DNA Melting Temperature (Tm) Calculator
Estimate the melting temperature (Tm) of a DNA oligonucleotide from its sequence using the Wallace rule, a GC-based formula, or a salt-adjusted approximation. Enter a DNA sequence to calculate GC content, estimated Tm and supporting calculation details with transparent step-by-step results.
DNA Melting Temperature (Tm) Calculator
Choose the estimation method that matches the level of approximation you need. The simple Wallace rule is intended for short oligonucleotides, while the GC-based and salt-adjusted methods provide additional sequence and solution-condition considerations.
Calculation Result
Step-by-Step Calculation
What Is DNA Melting Temperature (Tm)?
DNA melting temperature, usually written as Tm, is the temperature at which a DNA duplex reaches a defined melting transition under specified conditions. In a simplified description, it is the temperature at which approximately half of the duplex population is separated into single strands and half remains paired.
Tm is not determined by sequence alone. It depends on base composition, sequence context, salt concentration, strand concentration, duplex structure and other experimental conditions. For this reason, a Tm calculator provides an estimate rather than a universal fixed temperature for a sequence.
FreeCalz presents several common approximation approaches so the assumptions are visible instead of hiding the calculation behind a single unexplained number.
DNA Melting Temperature Formula
Different Tm formulas are appropriate for different levels of approximation. The three methods in this calculator are deliberately presented separately because they make different assumptions.
Tm (°C) = 2 × (A + T) + 4 × (G + C)Tm (°C) = 64.9 + 41 × (G + C − 16.4) ÷ N
where N = total number of nucleotidesTm (°C) = 81.5 + 16.6 × log10([Na+]) + 0.41 × GC% − 675 ÷ N
where [Na+] is entered in mol/L and GC% is the percentage of G and C basesThese formulas are approximation models, not interchangeable definitions of Tm. More detailed oligonucleotide calculations can use nearest-neighbor thermodynamic parameters and explicit experimental conditions.
How the Wallace Rule Calculates DNA Tm
The Wallace rule is a convenient rule of thumb for short DNA oligonucleotides. It assigns 2°C to each adenine (A) or thymine (T) base and 4°C to each guanine (G) or cytosine (C) base.
Tm = 2(A + T) + 4(G + C)For example, a sequence containing 8 A/T bases and 6 G/C bases gives:
Tm = 2 × 8 + 4 × 6 = 16 + 24 = 40°C
GC Content and DNA Melting Temperature
GC content is the percentage of nucleotides in a DNA sequence that are guanine or cytosine. A higher GC fraction is commonly associated with a higher estimated melting temperature, although sequence context and solution conditions also influence the actual duplex melting behavior.
GC% = (G + C) ÷ N × 100| Base | Count | Contribution in Wallace Rule |
|---|---|---|
| Adenine (A) | A | 2°C per base |
| Thymine (T) | T | 2°C per base |
| Guanine (G) | G | 4°C per base |
| Cytosine (C) | C | 4°C per base |
GC percentage is useful as a descriptive sequence property, but it is not a complete substitute for a thermodynamic Tm calculation.
Understanding the Salt-Adjusted Tm Method
DNA has a negatively charged phosphate backbone. Monovalent cations in solution can reduce electrostatic repulsion between DNA strands and influence duplex stability. Consequently, many practical Tm approximations include a salt correction.
Tm = 81.5 + 16.6 log10([Na+]) + 0.41(GC%) − 675/N
[Na+] = salt concentration in mol/LThe calculator accepts salt concentration in mM and converts it to mol/L before applying the logarithm. For example, 50 mM is entered as 50 and internally converted to 0.050 M.
Worked Example: Wallace Rule
Example sequence: ATGCGTACGTTAGC
Step 1: Sequence length = 14 nucleotides.
Step 2: A + T = 7 and G + C = 7.
Step 3: Tm = 2 × 7 + 4 × 7 = 42°C.
Step 4: GC% = 7 ÷ 14 × 100 = 50%.
Worked Example: Basic GC-Based Formula
Example sequence: ATGCGTACGTTAGC (N = 14; G + C = 7).
Step 1: Substitute the sequence counts into the formula: 64.9 + 41 × (7 − 16.4) ÷ 14.
Step 2: The resulting estimate is approximately 37.4°C.
Interpretation: This estimate differs from the Wallace result because the formulas use different approximations and assumptions.
Worked Example: Salt-Adjusted GC Method
Example inputs: ATGCGTACGTTAGC; GC% = 50%; sequence length = 14; monovalent salt = 50 mM = 0.050 M.
Step 1: log10(0.050) ≈ −1.3010.
Step 2: Tm = 81.5 + 16.6(−1.3010) + 0.41(50) − 675/14.
Step 3: The resulting estimate is approximately 44.8°C.
Interpretation: The salt-adjusted value is condition-dependent and should be interpreted only under the assumptions of this approximation.
Why DNA Tm Depends on More Than GC Content
Two sequences can have the same length and the same GC percentage but have different melting behavior. The order of bases affects neighboring interactions, and experimental conditions influence duplex stability.
- Sequence length: Length affects the balance of base-pairing and thermodynamic terms.
- GC content: GC fraction is associated with duplex stability but does not describe sequence context.
- Salt concentration: Ionic conditions influence interactions involving the charged DNA backbone.
- Strand concentration: Duplex formation depends on strand concentration and the equilibrium between duplex and single-stranded states.
- Sequence context: Neighboring base pairs contribute to thermodynamic stability.
- Mismatches: Non-complementary bases can alter duplex stability.
- Secondary structure: Hairpins and other structures can affect accessibility and apparent behavior in an experiment.
DNA Tm for PCR Primers
Melting temperature is commonly considered during PCR primer design because primer annealing conditions are related to the stability of primer-template duplexes. However, a simple Tm formula is only one part of primer design.
Practical primer design may also consider primer length, GC distribution, sequence specificity, self-complementarity, hairpins, primer-dimer potential, mismatches, target sequence context and the exact reaction chemistry.
Why Different DNA Tm Calculators Can Give Different Results
Different calculators can produce different Tm estimates for the same sequence because they may use different models and input assumptions. A result should therefore be interpreted together with the formula and conditions used to obtain it.
| Factor | Why It Can Change the Estimate |
|---|---|
| Formula/model | Simple rules and thermodynamic models make different approximations. |
| Salt composition | Different ionic conditions change duplex stability. |
| DNA concentration | Duplex equilibrium depends on strand concentration. |
| Sequence context | Neighboring bases can contribute differently to thermodynamic stability. |
| Mismatches | Non-complementary bases can reduce or otherwise alter duplex stability. |
How to Use the DNA Melting Temperature Calculator
- Enter a DNA sequence containing only A, T, G and C.
- Choose Wallace Rule for a quick short-oligonucleotide estimate.
- Choose Basic GC% Formula for a length-and-GC-based approximation.
- Choose Salt-Adjusted GC Method when you want to include an effective monovalent salt concentration in the simplified calculation.
- If using the salt-adjusted method, enter salt concentration in mM.
- Click Calculate Tm.
- Review the estimated Tm, GC content, sequence length, base counts and four calculation steps.
Spaces and line breaks are removed automatically so that a sequence copied from a document can be entered without manual formatting cleanup.
Methodology, Transparency and Limitations
This FreeCalz calculator performs mathematical estimates from a user-supplied DNA sequence and, for the salt-adjusted method, an entered monovalent salt concentration. It does not experimentally measure DNA melting behavior.
The calculator intentionally separates the three approximation methods so users can see which formula produced the result. The Wallace rule uses base counts only. The basic GC formula uses GC content and sequence length. The salt-adjusted method additionally applies a simplified monovalent-salt correction.
- The sequence must contain only the standard DNA bases A, T, G and C after whitespace is removed.
- The Wallace rule is a rule of thumb and is most useful as a quick estimate for short oligonucleotides.
- The GC-based formula is an empirical approximation and is not a full nearest-neighbor thermodynamic calculation.
- The salt-adjusted formula is a simplified approximation using effective monovalent salt concentration.
- Actual Tm can depend on strand concentration, buffer composition, ionic species, sequence context, mismatches and secondary structure.
- For critical experimental or regulated applications, use a validated method appropriate to the specific assay and reaction conditions.
Calculation methodology reviewed: September 2026
Purpose: Educational and informational DNA sequence calculations.
Transparency: Formulas and intermediate steps are displayed rather than presenting the result as an unexplained number.
Frequently Asked Questions
What is DNA melting temperature (Tm)?
Tm is a temperature describing a defined DNA duplex melting transition under specified conditions. It is commonly described as the temperature at which about half of the duplex population is melted.
What is the Wallace rule?
The Wallace rule estimates Tm as 2 × (A + T) + 4 × (G + C). It is a simple approximation commonly used for short oligonucleotides.
Does higher GC content usually increase Tm?
Higher GC content is commonly associated with higher estimated Tm, but GC percentage alone does not capture all sequence and solution effects.
Does salt concentration affect DNA Tm?
Yes. Ionic conditions influence DNA duplex stability, which is why salt-adjusted formulas include an ionic-strength or salt term.
What salt concentration should I enter?
Enter the effective monovalent salt concentration represented by the conditions you are modeling, in mM. The calculator converts the value to mol/L before applying its simplified salt correction.
Can I use this calculator for PCR primer design?
It can provide educational estimates, but primer design should account for the complete reaction and sequence context using an appropriate validated design method.
Why do two calculators give different Tm values?
They may use different formulas, thermodynamic parameter sets, salt corrections, concentration assumptions or sequence treatments. Compare the underlying method and inputs, not only the final number.
Is Tm a fixed property of a DNA sequence?
No. Tm depends on both sequence and experimental conditions such as salt, strand concentration, duplex structure and mismatches.
Does this calculator account for mismatches or hairpins?
No. These simplified methods do not model all structural and thermodynamic effects. Use a more detailed design method when those factors matter.
Is this calculator suitable for clinical diagnosis?
No. It is an educational mathematical tool and does not diagnose conditions, interpret patient results or replace validated laboratory procedures.
References and Scientific Sources
The calculator uses commonly published approximation concepts for DNA melting temperature, GC content and salt effects. The sources below are provided so readers can independently review DNA hybridization and oligonucleotide-design methodology.
Integrated DNA Technologies — OligoAnalyzer
IDT’s official oligonucleotide analysis resource provides Tm and related oligonucleotide calculations and is useful for comparing simple estimates with more detailed design workflows.
NCBI Bookshelf — Molecular Biology of the Cell / DNA Hybridization Concepts
NCBI Bookshelf provides authoritative educational material on DNA structure, hybridization and factors affecting nucleic-acid stability.
Primer3 — Primer Design Documentation
Primer3 is a widely used open-source primer-design resource that documents primer thermodynamics and Tm-related design considerations.
SantaLucia, J. — A Unified View of Polymer, Dumbbell, and Oligonucleotide DNA Nearest-Neighbor Thermodynamics
This foundational scientific work describes nearest-neighbor thermodynamic treatment of DNA duplex stability, illustrating why detailed Tm estimation can differ from simple GC-counting rules.
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DNA Melting Temperature (Tm) Calculator Disclaimer
This calculator is provided for educational and informational purposes. It estimates DNA melting temperature from user-entered sequence information using simplified formulas. It does not experimentally measure melting temperature, guarantee PCR or hybridization performance, diagnose medical conditions, or replace validated laboratory methods. For research, clinical, regulated or otherwise critical applications, use the applicable experimental protocol, validated primer-design method, manufacturer’s instructions and qualified professional judgment.