GC Content Calculator for DNA Primers

Background.

Ask a lab mate what GC content a primer should have and the answer comes back instantly: 40 to 60 percent. That band is real. Lorenz's 2012 PCR protocol in the Journal of Visualized Experiments gives it as the optimum, alongside a length of 15 to 30 bases and a G or C at the 3' end. But Primer3, one of the most widely used primer-design programs, accepts anything from 20 to 80 percent GC by default, wants 18 to 27 bases, and does not require a 3' G or C at all. A primer at 35 percent fails the first test and passes the second, so both verdicts are shown here, each with its source named.

Composition matters because the two kinds of base pair are not equal. A G·C pair is held by three hydrogen bonds and an A·T pair by two, and G and C also stack more strongly against their neighbours. A GC-rich primer therefore grips its template harder and melts at a higher temperature, which sounds like a virtue until it starts holding on to sites where it is mismatched. A GC-poor primer has the opposite trouble: it has to be made longer to bind at a usable temperature, and longer oligos fold back on themselves more readily.

Paste a sequence and you get its GC and AT percentages, the length, the number of G and C bases in the last few positions at the 3' end, the longest run of a single base, the Wallace dissociation temperature, and the molecular weight with micrograms per nanomole. A short written assessment sets each result against the published guidance.

The universal M13 forward primer used in the worked example makes a good test case. It is 17 bases long, one short of Primer3's default minimum; it ends in T, so it has no clamp; and sequencing facilities have used it for decades without trouble. Design rules describe tendencies. A primer that breaks one of them is a primer to look at more carefully, not one to throw away.

What is gc content calculator (primer)?

GC content is the share of bases in a DNA sequence that are guanine or cytosine, written as a percentage of the total length. It depends only on composition, not order: GGGCCCAAATTT and GCATGCATGCAT are both exactly 50 percent GC although the first contains runs a polymerase can slip on and the second does not. For a sequence made only of A, C, G and T, the GC and AT percentages always add up to 100. A GC clamp is a separate idea about position: a G or C at the very 3' end, the end the polymerase extends from, where a strongly paired base keeps the primer engaged with its template. The molecular weight reported alongside is the average mass of one single-stranded molecule, as the free acid with no counter-ions, which is what you need to move between the nanomoles on a synthesis report and the micrograms on a balance.

How to use this calculator.

1. 01Paste the sequence into the DNA sequence box, written 5' to 3'. Spaces, line breaks, hyphens, digits and 5'/3' labels are removed and lower case is accepted, so a sequence copied from an order sheet or a methods table usually goes in unchanged. From a FASTA file, copy the sequence lines but not the header.
2. 02Set the 5' end chemistry. Leave it on free 5'-hydroxyl for an ordinary synthesis; choose 5'-phosphate only if you ordered the oligo phosphorylated, as you would for ligation or 5' labelling. The choice moves the molecular weight by 79.979 g/mol and changes nothing else.
3. 03Leave the GC clamp window at 5 unless your lab's checklist uses a different span; anything from 1 to 10 bases is allowed. It decides how many 3'-end bases are scanned when counting G and C.
4. 04Read GC content and length in the design notes against both sources. Between 20 and 40 percent, or between 60 and 80, you are outside Lorenz's optimum but still inside what Primer3 returns by default.
5. 05To work with a dried oligo, multiply the synthesis yield in nanomoles by the mass per nanomole to get micrograms in the tube.
6. 06Take your annealing temperature from a nearest-neighbour Tm calculation rather than from the Wallace figure, which comes from different conditions.

The formula.

GC content is a count divided by a length: GC% = 100 × (nG + nC) ÷ L, where nG and nC are the numbers of guanines and cytosines and L is the number of bases after formatting is removed. AT content is the same calculation with nA and nT. Nothing is estimated in either figure.

The Wallace rule, Td = 2(nA + nT) + 4(nG + nC), gives each A or T two degrees and each G or C four. It comes from Wallace and colleagues' 1979 hybridisation experiments, in which short oligonucleotides were bound to a membrane in roughly 0.9 M sodium. It has no term for salt, for strand concentration or for which base sits next to which, so swapping any A or T for a G or C raises it by exactly 2 °C wherever the swap happens. A 20-mer at 40, 50 and 60 percent GC comes out at 56, 60 and 64 °C.

The clamp count looks at the last W bases, W being the window you set, and counts the G and C among them; separately, the assessment reports whether the final base itself is G or C. The run check finds the longest stretch of one repeated base and flags anything longer than four, matching Lorenz's AAAAA and CCCCC examples.

Molecular weight is built from the IUPAC standard atomic weights H 1.008, C 12.011, N 14.007, O 15.999 and P 30.974. Inside a chain each nucleotide contributes a residue, a nucleoside monophosphate minus one water: dA (C₁₀H₁₂N₅O₅P) 313.210, dC (C₉H₁₂N₃O₆P) 289.184, dG (C₁₀H₁₂N₅O₆P) 329.209 and dT (C₁₀H₁₃N₂O₇P) 304.195 g/mol. Summing the residues and adding one water (18.015) gives an oligo carrying a 5'-phosphate. A normal synthesis has a free 5'-hydroxyl instead, so HPO₃ (79.979) comes off. The net end correction, 18.015 − 79.979 = −61.964, is the −61.96 constant found in suppliers' oligo mass formulas, here derived rather than borrowed. Mass per nanomole is the molecular weight divided by 1,000, in µg/nmol.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Lorenz, T. C. (2012). 'Polymerase Chain Reaction: Basic Protocol Plus Troubleshooting and Optimization Strategies.' Journal of Visualized Experiments 63:e3998, doi:10.3791/3998. PRIMARY SOURCE, peer-reviewed, open access. Supplies verbatim every design threshold implemented on this page: 'Primer length should be 15-30 nucleotide residues (bases)'; 'Optimal G-C content should range between 40-60%'; 'The 3' end of primers should contain a G or C in order to clamp the primer and prevent breathing of ends'; and that 'single base runs (e.g., AAAAA or CCCCC) should be avoided as they can cause slipping along the primed segment'. Fetched and verified 2026-07-29.
2. [2]Untergasser, A., Cutcutache, I., Koressaar, T., Ye, J., Faircloth, B. C., Remm, M. & Rozen, S. G. (2012). 'Primer3 — new capabilities and interfaces.' Nucleic Acids Research 40(15):e115, doi:10.1093/nar/gks596, and the Primer3 manual of published default parameters. Consulted specifically to check the primary source's thresholds, and it PARTLY DISAGREES, which is why both are shown. Primer3's published defaults are PRIMER_MIN_SIZE 18, PRIMER_OPT_SIZE 20, PRIMER_MAX_SIZE 27, PRIMER_MIN_GC 20.0, PRIMER_MAX_GC 80.0 and PRIMER_GC_CLAMP 0 — a far wider GC window than Lorenz's optimum, a narrower size window, and no 3' clamp requirement at all. Manual fetched and every default verified 2026-07-29.
3. [3]Wallace, R. B., Shaffer, J., Murphy, R. F., Bonner, J., Hirose, T. & Itakura, K. (1979). 'Hybridization of synthetic oligodeoxyribonucleotides to Phi chi 174 DNA: the effect of single base pair mismatch.' Nucleic Acids Research 6(11):3543–3557. PEER-REVIEWED, the origin of the Td = 2(A+T) + 4(G+C) rule reported on this page. Cited with its conditions attached — the work used oligonucleotides hybridised to immobilised DNA in high salt, around 0.9 M sodium — because those conditions are why the Wallace value is not a solution melting temperature and should not be used to set an annealing temperature. Checked 2026-07-29.
4. [4]Commission on Isotopic Abundances and Atomic Weights (CIAAW), IUPAC — Standard Atomic Weights 2021 (table page carrying selective 2024 updates that do not affect H, C, N, O or P). STANDARDS BODY. Every nucleotide residue mass on this page is derived from the abridged standard atomic weights H 1.008, C 12.011, N 14.007, O 15.999 and P 30.974, whose CIAAW intervals were read directly from this table on 2026-07-29: H [1.00784, 1.00811], C [12.0096, 12.0116], N [14.00643, 14.00728], O [15.99903, 15.99977], P 30.973761998(5). The interval form is the reason the page reports average masses to a stated precision rather than implying more.
5. [5]SantaLucia, J. Jr. (1998). 'A unified view of polymer, dumbbell, and oligonucleotide DNA nearest-neighbor thermodynamics.' Proceedings of the National Academy of Sciences 95(4):1460–1465. PEER-REVIEWED, open access. Cited here as the contrast case rather than as an implemented model: it is the nearest-neighbour treatment that supersedes the Wallace rule for melting-temperature prediction, and it is what our annealing temperature calculator uses. For the M13 primer in this page's worked example the two approaches give 52 °C and 55.0 °C respectively — the gap is the point. Fetched 2026-07-29.