their molecular weights are approximately proportional to their lengths.
The length ratio in the formula corrects for that difference. Multiplying by the selected molar ratio then scales the insert count. A 3:1 setting means three insert molecules for each vector molecule in the theoretical mixture.
From mass to a reaction plan
The result is the insert mass to add, not a liquid volume. To obtain a volume, divide that mass by the measured insert concentration using compatible units. For example, a 50 ng insert requirement at 10 ng/µL corresponds to 5 µL of insert solution.
Check that the DNA volumes fit within the complete reaction volume after accounting for vector, buffer, enzyme, and any other required components. A correct mass ratio does not override the concentration or buffer requirements of the ligation protocol.
Choosing a ratio
The default ratio is a convenient example, not a universal optimum. End structure, fragment length, DNA quality, concentration, and the intended assembly strategy influence a useful ratio. Compare the estimate with the enzyme supplier’s protocol and adjust the experiment accordingly.
The equation needs a consistent length scale. Choose base pairs or kilobases beside each length field; the calculator converts both before applying the length ratio.
Frequently asked questions
Why can insert and vector masses be equal at a 3:1 ratio?
If the insert is one-third as long, each insert molecule has roughly one-third of the mass. Three insert molecules then weigh about as much as one vector molecule.
Does this include ligase or buffer?
No. It calculates DNA masses only; reagent volumes follow the chosen reaction protocol.
Can I use a ratio other than three?
Yes. Enter any positive ratio supported by your experimental plan.
Sources and further reading
- University of Nebraska–Lincoln, Herman Lab — DNA Ligation and Molar Ratios ↗
Uses DNA length and mass to set insert-to-vector molar ratios; an experimental protocol must still determine suitable reaction conditions.
- National Institute of Standards and Technology — SI conversion factors ↗
Reference for physical unit conversions. The calculator converts measurements before applying its formula.
Our explanations are written for BiologyCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.