Biology calculators

GC Content Calculator

Updated Sep 22, 2026 By Infinity Calculator
Rate Formulas

Sequence Input

Accepts a raw sequence, a single FASTA entry, or multi-FASTA. IUPAC degenerate codes (R, Y, S, W, K, M, B, D, H, V, N) are supported. Input is read case-insensitively; minimum 10 and maximum 100,000 nucleotides per sequence.
Valid nucleotides: 0 Quick GC preview: 0.00% Sequences detected: 1 Running preview only — press Calculate for the full analysis.

Analysis Options

Sequence type
Auto-detect uses the presence of U vs. T.
Any IUPAC letters, e.g. GC, AT, AU, GCU. T and U are treated as equivalent.
Minimum 10; must be smaller than the sequence length.
All Tm outputs are converted to this unit.
Silently strips spaces, line breaks, digits and gap characters (- . * _ :) before analysis.
When unchecked, GC% uses only A, C, G, T/U counts as the denominator.
Primary Result
Per-Base Breakdown
Step-by-Step Solution
GC Content Distribution (Sliding Window)
Base Composition
Analyze a Sub-Region
Sequence Viewer
CpG Island Analysis

Introduction

DNA is made of four bases: A, T, G, and C. GC content is the percent of bases in a sequence that are G (guanine) or C (cytosine). This free GC content calculator counts those bases and shows the answer as a percent.

GC content matters because G and C pair with three hydrogen bonds, while A and T pair with only two. So DNA with more G and C holds together more tightly and needs more heat to split apart. That is why scientists check GC content when they design PCR primers, pick annealing temperatures, or compare genes between species.

Paste a raw DNA or RNA sequence, or a FASTA file with many sequences. The tool then gives you:

  • GC% and AT% for each sequence
  • A count and percent for every base, including IUPAC codes like N, R, and Y
  • Melting temperature (Tm) in °C, °F, or K
  • A sliding window chart that shows where GC is high or low
  • CpG island detection and a color-coded sequence viewer

You can also pick your own bases to count, check just one part of the sequence, and download the results as a CSV file. Step-by-step math is shown, so you can see how each number was found.

How to use our GC Content Calculator

Paste a DNA or RNA sequence, pick a few options, and press Calculate. The tool shows your GC content percent, AT content, base counts, melting temperature (Tm), a sliding window GC chart, and any CpG islands it finds.

DNA / RNA sequence or FASTA input: Type or paste your sequence in the big box. You can use a plain sequence, one FASTA entry, or many FASTA entries at once. IUPAC codes like N, R, and Y are fine. Each sequence needs at least 10 bases and no more than 100,000.

Sequence type: Choose Auto, DNA, or RNA. Auto looks for U or T and picks for you. Pick DNA or RNA yourself if you already know.

Calculate content for bases: Enter the letters you want counted. Leave it as GC for normal GC content, or try AT, AU, or any other IUPAC letters. T and U count as the same base.

Sliding window size (bp): Set how many bases each window covers on the GC chart. Use 10 or more, and keep it smaller than your sequence. Small windows show fine detail; big windows show smooth trends.

Melting temperature unit: Pick Celsius, Fahrenheit, or Kelvin. Every Tm result switches to that unit.

Auto-clean input: Keep this checked to drop spaces, line breaks, numbers, and gap marks like - . * _ before the math runs.

Include degenerate bases in the denominator: Check this to count codes like N and R in the total. Leave it off to use only A, C, G, and T/U as the total.

Sub-region start and end: After you calculate, type a start and end position and click Analyze region to get GC content for just that part of the sequence. It also gets marked in the sequence viewer.

What Is GC Content?

DNA is made of four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). In RNA, uracil (U) takes the place of T. GC content is the percent of bases in a sequence that are either G or C. It is one of the most basic facts you can learn about a piece of DNA or RNA.

The GC Content Formula

GC content is simple math:

GC% = (G + C) ÷ (A + T + G + C) × 100

So a 100-base sequence with 30 G's and 25 C's has a GC content of 55%. The leftover part is the AT content, which is 45%.

Why GC Content Matters

G and C pair with three hydrogen bonds. A and T pair with only two. More G and C pairs means the DNA strands hold together more tightly. This changes how the DNA acts in the lab and in the cell:

  • Melting temperature (Tm): High GC DNA needs more heat to split into two strands. This matters a lot for PCR primers.
  • Primer design: Most good primers land near 40% to 60% GC. Very low GC primers bind weakly. Very high GC primers can stick to the wrong spots or form hairpins.
  • Gene finding: In many genomes, coding regions and gene promoters have higher GC than the DNA around them.
  • Species clues: Different organisms have very different average GC. Some bacteria sit near 25%, others near 75%. Human DNA averages about 41%.

Melting Temperature Rules

Two common quick formulas are used here:

  • Wallace rule (best for short primers, 14 bases or fewer): Tm = 2(A + T) + 4(G + C)
  • Basic formula (better for longer sequences): Tm = 64.9 + 41 × (G + C − 16.4) ÷ length

Both are estimates. They ignore salt level, primer amount, and mismatches, so treat them as a starting point, not a final answer.

CpG Islands

A CpG site is a C followed by a G on the same strand. In most of the genome these are rare. But short stretches called CpG islands are packed with them, and they often sit at the start of genes. The standard rule (Gardiner-Garden and Frommer) calls a region a CpG island when it is at least 200 bases long, has at least 50% GC, and has a CpG observed/expected ratio above 0.6. CpG islands are important because methylation at these sites can switch genes off.

Degenerate (IUPAC) Bases

Sometimes a sequence uses letters other than A, C, G, T, and U. These IUPAC codes stand for more than one possible base, such as R (A or G), Y (C or T), S (G or C), W (A or T), and N (any base). They show up in consensus sequences and in reads where the base was unclear. You can choose to count them in the total or leave them out.

Reading Your Results

GC ContentWhat It Suggests
Below 35%AT-rich. Common in intergenic DNA, some bacteria, and AT-rich genomes like Plasmodium.
35% to 60%Balanced. Typical for most genes and good for primer design.
Above 60%GC-rich. Often promoters, CpG islands, or GC-rich organisms. Can be hard to amplify by PCR.

Sliding Windows

One average number hides a lot. A sliding window moves a short frame (say 100 bases) along the sequence and measures GC in each step. This shows where GC rises and falls, which helps you spot promoters, CpG islands, repeats, and joins between different pieces of DNA.

Formulas used

Target base content (e.g. GC%)
\text{GC\%} = \frac{n_{G} + n_{C}}{n_{\text{denom}}} \times 100
AT content
\text{AT\%} = \frac{n_{A} + n_{T/U}}{n_{\text{denom}}} \times 100
Denominator (with or without degenerate bases)
n_{\text{denom}} = n_{A} + n_{C} + n_{G} + n_{T/U} \; (+\; n_{\text{degenerate}})
GC/AT ratio
R_{GC/AT} = \frac{n_{G} + n_{C}}{n_{A} + n_{T/U}}
Melting temperature — Wallace rule (length \le 14)
T_m = 2\,(n_{A} + n_{T/U}) + 4\,(n_{G} + n_{C})
Melting temperature — basic formula (length > 14)
T_m = 64.9 + 41 \times \frac{(n_{G} + n_{C}) - 16.4}{n_{A} + n_{T/U} + n_{G} + n_{C}}
CpG observed/expected ratio
\frac{O}{E} = \frac{n_{CpG} \times L}{n_{C} \times n_{G}}
Temperature unit conversion
T_{^{\circ}F} = T_{^{\circ}C} \times \frac{9}{5} + 32, \qquad T_{K} = T_{^{\circ}C} + 273.15

Frequently asked questions

What is a good GC content for PCR primers?

Most primers work best at 40% to 60% GC. That range gives a melting temperature near 55 to 65 °C, which fits normal PCR programs.

  • Under 40% GC: the primer binds weakly and may need a lower annealing temperature.
  • Over 60% GC: the primer can stick to the wrong places or fold on itself.

Also try to keep the forward and reverse primer Tm values within about 5 °C of each other.

What is a GC clamp in primer design?

A GC clamp is one or two G or C bases at the 3' end of a primer. Because G-C pairs have three hydrogen bonds, they grip the template tightly at the exact spot where the polymerase starts building.

Use one or two G/C bases in the last five positions. More than three in a row is too sticky and can cause false binding.

Why is GC-rich DNA hard to amplify by PCR?

GC-rich DNA holds together tightly and can form hairpins and other folds. The strands may not fully separate during denaturation, so the primers cannot reach the template.

Common fixes:

  • Add DMSO (about 3 to 10%) or betaine (about 1 M) to weaken the bonds.
  • Raise the denaturation temperature or lengthen that step.
  • Use a hot-start or high-fidelity polymerase made for GC-rich targets.

What is the average GC content of the human genome?

The human genome averages about 41% GC. But the value is not the same everywhere. Long blocks called isochores range from roughly 35% to over 55% GC.

Chromosome 19 is the most GC-rich human chromosome at around 48%, while chromosome 4 sits near 38%. Gene-packed regions usually have higher GC than gene-poor regions.

Why do bacteria have such different GC content?

Bacterial GC content ranges from about 25% to 75%. The value is fairly stable inside a species, so it has long been used as a clue in classifying bacteria.

Differences come from mutation bias, DNA repair systems, and genes picked up from other organisms. For example, Escherichia coli is near 50%, Mycoplasma near 25%, and Streptomyces above 70%.

Does high GC content mean an organism lives in a hot place?

Not for whole genomes. Many heat-loving microbes have low genomic GC, so total GC does not predict growth temperature.

The link is real for structural RNA, though. Ribosomal RNA and transfer RNA from heat-loving species do carry more G and C, because those extra hydrogen bonds keep the RNA folded at high temperature.

What is GC skew and how is it different from GC content?

GC content counts G and C together. GC skew compares them: (G − C) ÷ (G + C) on one strand.

In many bacterial genomes the skew flips sign at the origin of replication and again at the terminus. Because of that, biologists use GC skew plots to find where DNA copying starts, something plain GC content cannot show.

Why do coding regions often have higher GC content?

Two main reasons:

  • Codon choice: the third base of a codon can often change without changing the protein. Many species favor G or C there.
  • Repair bias: the repair machine that fixes mismatches during recombination tends to swap A/T for G/C, which slowly raises GC in busy, gene-rich regions.

This is why a jump in GC on a sliding window plot can point to a gene or a promoter.

Does GC content affect DNA sequencing results?

Yes. Regions that are very GC-rich or very AT-rich usually get fewer sequencing reads. This is called coverage bias, and it mostly comes from the PCR step used to build sequencing libraries.

Low coverage means variants in those regions are easier to miss. PCR-free library preps and careful enzyme choice cut down the bias.

How do I find AT content if I know the GC content?

Subtract from 100. AT% = 100 − GC%

So a sequence with 58% GC has 42% AT. This works because A, T, G, and C are the only four bases counted. If the sequence has unclear bases like N, the two numbers may not add to exactly 100 unless those bases are left out of the total.

Why are CpG islands found at the start of so many genes?

About 60–70% of human gene promoters sit inside a CpG island. When the C bases in an island stay unmethylated, the DNA stays open and the gene can be switched on.

When those C bases get methylated, the gene is usually turned off. Tumor cells often methylate CpG islands in front of protective genes, which is why these regions are studied so closely in cancer research.

How does GC content change DNA melting temperature?

Each extra percent of GC raises the melting temperature by roughly 0.4 °C for long DNA. A G-C pair has three hydrogen bonds, and stacking between neighboring G/C bases is also stronger, so more heat is needed to pull the strands apart.

Salt matters too. Raising salt shields the negative backbone and pushes Tm higher, so always note the buffer when you compare Tm values.

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