Ligation Calculator
Find the insert DNA mass needed for a molecular cloning ligation reaction from vector mass, fragment lengths, and your target insert:vector molar ratio.
🔗 What is the Ligation Calculator?
The ligation calculator finds how much insert DNA mass, in nanograms, you need to add to a molecular cloning ligation reaction to hit a target insert:vector molar ratio. Ligation joins a linearized vector and an insert fragment using DNA ligase, and the efficiency of that reaction depends on how many insert molecules are available per vector molecule, not on how many nanograms of each DNA you mix together.
This calculator is used constantly in molecular biology labs. Researchers cloning a gene of interest into a plasmid use it before every ligation setup to decide how much insert to add relative to a fixed vector mass. Students in molecular cloning courses use it to understand why mass ratios and molar ratios give very different results when fragment sizes differ. Core facilities and protocol writers use the same formula to standardize recommended reagent volumes across many different insert and vector combinations.
A common misconception is that mixing equal nanogram amounts of insert and vector gives a 1:1 reaction. It does not, a short insert has far more individual molecules per nanogram than a long vector, so equal mass at very different lengths can mean a 10:1 or even 50:1 molar excess of insert without anyone intending it. Because DNA ligase joins molecule ends, molar ratio, not mass ratio, is the variable that actually controls ligation outcome.
This calculator is useful because it removes that arithmetic step entirely: enter your vector mass, both fragment lengths, and the molar ratio you want, and it returns the exact insert mass in nanograms to add, the molar amount of each fragment in femtomoles, and a quick comparison table across the most commonly used ratios.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Standard 3:1 Sticky-End Ligation
50 ng of a 3,000 bp vector, 500 bp insert, target 3:1 insert:vector ratio
Example 2 — High 5:1 Ratio for a Large Vector
100 ng of a 5,000 bp vector, 1,000 bp insert, target 5:1 ratio
Example 3 — Small Insert into a Large Vector
20 ng of a 6,000 bp vector, 200 bp insert, target 3:1 ratio
❓ Frequently Asked Questions
🔗 Related Calculators
What is the ligation calculator formula?
Insert mass (ng) = Vector mass (ng) x (Insert length / Vector length) x Molar ratio. This scales the vector mass by the length ratio of insert to vector, then multiplies by however many insert molecules you want per vector molecule.
Why do I need a molar ratio, not just a mass ratio?
DNA ligase joins molecule ends, not nanograms. A 500 bp insert and a 5,000 bp vector at equal mass have very different numbers of molecules present because a shorter fragment has far more molecules per nanogram, so matching molar amounts (not mass) is what actually controls how often an insert end meets a vector end.
What insert:vector molar ratio should I start with?
3:1 is the standard default for a first attempt at a sticky-end ligation. If you get very few colonies, try 1:1 and 5:1 in parallel; blunt-end ligations often need higher ratios such as 5:1 to 10:1.
How is the femtomole (fmol) amount calculated?
Amount (fmol) = [mass (ng) x 10^6] / [length (bp) x 650], using the standard average molecular weight of 650 g/mol per base pair of double-stranded DNA. This converts a mass measurement into a molar amount so vector and insert can be compared on the same molecular basis.
Does this calculator account for vector dephosphorylation or restriction efficiency?
No. It only computes the molar-ratio-based insert mass from the lengths and vector mass you provide. Actual ligation success also depends on complete restriction digestion, matched or compatible ends, vector dephosphorylation (if self-ligation is a concern), ligase activity, and incubation conditions.
Can I use this for Gibson Assembly or other overlap-based cloning?
The same insert:vector molar-ratio logic applies to most cloning methods including Gibson Assembly, though Gibson Assembly protocols commonly recommend molar ratios around 2:1 to 3:1 insert:vector and require homologous overlap sequences rather than restriction sites, so treat this as a starting molar-ratio estimate, not a complete Gibson protocol calculator.
What if my vector and insert are different topologies, such as a PCR product insert into a circular vector?
The mass-to-length-to-molar-amount math is identical regardless of whether the insert or vector is linear or circular at the time of measurement; only the final ligated product's total length and topology change, so this calculator still applies to standard PCR-product-into-vector cloning.
Why did my ligation with the calculated insert mass still fail?
A correctly calculated molar ratio does not guarantee ligation success. Common causes of failure include incomplete restriction digestion, degraded or dephosphorylated insert ends, low ligase activity, incorrect buffer or ATP concentration, or vector re-circularization without insert, check these before assuming the ratio itself was wrong.
What is a typical vector mass to use in a ligation reaction?
Most standard ligation protocols use 25 to 100 ng of vector per 10 to 20 microliter reaction, with insert mass then calculated from the desired molar ratio via insert mass = vector mass x (insert length / vector length) x molar ratio.
Does the 650 g/mol per base pair constant change for single-stranded DNA or RNA?
Yes, 650 g/mol/bp is the standard average molecular weight for double-stranded DNA. Single-stranded DNA averages closer to 330 g/mol per nucleotide, and RNA (also single-stranded, with an extra oxygen) averages closer to 320 to 340 g/mol per nucleotide, so this calculator's constant applies specifically to dsDNA vector and insert fragments.
Can I use this calculator for TA cloning or Gateway cloning?
Yes, the underlying insert:vector molar ratio math is the same for any ligation-based cloning strategy, including TA cloning. Gateway cloning uses a recombinase reaction rather than DNA ligase, but many Gateway protocols still reference an insert:vector molar ratio in the same 1:1 to 3:1 range, so the calculation remains a useful estimate.