GC Content Calculator
Paste a sequence, get its G-plus-C share: a pure count that sets how tightly a duplex melts and fingerprints genomes from AT-rich Mycoplasma to the 72% Streptomyces.
GC Content Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Unmodified A-C-G-T strings only — no ambiguity codes
- Genome landmarks quoted approximate
In short: TAATGCC counts out A 2, T 2, C 2, G 1 — three of seven bases are G or C, so GC 42.857143%, a notch below the human average of ≈41%. The counts also say something structural: G 1 against C 2 means this string cannot be a whole duplex by itself — paired strands carry G and C equally. Push it into the Tm page and the same seven letters price a melt near the mid-30s °C on the bench rules.
Formula
GC% = (G + C) / (A + T + G + C) × 100
GC content is a division, not a measurement: count the guanines and cytosines, divide by every base in the string. In double-stranded DNA the count is the same on either strand — every G pairs a C — which is why a whole duplex always carries G and C in equal numbers and why the census card checks that balance.
Worked Example
- Paste the DNA sequence (A, C, G, T).
- Read the GC percentage and the letter census.
- Check the duplex balance: does G equal C?
- Follow the link to price the melting temperature.
Defaults: TAATGCC → GC 42.857143% (3 of 7). ATCGATCG → 50.000000%. GCGCGC → 100.000000%. ATATAT → 0.000000% — and the Tm page prices that pure-AT string at 16.871429 °C, barely melted at room temperature.
Strengths & Limits Of This Model
Where this engine is strong
- Pure count, zero convention risk
- The duplex balance check is free
Where it stops
- No sliding-window view
- No GC skew or strand bias
Practical Use Cases
Primer design
GC clamp and annealing sanity
Genome forensics
which organism reads this GC
Teaching
counts before chemistry
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
GC Content Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does GC content matter so much?
Because it is the one number every downstream experiment inherits. G-C pairs hold three hydrogen bonds where A-T pairs hold two, so GC-rich duplexes melt hotter, anneal tighter and amplify differently. Genomes carry long-run GC signatures too — the human average sits near 41%, E. coli near 50%, and Streptomyces coelicolor near 72% (all approximate) — which is why a sudden GC shift in a sequence can hint at horizontal gene transfer.
Should I count one strand or both?
For a double-stranded molecule the question answers itself: the strands are complementary, so counting either gives the same GC share. The census card makes the check concrete — a string whose G count disagrees with its C count cannot be a whole duplex on its own. Single-stranded probes and amplicon intermediates are exactly the strings that fail that check, and the page says so instead of pretending.
What about N, R and the other ambiguity codes?
They are refused — by name and position. The bench rules that consume GC content want a plain A-C-G-T string; an ambiguity code inside a primer is a mixed synthesis that no single percentage can price. Paste the concrete sequence you actually ordered and the arithmetic is exact again.
My sequence has spaces and dashes from a FASTA export.
Formatting survives the paste: spaces, commas, periods and dashes are stripped before the census runs, and case does not matter — actg counts the same as ACTG. Digits are refused, because a number inside a sequence is usually a coordinate that wandered in from a scaffold name, and counting it as a base would be a lie.
Which genomes are genuinely high- or low-GC?
The span is enormous: bacterial genomes run from below 25% in some AT-rich Mycoplasma to about 75% in Micrococcus and the streptomycetes (values approximate). Humans average near 41% but range roughly 35–60% across 100 kb windows — isochores, not noise. The band card places your string among those landmarks rather than pretending every percentage is equally ordinary.
Is GC content the same as melting temperature?
They are cousins, not twins. GC content is a static count; melting temperature adds salt, length and thermodynamics on top of the count. A 4-mer and a 4,000-base region can share a GC percentage and melt hundreds of degrees apart. The primer Tm page turns this count into a temperature with the bench rules; this page keeps to the arithmetic every one of those rules starts from.
Why does the page refuse single letters less than a full count?
It does not — a one-base string counts honestly (a lone G reads 100.000000%). The refusals are reserved for things that are not sequences: empty pastes, ambiguity codes, digits. A single base is a legitimate, if boring, census; the arithmetic does not pretend otherwise.
How do I use this in primer design?
Aim each primer near 40–60% GC, keep the pair within about 5 °C of each other in Tm, and give the 3′ end its famous GC clamp — one or two G/C letters that anchor the extension — without stacking three of them. The census card gives you the raw counts to check all three by eye before the Tm page prices them.