How to Use the PCR Product Size Calculator
Step 1: Enter the base-pair coordinate where your forward primer binds on the template and the start coordinate of the reverse primer binding site.
Step 2: The calculator subtracts the forward primer position from the reverse primer position and adds 1 bp to estimate the amplicon size.
Step 3: Review the predicted size category to guide gel selection and band interpretation.
About Amplicon Size Estimation
PCR product size is a key planning value for agarose gel electrophoresis, extension timing, and expected fragment visualization. A practical estimate helps you choose an appropriate gel percentage before running the assay.
Product size (bp) = Reverse primer start − Forward primer start + 1
// Example:
Forward start = 120 bp
Reverse start = 820 bp
Product size = 820 − 120 + 1 = 701 bp
Interpreting Product Size
Small PCR products (under 500 bp) often resolve well on 1.5–2.0% agarose, medium fragments around 500–2000 bp on 1.0–1.5%, and larger fragments above 2000 bp on 0.7–1.0% agarose. Use the calculator output as a quick planning estimate before you run the gel.