Primer Tm Calculator (Nearest-Neighbor)
Calculate PCR primer melting temperature using nearest-neighbor thermodynamics. More accurate than Wallace rule for primers 15-30 bases. Uses unified NN parameters for primer design.
🧬 PCR Design📐 Tm = dH / (dS + R ln[primer]) - 273.15🧬 Biology
Number of A+T bases in primer
Number of G+C bases in primer
Primer length (total bases)
Primer concentration (nM)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Primer Tm Calculator (Nearest-Neighbor) | — | Tm = dH / (dS + R ln[primer]) - 273.15 | degrees C |
Step-by-Step Examples
Example 1
Standard PCR Primer
20-mer primer: A+T=9, G+C=11, c=250nM.
- GC% = 11/20 = 55%
- Tm (NN estimate) ≈ 62 degrees C
- Annealing temp: ~57 degrees C
- Wallace rule: 2(9)+4(11)=62 degrees C (similar)
✓ Tm ≈ 62 degrees C, anneal 57 degrees C
Example 2
AT-Rich Primer
18-mer: A+T=12, G+C=6, 250nM.
- Low GC = lower Tm
- Tm ≈ 52 degrees C
- Wallace: 2(12)+4(6)=48 degrees C
- NN more accurate for AT-rich sequences
- Anneal temp: 47-52 degrees C
✓ Tm ≈ 52 degrees C
Example 3
GC-Rich Primer
22-mer: A+T=6, G+C=16, 250nM.
- High GC = high Tm
- Tm ≈ 71 degrees C
- May form hairpin structures — check secondary structure
- Anneal at 66 degrees C
✓ Tm ≈ 71 degrees C, check for hairpins
Real-World Applications
PCR Design
Accurate Tm guides annealing temperature selection for specific amplification.
Multiplex PCR
All primers in multiplex must have similar Tm for simultaneous amplification.
qPCR Assays
Consistent Tm ensures reliable quantitative results across plates and runs.
Primer Ordering
Check Tm before ordering primers — avoid expensive redesign.
Common Mistakes to Avoid
⚠️
Using one Tm formula for all lengths
Wallace rule for <15 bp; SantaLucia NN for 15-30 bp; different corrections for long probes. Match formula to primer length.
⚠️
Ignoring primer concentration
NN Tm depends on primer concentration. Standard assumption is 250nM primer in PCR. Lower concentration reduces Tm slightly.
⚠️
Not checking self-complementarity
Primers can form hairpins or primer-dimers that reduce efficiency. Always check with Primer3 or OligoAnalyzer even if Tm is fine.
Frequently Asked Questions
What primer concentration should I use? ▾
Standard: 0.1-0.5 uM (100-500 nM) final concentration in PCR. Default for Tm calculation: 250 nM. Higher [primer] slightly increases Tm.
How much does GC content affect Tm? ▾
Each 1% increase in GC raises Tm ~0.4 degrees C (GC term in MSD equation). Going from 40% to 60% GC raises Tm ~8 degrees C.
Should I use Wallace or NN method? ▾
NN (nearest-neighbor) is more accurate for most primers. Wallace (Tm = 2AT + 4GC) is a quick estimate. Both are approximations; actual Tm varies with primer sequence context.
What is a good primer GC content? ▾
Optimal: 40-60% GC. Avoid runs of >4 identical bases. Aim for GC clamp (G or C at 3' end) to stabilize primer-template binding at extension start.
How close should paired primer Tm values be? ▾
Within 5 degrees C of each other. Primers with very different Tm give unequal amplification efficiency at a single annealing temperature.