Primer & Oligo Design Answers After Choosing a Calculator

Use the Tm Calculator as the primary action page for vendor-style Tm checks. It lets you match salt, Mg²⁺, DMSO, and oligo concentration assumptions before you compare values from another calculator.

Example conditions: 50 mM Na⁺, 0 mM Mg²⁺, 250 nM oligo concentration. Run the Tm Calculator or review the calculation methodology.

Open Tm Calculator for the actual result

Check the method, salt assumptions, oligo concentration, and whether Mg²⁺ or DMSO corrections are enabled. Most Tm disagreements come from condition mismatch, not from the sequence itself.

What differs between tools is:

• Salt correction formula — use a method suitable for your PCR buffer
• Batch review — use batch mode when primer pairs or pools must be checked together
• Privacy — Browser by default; History/Library/API may save

For the calculation itself, open the Tm Calculator. Use this page for follow-up interpretation.

Open Tm Calculator for the actual result

Use the Primer Analyzer when you need a combined primer review that includes Tm, GC%, molecular weight, hairpins, self-dimers, hetero-dimers, and mismatch checks in one report.

For a detailed comparison, see OligoPool vs IDT — Full Feature Comparison. For immediate analysis, open the Primer Analyzer.

Open Tm Calculator for the actual result

Browser by default; History/Library/API may save. When Dashboard History save is enabled, History stores the full input snapshot so you can restore a run. Usage analytics stores aggregate events, not sequence text or result rows. Use local downloads instead when you want a fully local handoff.

Open Tm Calculator for the actual result

The nearest-neighbor (NN) thermodynamic method is commonly preferred for primers and probes because it accounts for stacking interactions between adjacent base pairs, not just base composition.

The Tm Calculator uses SantaLucia 1998 with Owczarzy 2008 salt correction for settings where those assumptions fit the experiment.

Calculate Tm with matched buffer settings

Differences between calculators can come from: 1) Different parameter sets — SantaLucia 1998 vs older Breslauer 1986, 2) Different salt corrections — Owczarzy 2004/2008 vs older SantaLucia 1998 formula, 3) Assumed oligo concentration, 4) Initiation parameter handling — how terminal base pairs are counted.

Bottom line: If your result differs from another tool, first check salt, magnesium, dNTP, additive, concentration, self-complementarity, and method settings before interpreting the difference.

Calculate Tm with matched buffer settings

Use your actual buffer concentrations for a condition-matched Tm estimate. Common defaults:

Critical: Changing Na⁺ from 50→100 mM shifts Tm by +3-5°C. Changing Mg²⁺ from 1.5→3 mM shifts Tm by +1.5-2.5°C. Always match your actual conditions.

Calculate Tm with matched buffer settings

For many standard PCR primer designs, 55-65°C is a useful starting window and primer pairs are often kept within about 5°C of each other. A Taq-style starting Ta is often a few degrees below the lower primer Tm, but polymerase, buffer, additives, target, and assay format can change the recommendation. Use the current manufacturer protocol or calculator for enzyme-specific guidance, then optimize with a gradient. qPCR probe placement also depends on the assay chemistry and manufacturer guidance.

Calculate Tm with matched buffer settings

Standard nearest-neighbor parameters are for unmodified DNA. Terminal and backbone modifications can change duplex stability by an amount that depends on the chemistry, position, sequence, and assay conditions. The calculator supports unmodified DNA/RNA. For modified oligos, use the result only as a baseline and consult modification-specific data or empirical testing.

Calculate Tm with matched buffer settings

Use these ΔG values as practical review points, then weigh the structure location and assay conditions:

Check your sequences with the Secondary Structure Predictor. For a detailed tutorial, see How to Detect and Fix Secondary Structures.

Run Secondary Structure Predictor

Strategies ordered by effectiveness:

1. Shift the primer binding site by 2-5 bases upstream or downstream — often eliminates the hairpin without changing target specificity
2. Introduce silent mutations (for gene assembly oligos) — change G→A or C→T at non-critical positions to break the complementary stem
3. Shorten the primer — remove bases involved in the self-complementary region (maintain Tm >55°C)
4. Add or adjust DMSO only within the polymerase protocol — additives can change both apparent Tm and amplification behavior
5. Test annealing temperature empirically with a gradient appropriate for the polymerase and buffer

For 3' end dimers specifically: avoid 3' ends with >3 consecutive G/C bases. This is the single most impactful rule for preventing primer-dimer artifacts.

Run Secondary Structure Predictor

A hairpin is when a single strand folds back on itself, forming a stem-loop structure (intramolecular). A self-dimer is when two copies of the same sequence bind to each other (intermolecular). A cross-dimer (hetero-dimer) is when two different sequences (e.g., forward and reverse primers) bind to each other. All three compete with the intended target binding, reducing PCR efficiency. 3' end involvement is most critical — dimers at the 3' end are extended by polymerase, creating primer-dimer artifacts visible as ~40-100 bp bands on gels.

Run Secondary Structure Predictor

Not always. Use ΔG bands as screening signals rather than universal pass/fail rules. Structures in a 5' overhang may matter less than structures in the template-binding region, and temperature context matters. A structure predicted at one temperature may behave differently at the actual annealing temperature. Give special attention to extendable 3' end complementarity, then interpret the result in the assay context and verify experimentally.

Run Secondary Structure Predictor

Follow this systematic 5-step checklist:

For detailed guidance with examples, see the primer validation checklist.

Open Primer Analyzer for full primer review

For PCR primers: 40-60% is ideal, with 45-55% being optimal. For qPCR probes: 50-60% provides good stability. For oligo pools/libraries: keep GC uniform across all sequences to avoid representation bias. Avoid extremes: <30% GC produces unstable duplexes with low Tm; >70% GC promotes secondary structures (G-quadruplexes, hairpins) that reduce synthesis yield and binding efficiency. Use the GC Content Analyzer to check your sequences before ordering.

Open Primer Analyzer for full primer review

It depends on your application:

• Single primer Tm: Tm Calculator — nearest-neighbor with salt correction
• Primer pair analysis: Primer Analyzer — Tm, dimers, and specificity in one tool
• Hairpin/dimer check: Secondary Structure Predictor — ΔG calculations with visual output
• Library/pool QC: Batch Sequence QC — validate 1,000 sequences free or 50,000 with Tool Membership
• Resuspension/dilution: Dilution Calculator — concentration calculations
• Everything at once: Oligo Properties Calculator — Tm, GC%, MW, ε₂₆₀ in one view

Open Primer Analyzer for full primer review

Formula: Volume (µL) = nmoles / (target µM) × 1000. Example: 100 nmol oligo to 100 µM stock = 100/100 × 1000 = 1000 µL (1 mL) in TE buffer (10 mM Tris, 0.1 mM EDTA, pH 8.0). For accurate quantification after resuspension, measure A₂₆₀ absorbance — 1 OD₂₆₀ ≈ 33 µg/mL for single-stranded DNA. Use the Molecular Weight Calculator to get the extinction coefficient (ε₂₆₀) for precise concentration determination via Beer-Lambert law: Concentration = A₂₆₀ / (ε₂₆₀ × path length).

Open Primer Analyzer for full primer review

An oligo pool is a complex mixture of thousands to millions of unique oligonucleotides synthesized in a single reaction using array-based synthesis. Use pools when you need >100 unique sequences for the same experiment (CRISPR libraries, NGS panels, mutagenesis, gene assembly). For <100 sequences or when you need each oligo individually (PCR primers, probes), order individual oligos instead.

Start with the Oligo Pool Guide for the full design, QC, vendor, and ordering path. Use Oligo Pool vs Individual Oligos when the first decision is whether pooled synthesis is appropriate.

Open the Oligo Pool Guide

Use the Oligo Pool Vendor Comparison for vendor selection and the vendor specification notes for documented product-page details.

Open the Oligo Pool Guide

Run these checks before ordering to avoid costly redesign:

1. Upload all sequences to Batch Sequence QC — checks GC%, Tm, homopolymers, length
2. Target >90% pass rate (for CRISPR screens, aim >95%)
3. Check Tm uniformity — pool Tm CV should be <3°C for functional assays
4. Run secondary structure check on flagged sequences
5. Use Vendor Format Adapter to generate the correct order file format

Open the Oligo Pool Guide

Run Batch Sequence QC before export

We support FASTA (single-line and multi-line), CSV, TSV, and plain text (one sequence per line). The Format Converter tool auto-detects input format and converts between them. For vendor ordering: use the Vendor Format Adapter to generate IDT, Twist, GenScript, or Dynegene-specific order files.

Run Batch Sequence QC before export

Yes. Most tools support CSV/Excel export. Batch QC generates comprehensive reports with per-sequence details and pool-level statistics. The Vendor Format Adapter exports ready-to-upload order files for each supported vendor.

Run Batch Sequence QC before export

Run Batch Sequence QC before export