About the GC Content Calculator
GC Content Calculator measures the share of G and C bases in a DNA sequence. Paste a sequence, as plain letters or in FASTA format, and the page counts A, T, G and C, gives the GC percentage and the AT percentage, notes any other letters and estimates the melting temperature: the Wallace rule for short sequences under 14 bases and a basic formula for longer ones. GC content matters for primer design, gene analysis and comparing genomes, since G-C pairs have three hydrogen bonds and make DNA more stable than A-T pairs.
Updated October 2026
How to use the GC Content Calculator
- 1Paste your DNA sequence, plain or with a FASTA header.
- 2Read the GC percentage and base counts.
- 3Check the melting temperature estimate.
- 4Look at the Other letters count if the sequence has ambiguity codes.
- 5Copy the result.
What the percentage tells you
GC content describes the composition of DNA. A stretch with 60% GC is held together more strongly than one with 30% GC, so it needs more heat to separate. In genomes, GC content differs between species, between genes and between regions of one chromosome, and scientists use it to compare organisms and to spot unusual segments.
For primer design
A pair of PCR primers should have similar melting temperatures, usually within 2 to 3 degrees, and GC content in the middle range. Check each primer here, then use a full primer design tool for exact values, because real melting temperatures depend on salt and primer concentration.
- Length: 18 to 25 bases
- GC content: 40% to 60%
- A G or C at the 3′ end helps binding
- Avoid long runs of one base
Limits of the estimate
The formulas used here are simple and ignore salt, concentration and the order of the bases. They are good enough for a quick comparison. For publication or critical work use a nearest-neighbour calculator.
Related tools
Make the other strand with the Reverse Complement Calculator, translate the sequence with the DNA to Protein Translator and work out percentages with the Percentage Calculator.
Features
- GC and AT percentage
- Counts of A, T, G, C and other letters
- Melting temperature estimate
- FASTA, spaces and numbers ignored
- U read as T
- Private: nothing is uploaded
Frequently asked questions
What is GC content?
The percentage of bases in a DNA or RNA sequence that are guanine (G) or cytosine (C). It is calculated as (G + C) divided by the total of A, T, G and C.
What is a good GC content for a PCR primer?
Between about 40% and 60%, with a G or C near the 3′ end. Too low gives weak binding and too high can give mispriming.
Why does GC content matter?
G-C pairs are held by three hydrogen bonds and A-T pairs by two, so sequences rich in G and C melt at a higher temperature and are more stable.
How is the melting temperature estimated?
For under 14 bases: Tm = 2 × (A + T) + 4 × (G + C). For longer ones: Tm = 64.9 + 41 × (G + C − 16.4) / length. These are rough guides for primers.
What is the average GC content of the human genome?
About 41%, with large regional variation. Bacteria range from about 25% to 75%.
What happens with N and other codes?
Ambiguous letters are not counted as G or C or as part of the denominator, and the melting temperature is not shown because it would be unreliable.
Does it accept RNA?
Yes. U is counted as T.
Can I use it for a whole genome?
The page handles sequences of many thousands of bases, but a genome of millions or billions of bases is better handled with bioinformatics software. For genes, plasmids and primers it is ideal.
What does a very high or low GC content mean?
Extremes can make DNA hard to copy and sequence, because G-C rich stretches form stable structures and A-T rich stretches separate easily. Labs check GC content before ordering synthetic genes.
Is my sequence uploaded?
No. It is analysed in your browser.