How to Use the AT/GC Ratio Calculator
Step 1: Enter your DNA sequence in the input box above.
Step 2: Click Calculate Ratio. The tool shows AT% vs GC% as large numbers and a visual split bar showing the proportion of each.
Step 3: The AT/GC ratio number tells you how many AT bases there are for every GC base. A ratio of 1.0 means equal amounts of AT and GC.
Understanding AT/GC Ratio
The AT/GC ratio is a fundamental property of DNA sequences. It affects thermal stability, DNA replication, gene expression, and PCR conditions.
AT count = A + T bases
GC count = G + C bases
AT/GC Ratio = AT count / GC count
// Example: ATGCATGC (8 bases)
AT = 4, GC = 4
AT/GC Ratio = 4/4 = 1.0 (balanced)
Ratio Interpretation Guide
A ratio less than 1.0 means GC-rich sequence â more stable, higher melting temperature. A ratio greater than 1.0 means AT-rich sequence â less stable, lower melting temperature. A ratio of exactly 1.0 means perfectly balanced sequence.
Biological Significance
GC base pairs form 3 hydrogen bonds while AT pairs form only 2. This makes high GC sequences more thermally stable. Many promoter regions are AT-rich (TATA box) while coding regions tend to have balanced or higher GC content. Organisms like Mycobacterium tuberculosis have very high GC content (around 65%) while others like Plasmodium falciparum are extremely AT-rich (around 80% AT).