Tm Calculator
Calculate PCR primer melting temperature from nearest-neighbour thermodynamics, with salt correction.
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What is this?
A Tm Calculator works out the melting temperature of a DNA primer - the temperature at which half of it is bound to its template and half has come away. It is the value a PCR annealing temperature is chosen from, and it depends on the sequence, the primer concentration and the salt in the buffer.
How to Use the Tm Calculator
The melting temperature of a primer is the temperature at which half of it is bound to its template and half has come away. It is the number every annealing step is designed around: too high and the primer never binds, too low and it binds in the wrong place.
This calculator uses nearest-neighbour thermodynamics rather than a base-counting rule. Each overlapping pair of bases carries its own measured enthalpy and entropy, which keeps it accurate for the 18-30 base primers used in PCR where the older "2 + 4" rule drifts. It also corrects for magnesium as well as sodium and potassium, which matters because Mg2+ is a far more effective stabiliser per ion and a monovalent-only estimate reads several degrees low in a real PCR buffer.
1. Pick a Reaction Preset
Each preset fills in the buffer for a common setup - standard Taq PCR, qPCR, high-fidelity Phusion or Q5, colony PCR, mutagenesis, GC-rich templates, low-salt buffer or a magnesium-heavy multiplex. The preset also decides which annealing-temperature rule is applied, since high-fidelity enzymes use a different offset from Taq.
2. Paste Your Forward Primer
Type or paste the sequence 5' to 3'. Spaces, line breaks and digits are stripped automatically, so a sequence copied straight out of a file or an order form works as is.
3. Add the Reverse Primer
Optional, but worth doing. Entering both gives you the gap between the two Tm values and a 3' complementarity check, which together decide whether a pair works.
4. Adjust the Buffer if Needed
Monovalent salt (Na+ plus K+), magnesium, dNTPs, Tris and primer concentration are all editable. dNTPs matter because they bind magnesium and take it out of play, so raising dNTPs lowers Tm.
5. Read the Result
You get the nearest-neighbour Tm for each primer, two older estimates for comparison, the thermodynamics behind the number, structure checks for hairpins and dimers, and a suggested annealing temperature. Export the whole thing to CSV if you want it in your notes.
Key Formulas Used in the Calculator
Melting Temperature
Tm(1M NaCl)=ΔS+Rln(xCT)ΔH×1000
R is the gas constant, 1.987 cal/(K·mol), and CT is strand concentration in mol/L. The divisor x is 4 for a normal primer whose strands are at equal concentration and 1 when the oligo is self-complementary. This is Equation 3 of SantaLucia 1998.
Nearest-Neighbour Sum
ΔH=∑ΔH(each base pair step)+ΔH(initiation at both ends)
The sequence is walked two bases at a time, so a 20-mer contributes 19 overlapping steps, each with its own measured enthalpy and entropy. An initiation term is added for each end of the duplex, and it differs depending on whether that end is a G·C or an A·T pair.
Salt Correction
ΔS([Na+])=ΔS(1M NaCl)+0.368×(N−1)×ln[Na+]
Equation 8 of SantaLucia 1998. N - 1 is the number of nearest-neighbour steps and [Na+] is the monovalent-equivalent concentration in mol/L. Enthalpy is treated as salt-independent.
Magnesium as a Sodium Equivalent
[Na+]eq=[Na+]+[K+]+2[Tris]+120×[Mg2+]−4[dNTPs]
Magnesium screens the charge on the DNA backbone far more effectively than sodium, so it is scaled up before being added to the monovalent total. dNTPs are subtracted first because they chelate magnesium out of play - and the entry is per nucleotide, so all four of dATP, dCTP, dGTP and dTTP count, which is why the figure is multiplied by four. Tris carries charge and contributes half its concentration.
Symmetry Correction
If Sequence=Its Own Reverse Complement,ΔS=ΔS−1.4
A self-complementary oligo anneals to itself rather than to a separate partner strand, so it takes an entropy penalty of 1.4 cal/(K·mol) and the concentration divisor drops from 4 to 1.
Benefits
Uses the SantaLucia 1998 unified nearest-neighbour parameters, not a base-counting rule
Corrects for magnesium and monovalent salt together, via a sodium-equivalent ionic strength
Accounts for dNTPs chelating magnesium, which a naive salt entry misses
Presets for standard PCR, qPCR, high-fidelity, colony, mutagenesis and multiplex
Shows the Wallace and salt-adjusted estimates side by side so you can see them drift
Flags hairpins, self-dimers, 3' complementarity and GC clamp problems
Suggests an annealing temperature using the right offset for the chosen polymerase
Exports every value to CSV
When & Where to Use
Choosing an annealing temperature for a PCR run
Checking that a new primer pair is thermally matched
Diagnosing a PCR that gives no product or nonspecific bands
Comparing candidate primers during design
Working out how much a change in magnesium shifts your Tm
Checking a primer ordered from a supplier before using it
Screening a pair for primer-dimer risk before ordering
Who Should Use This Calculator
The Tm Calculator is for molecular biologists, students and lab technicians designing or troubleshooting PCR, qPCR, sequencing and cloning primers - anyone who needs an annealing temperature grounded in the sequence and the actual buffer rather than a rough guess.
Tips to Get the Best Deal
Design a pair whose Tm values are within about 5 °C of each other
Keep GC content roughly between 40% and 60%
Aim for a G or C at the very 3' end; the last two bases are what the polymerase grips
Raising magnesium raises Tm; raising dNTPs lowers it by mopping magnesium up
Tm is not the annealing temperature - apply your polymerase's offset
Check 3' complementarity between the pair before ordering, not after
The 2 + 4 rule drifts on PCR-length primers; nearest-neighbour does not
Frequently Asked Questions (FAQs)
What is primer melting temperature?
Tm is the temperature at which half of the primer molecules are bound to their template and half are free in solution. It depends on the sequence, its concentration and the ions around it, and it is the number an annealing temperature is chosen from.
Is Tm the same as the annealing temperature?
No. Tm is an equilibrium point; the annealing temperature is chosen from it, and the offset depends on the enzyme. Taq-style protocols commonly run about 5 °C below the lower of the two primer Tm values, while NEB advise roughly the lower Tm plus 1 °C for Q5 and Phusion. The calculator applies whichever rule matches the preset you picked.
Does magnesium affect Tm?
Strongly, and it raises it. Mg2+ is far more effective per ion than sodium or potassium at screening the charge on the DNA backbone, so it is converted to a sodium-equivalent concentration before the salt correction is applied. Going from 1.5 to 3 mM Mg2+ typically moves Tm by more than a degree, which is why a monovalent-only calculator reads low for real PCR buffers.
Why do dNTPs lower the Tm?
dNTPs chelate magnesium tightly, so some of the magnesium you added is not available to stabilise the duplex. The calculator subtracts them from magnesium before converting what is left into a sodium equivalent. Note that the entry is per nucleotide: at 0.2 mM each the reaction holds 0.8 mM of dNTPs in total, so 0.8 mM of magnesium is spoken for.
What method does this calculator use?
The unified nearest-neighbour parameters and the Equation 8 salt correction from SantaLucia, PNAS 95:1460-1465 (1998), with magnesium folded in as a sodium equivalent in the manner of von Ahsen et al., Clin Chem 47:1956-1961 (2001). The same nearest-neighbour table sits behind NEB's Tm Calculator, Primer3 and IDT's OligoAnalyzer.
Why does this differ from the 2 + 4 rule?
The Wallace rule adds 2 °C per A or T and 4 °C per G or C. It ignores the order of the bases, the primer concentration and the buffer, all of which genuinely change Tm. It is reasonable for oligos under about 14 bases and drifts increasingly for PCR-length primers. Both it and the older salt-adjusted formula are shown alongside the nearest-neighbour value so you can see the size of that drift.
What is a GC clamp and why does it matter?
A G or C among the last few bases at the 3' end binds more tightly than an A or T, helping the polymerase start cleanly. The headline figure counts the last two bases, since those are what the polymerase grips; the warnings also watch the last five, because four or five G/C there makes the primer stick to partial matches and misprime.
What do the hairpin and self-dimer numbers mean?
They report the longest run of complementary bases the primer can form with itself - folded back into a hairpin, or paired with another copy of itself. Both compete with template binding. Runs of four or more base pairs are worth attention, particularly when they sit at the 3' end where they can be extended. The self-dimer figure tests base pairing once, so a poly-A primer correctly scores zero rather than pairing A with A.
Why won't it accept N or R in my sequence?
Nearest-neighbour parameters were measured for the sixteen A/C/G/T dinucleotide steps. There are no published values for ambiguity codes, so any number produced for them would be invented rather than calculated. Substitute the specific base you intend to order.
What is a self-complementary primer?
One whose sequence reads identically to its own reverse complement, such as GAATTC. It anneals to a copy of itself rather than to a separate partner strand, so the calculation takes an entropy penalty of 1.4 cal/(K·mol) and uses a concentration divisor of 1 instead of 4. The calculator detects this automatically.
Pro Tips
Design primer pairs whose Tm values sit within about 5 °C of each other.
Tm is not the annealing temperature - apply your polymerase's offset to it.
Magnesium raises Tm strongly; dNTPs lower it by chelating that magnesium.
Aim for a G or C at the very 3' end, and no more than three among the last five.
Check 3' complementarity between a pair before ordering them.
The 2 + 4 base-counting rule drifts on PCR-length primers; nearest-neighbour does not.