Free PCR Product Size Calculator
Calculate the expected amplicon size from your forward and reverse primer positions. Instantly determine PCR product length in base pairs. No login required.
VigyanLLM's PCR product size calculator determines the expected amplicon length by computing the distance between your forward primer start position and reverse primer end position on the template. This is essential for PCR optimization, gel electrophoresis planning, and primer validation.
| Input | Description |
|---|---|
| Forward primer start | Position where the forward primer binds (1-based) |
| Reverse primer end | Position where the reverse primer ends (1-based) |
| Primer lengths | Lengths of forward and reverse primers (optional, for full calculation) |
How PCR Product Size Is Calculated
The PCR product size equals the distance from the forward primer start position to the reverse primer end position, inclusive. When primer lengths are provided, the formula is: Product Size = Reverse End Position minus Forward Start Position plus 1. This gives you the exact base pair length of the amplified fragment, which determines the expected band position on an agarose gel. For example, if your forward primer starts at position 100 and your reverse primer ends at position 500, the PCR product is 401 bp.
Why PCR Product Size Matters
- Gel electrophoresis: The product size determines which DNA ladder band your sample should align with
- Primer validation: If the calculated size does not match your expected result, one or both primers may be binding to the wrong location
- PCR optimization: Larger products may require longer extension times and specialized polymerases
- qPCR efficiency: Product sizes between 70 and 200 bp are ideal for quantitative PCR applications
About VigyanLLM
VigyanLLM is India's first sovereign, on-premises biomedical AI platform. The full primer design tool includes automated PCR product size calculation as part of its 24-parameter validation pipeline, along with BLAST specificity checking, dimer prediction, and melting temperature analysis. Try the full primer designer. Free for Indian universities.
Frequently Asked Questions About PCR Product Size
Everything you need to know about calculating PCR amplicon size
How is PCR product size calculated?
The PCR product size is the number of base pairs between the forward primer start position and the reverse primer end position, inclusive. Formula: Product Size = Reverse End minus Forward Start plus 1.
What is an ideal PCR product size?
For standard PCR, products between 200 and 2000 bp work well with Taq polymerase. For qPCR, smaller products of 70 to 200 bp are preferred for optimal efficiency. Products over 3000 bp may require specialized high-fidelity polymerases and longer extension times.
Are primer positions 0-based or 1-based?
This calculator uses 1-based positions, which is the standard convention in molecular biology (matching NCBI GenBank, UCSC Genome Browser, and most primer design tools). If your positions are 0-based, add 1 to each value before entering them.
What if my calculated size does not match my gel result?
A mismatch could indicate non-specific primer binding (primers annealing to off-target sites), incorrect template annotation, or the presence of introns in genomic DNA templates. Verify primer specificity using BLAST and check your template sequence coordinates.
Is this calculator free?
Yes, completely free with no usage limits. No login or account required. All calculation happens in your browser and no data is sent to any server.