GC Content Calculator for DNA Primers

Background.

This calculator reports the composition of a DNA oligonucleotide: what fraction of it is G or C, how long it is, how many G and C bases sit at the 3' end, what it weighs, and how it measures up against published primer-design guidance. Everything above the design verdicts is exact arithmetic on the sequence you paste — there is no model and no approximation in a base count.

GC content matters because G·C base pairs are held together by three hydrogen bonds and A·T pairs by two, and because stacking interactions between adjacent G and C bases are stronger still. A GC-rich sequence therefore takes more energy to separate: it binds more tightly, melts at a higher temperature, and tolerates mismatches more readily. That last consequence is the one primer designers care about, because a primer that binds too tightly will also bind happily to sites it was never meant to find. A GC-poor primer has the opposite problem — it has to be made longer to reach a workable melting temperature, which increases the chance it folds on itself.

Where published guidance disagrees, this page shows both sources rather than picking one. Lorenz's 2012 PCR protocol in the Journal of Visualized Experiments states that optimal GC content ranges between 40 and 60 percent, that primer length should be 15 to 30 bases, and that the 3' end should carry a G or C to clamp the primer and stop the end 'breathing'. Primer3 — the program most primer designs actually come out of — ships defaults that tell a different story: its acceptance window for GC content is 20.0 to 80.0 percent, its size window is 18 to 27 bases, and its GC-clamp requirement defaults to zero, meaning no clamp is enforced at all. Those are not contradictions so much as different jobs: Lorenz is describing an optimum, Primer3 is setting the outer bounds of what it will return. But a reader shown only the first would discard perfectly workable primers, so both appear here and the assessment names which source each verdict comes from.

The temperature this page reports needs its own warning, and it is on the page rather than in a footnote. The Wallace rule — two degrees per A or T, four degrees per G or C — comes from Wallace and colleagues' 1979 hybridisation work, carried out at roughly 0.9 molar sodium with the oligonucleotide immobilised on a membrane. It is a serviceable rule of thumb for choosing hybridisation conditions for a short probe. It is not a solution melting temperature, it embeds a salt concentration around eighteen times higher than a PCR buffer, and it takes no account of which base sits next to which. For the number you actually programme into a thermocycler, use a nearest-neighbour calculation — our annealing temperature calculator does exactly that, and for the M13 primer used in the worked example below the two approaches land three degrees apart, which is a useful thing to see for yourself.

The molecular weight is derived rather than borrowed. Every nucleotide residue mass on this page is computed from the IUPAC standard atomic weights of hydrogen, carbon, nitrogen, oxygen and phosphorus, summed over the residue's molecular formula — so the familiar constant of −61.96 that appears in every supplier's oligo mass formula is reproduced here from first principles rather than copied. One convention has to be stated because suppliers differ: this page reports the free acid. If your synthesis report quotes a sodium salt, its figure will be heavier by roughly 22 grams per mole per phosphate, which for a 17-mer is about 350 g/mol, or seven percent. Knowing which convention each side used is the difference between a reconciliation and a mystery.

Finally, the scope. This is an oligonucleotide tool. It accepts DNA only, refuses degenerate and modified bases outright rather than guessing an average composition for them, and caps input at five thousand bases. Above about fifty bases the Wallace value and the clamp count stop meaning anything, and the page says so rather than quietly continuing to report them as though they did.

What is gc content calculator (primer)?

GC content is the proportion of bases in a nucleic acid sequence that are guanine or cytosine, conventionally expressed as a percentage of the total. For the sequence GTAAAACGACGGCCAGT, nine of the seventeen bases are G or C, so the GC content is 9 ÷ 17 = 52.9 percent. The remaining bases are adenine and thymine, giving an AT content of 47.1 percent; the two always sum to exactly 100 percent for an unambiguous DNA sequence. GC content is a property of composition alone — it does not depend on the order the bases appear in, so GGGCCCAAATTT and GCATGCATGCAT have identical GC content despite behaving very differently. Its importance comes from base-pair chemistry. A G·C pair forms three hydrogen bonds where an A·T pair forms two, and the stacking energy between neighbouring bases is generally larger for G and C. Both effects push in the same direction: raising GC content raises duplex stability, which raises melting temperature. That relationship is the basis of every simple melting-temperature rule, including the Wallace 2(A+T) + 4(G+C) formula this page reports, and it is why GC content ends up in every primer-design checklist. A GC clamp is a related but distinct idea: the presence of a G or C at the extreme 3' end of a primer, the end from which the polymerase extends. The reasoning is that the terminal base pair of a duplex is the one most likely to transiently open — to 'breathe' — and a G·C pair there breathes less than an A·T pair, so the 3' end stays engaged with the template while the polymerase gets to work. Molecular weight, the last thing this page reports, is unrelated to any of that and is simply the summed mass of the residues plus the terminal groups. It is what you need to convert between the nanomoles a synthesis report quotes and the micrograms a balance measures, or to work out how much buffer to add to a dried pellet to reach a target concentration.

How to use this calculator.

1. 01Paste the sequence 5' to 3'. Whitespace, hyphens, digits and 5'/3' end markers are stripped automatically, and lowercase is fine — a FASTA line pasted without its header will work as-is.
2. 02Set the 5' end chemistry. Leave it on free hydroxyl unless you specifically ordered a 5'-phosphorylated oligo, which is normal for primers destined for ligation or for 5' end-labelling. This changes only the molecular weight.
3. 03Leave the GC clamp window at 5 unless your lab uses a different convention. It controls how many 3'-terminal bases are scanned for G and C.
4. 04Read GC content against both published bands. Inside 40–60 percent you are in Lorenz's optimum. Between 20 and 40, or between 60 and 80, you are outside the optimum but still inside what Primer3 will return by default — a preference, not a failure. Outside 20–80 percent both sources agree there is a problem.
5. 05Check the length against both windows too. The two published sources genuinely disagree at the edges, and the assessment text names which one you have fallen outside.
6. 06Look at the 3' terminal base. Lorenz recommends it be a G or C. Primer3 does not require one. If your primer ends in A or T and everything else looks good, that is worth knowing but is rarely worth a redesign on its own.
7. 07Use the mass per nanomole to resuspend a dried oligo. A tube containing 25 nmol of a primer weighing 5.228467 µg/nmol holds about 131 µg; adding 250 µL of buffer gives a 100 µM stock.
8. 08Do not use the Wallace Td as an annealing temperature. It is reported because people search for it, and it is labelled with the conditions it came from. For a PCR annealing temperature use a nearest-neighbour calculation.

The formula.

Five calculations run on this page, and only the first two involve no assumptions at all.

GC% = 100 × (nG + nC) / L AT% = 100 × (nA + nT) / L Td = 2 × (nA + nT) + 4 × (nG + nC) clamp = count of G or C in the last W bases MW = Σ(residue masses) + H₂O − HPO₃ [5'-hydroxyl] MW = Σ(residue masses) + H₂O [5'-phosphate]

GC and AT percentages are exact division of integer counts. Note that AT content is computed from its own counts rather than as 100 minus GC content — that costs nothing and means the two figures independently verify each other, which a shipped test relies on.

The Wallace rule assigns two degrees to every A or T and four to every G or C, and adds them up. It has no concentration term, no salt term and no sequence-context term, which is exactly why it is fast to do in your head and why it disagrees with real melting temperatures. It is reported here labelled with the conditions Wallace and colleagues actually used in 1979: an oligonucleotide bound to a membrane in roughly 0.9 molar sodium.

The molecular weight is built from IUPAC standard atomic weights — hydrogen 1.008, carbon 12.011, nitrogen 14.007, oxygen 15.999, phosphorus 30.974 — summed over each residue's molecular formula. A residue is a nucleoside 5'-monophosphate minus a water, because that is the repeating unit inside a chain: dA is C₁₀H₁₂N₅O₅P at 313.210, dC is C₉H₁₂N₃O₆P at 289.184, dG is C₁₀H₁₂N₅O₆P at 329.209, and dT is C₁₀H₁₃N₂O₇P at 304.195. Adding one water (18.015) closes the chain at both ends and gives the mass of an oligo with a 5'-phosphate. Standard synthesis leaves a free 5'-hydroxyl instead, so a metaphosphate group (HPO₃, 79.979) is removed. The net terminal correction for a 5'-OH oligo is therefore 18.015 − 79.979 = −61.964 — which is the −61.96 constant every supplier's mass formula carries, derived here rather than assumed.

Rounding stage: FINAL ONLY. Counts are integers throughout. The percentages, the Wallace value and the masses are carried as full-precision Decimals and rounded once, at the moment each field is returned. Nothing is rounded and then reused, so the displayed GC content and the displayed AT content still sum to exactly 100.

Invalid-domain behaviour. An empty sequence is rejected. Any character that is not an unambiguous A, C, G or T is rejected with the offending characters named, and a U gets a message specifically pointing out that this is a DNA tool and that a deoxyribonucleotide mass is not an RNA mass. Sequences beyond five thousand bases are rejected because this is an oligonucleotide tool. A clamp window that is not a positive whole number is rejected; a window longer than the sequence is narrowed to the whole sequence and the narrowing is reported, rather than raising an error on a field the user never touched.

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. INDEPENDENT SECOND AUTHORITY — 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.