Molecular Cloning Calculators | Plasmid MW, Ligation & Transformation

Molecular cloning tools

Every tool below opens instantly and runs entirely in your browser — nothing is uploaded, stored or logged.

Ligation Ratio CalculatorCalculate the insert:vector mass for any molar ratio, or find the ratio you actually ligated — with the derivation, worked examples and MW cross-links.
Transformation Efficiency CalculatorConvert colony counts to transformation efficiency in CFU/µg, with the plated-volume dilution chain, typical ranges and links to the CFU calculator.
Plasmid Molecular Weight CalculatorTurn a plasmid size in bp into molecular weight, pmol/µg, molarity and copy number for cloning — the workflow bundle plain DNA MW tools leave out.
Restriction Enzyme Site FinderFind every restriction enzyme that cuts your DNA sequence — scans the full REBASE set in your browser, with cut positions and fragment sizes.

A cloning workflow is a chain of quantitative hand-offs: you need the plasmid’s molecular weight to turn a mass into moles, a molar ratio to decide how much insert to add to a ligation, and a colony count to find out whether the transformation actually worked. Each step is simple arithmetic, but the conversions between bp, ng, pmol and CFU/µg are where mistakes hide — and a wrong ratio or a mis-plated volume can cost a week. This collection covers the three calculations that come up most often at the bench. Every tool shows the equation, works through real numbers and cites where the constants come from, and everything runs locally in your browser, so a sequence you paste is never uploaded.

Plasmid Molecular Weight: bp to Moles and Molecules

The plasmid’s molecular weight is the master number the rest of the workflow depends on. Because double-stranded DNA has a nearly uniform mass per base pair, the plasmid molecular weight calculator estimates it from length on the cloning convention of 660 g/mol·bp (the sodium-salt average NEB and Promega use; the free-acid average is ~618, about 6% lower). From that single value it derives everything a cloning protocol asks for: pmol per µg and µg per pmol, the number of molecules per µg via Avogadro’s constant, and — if you enter a mass or a concentration — the amount in pmol and the molarity in nM and µM. It is the conversion that makes copy number computable, since dividing the plasmid mass per cell by the single-molecule mass gives copies per cell. For a residue-exact mass from a sequence rather than an average, the DNA molecular weight calculator uses the free-acid basis and is the right tool when composition matters. The how to calculate plasmid molecular weight guide explains the 660-versus-618 conventions, the mass-to-mole conversions and a full copy-number example.

Ligation Ratio: Balancing Insert and Vector by Moles

A ligation is driven by the molar ratio of insert to vector, not the mass ratio, and because molecular weight is proportional to length the two differ by the size ratio. The ligation ratio calculator solves this in both directions. In forward mode it takes your insert and vector sizes, the vector mass and a target ratio (3:1 for sticky ends, 1:1 for blunt, 6:1–10:1 for a small insert) and returns the exact ng of insert to add, plus the total DNA in the reaction. In reverse mode it takes the two masses you actually pipetted and reports the molar ratio you really set up — the diagnostic that explains a failed ligation. It needs the fragment lengths, which is where the plasmid molecular weight step feeds in. The how to calculate ligation ratio guide derives the formula, works through a pUC19 example and explains why a small insert needs so little mass.

Transformation Efficiency: From Colonies to CFU/µg

Transformation efficiency tells you how competent your cells were and whether a ligation is worth cloning: it is the number of colony-forming units per microgram of DNA. The catch is that you only plate a fraction of the recovery culture, so the transformation efficiency calculator walks the full volume chain — colonies counted, volume plated, total recovery volume and the ng of DNA added — to the µg actually represented on the plate, and divides to give CFU/µg. It reports the dilution factor and flags whether the result sits in the chemically competent range (≈10⁶–10⁸), the electrocompetent range (≈10⁹–10¹⁰) or suspiciously below. Because it is built on a plate count, it pairs naturally with the CFU calculator for general colony counting and dilution work. The how to calculate transformation efficiency guide covers the countable-range rule, the plated-volume correction and troubleshooting a low result.

Which calculator do I need?

Start from the number you are missing:

A plasmid size and you need moles, molecules or molarity → plasmid molecular weight calculator for MW in g/mol and kDa, pmol/µg, molecules/µg and the copy-number path.
Insert and vector sizes and you need the mass to add → ligation ratio calculator in forward mode for a target molar ratio, or reverse mode to check the ratio you set up.
A colony count and you need the efficiency → transformation efficiency calculator for CFU/µg with the plated-volume dilution chain.
A mass or concentration you need to convert to molarity → the molarity calculator, or the DNA concentration calculator to get µg/mL from an A260 reading first.
A dilution to make from a stock → the dilution calculator.
General plate counts and serial dilutions → the CFU calculator.

These tools chain the way a cloning experiment does: get the plasmid molecular weight from its length, use it to convert masses to moles and set a ligation ratio, then plate the transformation and compute the efficiency to see whether it worked. For sequence analysis upstream, hand off to the DNA & RNA calculators; for solution prep, see the laboratory calculators.

Molecular Cloning Tools

Plasmid Molecular Weight Calculator — bp to g/mol and kDa on the 660 convention, with pmol/µg, molecules/µg, molarity and the copy-number conversion.
Ligation Ratio Calculator — insert mass for a target insert:vector molar ratio, or the ratio your masses actually give, with the derivation shown.
Transformation Efficiency Calculator — colony count to CFU/µg through the plated-volume dilution chain, with typical-range interpretation.

Frequently asked questions

How is a plasmid’s molecular weight calculated from its length?

By multiplying the length in base pairs by the average mass of a base pair. Cloning tools use 660 g/mol·bp, the sodium-salt average, so a 2686 bp plasmid such as pUC19 is 2686 × 660 ≈ 1.77 × 10⁶ g/mol (1772.8 kDa). The free-acid average is about 618 g/mol·bp, roughly 6% lower; neither is wrong, but you must stay consistent within a calculation. For a residue-exact mass from a sequence, use the DNA molecular weight calculator instead.

What insert:vector molar ratio should I use for a ligation?

A 3:1 insert:vector molar ratio is the standard starting point for a sticky-end ligation; a 1:1 ratio is common for blunt ends, and a small insert often needs 6:1 to 10:1 because fewer insert molecules are present at the same mass. The ligation is driven by moles, not mass, so the mass of insert to add scales with the size ratio — a 1000 bp insert into a 2686 bp vector at 3:1 needs about 56 ng of insert for every 50 ng of vector.

Why does a small insert need so much less mass than the vector?

Because molecular weight is proportional to length, equal moles of a small fragment and a large vector have masses in the same ratio as their lengths. To reach a 3:1 molar ratio with a 1000 bp insert and a 2686 bp vector, the insert mass is 3 × (1000 / 2686) × vector mass — the size ratio (≈0.37) shrinks the mass the insert has to contribute. That is exactly the factor the ligation ratio calculator applies.

What is a good transformation efficiency in CFU/µg?

It depends on the cells. Chemically competent cells typically give 10⁶–10⁸ CFU/µg, while electrocompetent cells reach 10⁹–10¹⁰ CFU/µg. A routine cloning transformation with a supercoiled plasmid usually lands in the 10⁶–10⁸ range; a ligation mix transforms several-fold lower. An efficiency far below the expected range points to poor DNA quality, loss of competency or a failed heat-shock or electroporation step.

Do these calculators store or upload my plasmid sequence?

No. Every calculation runs locally in your browser. A sequence you paste is never sent to a server, stored or logged, and there is no login, account or payment.