How this instrument works
Molecular cloning ligations work in molar terms, not mass terms — you want a certain number of insert molecules for every vector molecule, not a certain number of nanograms of each. But pipetting happens in nanograms and microliters, so the practical problem is converting a molar ratio into a mass to weigh out. The standard ligation calculation solves that: it scales the vector's mass by the ratio of insert length to vector length (which converts vector mass into an equivalent insert mass at a 1:1 molar ratio), then multiplies by the desired insert:vector molar ratio to get the final insert mass.
The length-ratio step is doing real work, not just bookkeeping. A given mass of DNA contains more molecules if the DNA fragments are shorter, because each molecule of a short fragment weighs less than each molecule of a long one — the same 50 ng of a 1 kb fragment represents three times as many individual molecules as 50 ng of a 3 kb fragment. Multiplying vector mass by insert length over vector length corrects for exactly that difference in molecule size before the molar ratio is applied.
The molar ratio itself is a practical choice, not a fixed constant: 1:1 to 1:10 insert:vector are common starting points, and 3:1 is a frequently used default for standard subcloning, because a modest molar excess of insert improves the odds that a vector molecule finds and ligates to one insert molecule rather than religating to itself or picking up two inserts. Too high a ratio wastes insert and can favor concatemers; too low a ratio favors vector self-ligation and low colony yield, so this figure is usually the first thing worth adjusting when a ligation isn't producing enough correct clones.
- Enter the mass of vector DNA used into Vector mass (ng).
- Enter Vector length (kb) and Insert length (kb) — the sizes of your two DNA fragments in kilobases.
- Set Insert:vector molar ratio to your target, commonly 3 for standard subcloning.
- Read Insert mass needed (ng) — pipette that much insert DNA into the ligation reaction alongside the vector mass you entered.
- Vector length must be greater than zero and vector mass cannot be negative, or the instrument can't compute a mass.
Worked example — 50 ng of a 3 kb vector, 3:1 ratio
Enter 50 into Vector mass (ng), 3 into Vector length (kb), 1 into Insert length (kb), and 3 into Insert:vector molar ratio. Insert mass needed (ng) computes as (50 × 1 / 3) × 3 — the /3 and ×3 cancel algebraically, leaving exactly 50.0 ng of insert to add.
That 50 ng of a 1 kb insert contains three times as many molecules as 50 ng of the 3 kb vector, which is exactly the 3:1 molar excess of insert over vector this reaction was set up to achieve — matching the target ratio entered.
Questions
Why doesn't equal mass of vector and insert give an equal molar ratio?
Because DNA mass and DNA molecule count aren't the same thing once fragments differ in length — a shorter fragment packs more individual molecules into the same mass than a longer one does. Equal nanograms of a 1 kb insert and a 3 kb vector actually gives a 3:1 molar excess of insert, not a 1:1 molar ratio, which is exactly the length correction this calculator's formula applies before scaling by your target ratio.
What insert:vector molar ratio should I use?
1:1 to 1:10 insert:vector are all commonly used, with 3:1 a frequent default for standard subcloning of a single insert into a vector — a modest excess of insert improves the odds a vector finds one insert molecule to ligate rather than self-ligating or picking up several inserts. If colony yield is low, trying a higher ratio or a lower one is often the first troubleshooting step, since the ideal ratio varies by insert size and ligase efficiency.
Does this formula account for sticky ends versus blunt ends?
No — it only converts a molar ratio into a mass using fragment lengths, which applies the same way regardless of whether your ends are sticky or blunt. Blunt-end ligations are generally less efficient than sticky-end ligations and often benefit from a higher molar ratio or higher total DNA concentration to compensate, but that's a separate practical adjustment on top of, not a change to, the mass calculation itself.
My vector length includes only the linearized backbone, right?
Yes — Vector length (kb) should be the length of the linearized vector backbone after digestion or PCR amplification, not the length of the original uncut plasmid if any piece was removed. Using the wrong length here throws off the molar-ratio correction in the same proportion as the length error, since the formula scales directly with the vector-to-insert length ratio.
What if my calculated insert mass is too small to pipette accurately?
Scale both DNA amounts up together rather than pipetting an inaccurately small volume — since the formula is a ratio, doubling both your vector mass and your target insert mass proportionally keeps the same molar ratio while giving you a larger, more pipettable insert volume. Diluting a concentrated insert stock first, then pipetting a larger volume of the dilution, is the usual practical fix.