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GC Content Calculator

Biology

Calculate the GC content percentage of any DNA or RNA sequence. Paste a sequence of A/T/G/C or A/U/G/C letters and get instant base composition results.

Accepts A, T, G, C (DNA) or A, U, G, C (RNA). Whitespace is ignored; any other character is counted as invalid and excluded from the percentages.

GC Content

60%
60%
40%
GC (60%)
AT (40%)
Length
25 bases
G / C
8 / 7
A / T
5 / 5
Strand Type
DNA

What is a GC Content?

The GC Content Calculator analyzes a DNA or RNA sequence and computes the percentage of bases that are guanine (G) or cytosine (C), using GC% = (G count + C count) ÷ total length × 100. Paste or type a sequence into the text box, and the calculator instantly returns the GC content, the complementary AT (or AU) content, and a full base count breakdown.

GC content is one of the most basic and widely used descriptors of a nucleic acid sequence, relevant to sequence stability, PCR primer design, and genome characterization. To estimate the physical mass of a sequence based on its length, see the DNA/RNA Molecular Weight Calculator.

How to use this GC Content calculator

  1. Paste or type your sequence — into the text box, using standard A/T/G/C (DNA) or A/U/G/C (RNA) letters. Line breaks and spaces are fine.

  2. Read the GC content result — the highlighted result shows GC content as a percentage, with a proportional GC-vs-AT/AU bar underneath.

  3. Check the base breakdown — individual G, C, A, and T/U counts and the total valid sequence length are shown in the result card.

  4. Review any invalid character warning — if the calculator flags invalid characters, double-check your sequence for typos, ambiguity codes, or accidentally included header text.

Formula & Methodology

GC content formula:
GC% = (G count + C count) ÷ total valid length × 100

Base detection logic:
- The sequence is uppercased and whitespace is stripped.
- Each character is classified as G, C, A, T, or U; any other character is counted as invalid and excluded from the length used in the percentage.
- If the sequence contains U but no T, it's treated as RNA; otherwise it's treated as DNA.

Worked example:

Sequence: ATGCGCATCG (10 bases)

G count = 3, C count = 3 → GC = 6 bases

GC% = 6 ÷ 10 × 100 = 60%

Note: This calculator treats ambiguity codes (N, R, Y, W, S, K, M, etc.) as invalid characters rather than resolving them probabilistically, so sequences with many ambiguous bases will show a shorter valid length than the total characters entered.

Frequently Asked Questions

GC content is the percentage of bases in a DNA or RNA sequence that are guanine (G) or cytosine (C), calculated as (G count + C count) ÷ total sequence length × 100. It's a fundamental measure used to describe the composition and stability of a nucleic acid sequence.
GC base pairs are held together by three hydrogen bonds (versus two for A-T pairs), making GC-rich DNA regions more thermally stable and requiring higher temperatures to denature (melt). GC content also affects PCR primer design, genome sequencing difficulty, and can indicate evolutionary or functional characteristics of a genomic region.
The calculator automatically detects whether your sequence is DNA or RNA based on whether it contains T (thymine, found in DNA) or U (uracil, found in RNA) — it cannot contain both, since a real nucleic acid strand uses one or the other. The result card labels the output accordingly and shows AT content for DNA or AU content for RNA.
The calculator accepts the standard nucleotide letters A, T, G, C (DNA) or A, U, G, C (RNA), in either uppercase or lowercase. Whitespace and line breaks are ignored automatically, and any other character (like ambiguity codes N, R, Y, or stray symbols) is flagged as invalid and excluded from the percentage calculations.
GC content varies widely across life: human DNA averages around 41% GC, E. coli bacteria around 50%, and some extremophile bacteria and GC-rich organisms can exceed 70%, while AT-rich organisms like Plasmodium falciparum (the malaria parasite) can be below 20%. There's no single "normal" value — it reflects each organism's evolutionary history.
PCR primers are typically designed with 40–60% GC content to ensure they bind (anneal) to the template DNA at an appropriate, predictable melting temperature — primers with GC content far outside this range can bind too weakly (low GC) or too strongly and nonspecifically (high GC), reducing PCR efficiency.
Higher GC content generally means a higher melting temperature (Tm), because the extra hydrogen bond in each G-C pair requires more thermal energy to break compared to an A-T pair. This relationship is used in simplified Tm estimation formulas for short DNA sequences, like primers and probes.
Yes — this calculator strips whitespace and line breaks automatically before analyzing the sequence, so you can paste a multi-line sequence copied directly from a FASTA-formatted file (just leave out the header line starting with '>', since that isn't a nucleotide sequence).
Check for invalid characters flagged by the calculator — ambiguity codes (N, R, Y, W, S, K, M) or accidental non-nucleotide text (like a FASTA header) will be excluded from the valid sequence length, which changes the percentage calculation. Also confirm you haven't accidentally mixed T and U characters, which would produce an inconsistent DNA/RNA read.
Extremely high or low GC content regions are notoriously difficult to sequence and assemble accurately with many sequencing technologies, because they can form secondary structures or amplify unevenly during library preparation — a practical reason why GC content is checked early in genome assembly quality control.
This calculator works for sequences of any length you can paste into the text box, from short primers (15–30 bases) to much longer gene or plasmid sequences, though extremely large whole-genome files are better handled with dedicated bioinformatics software rather than a browser text box.
GC content doesn't directly change the average molecular weight formula this platform uses (which relies on sequence length and strand type), but real GC-rich versus AT-rich sequences do have slightly different exact molecular weights, since G/C and A/T nucleotides have different individual masses. See the [DNA/RNA Molecular Weight Calculator](/dna-rna-molecular-weight-calculator/) for an estimate based on sequence length.
Also known as
GC percentage calculatorDNA base composition calculatorsequence GC calculatorguanine cytosine content calculator