DNA Complement Calculator
DNA complement, length, and GC percent.
What it does
Base pairing mirrors every strand. Adenine meets thymine while guanine meets cytosine, so ATGCCGTA reflects as TACGGCAT. Length and GC share ride along as free bookkeeping for primers and records.
The tool above pairs strict DNA letters. Defaults give an 8 letter complement at 50 % GC. A 10 letter GGCCAAATTT run gives CCGGTTTAAA at 40 % GC. Lowercase folds up and spaces strip out, while any other letter stops with a clear message.
GC percent hints at stability because triple bonded pairs hold tighter than double bonded ones. It never sets melting temperature alone; nearest neighbor math with salts does that job. Length caps at 5000 letters keep pastes sane.
This page gives the forward complement only. It does not reverse for reverse complement, translate proteins, or allow ambiguity codes. For cloning orientation, reverse the output string after.
How to use
- Type or paste the DNA letters.
- Use only A, C, G, T.
- Read the complement first.
- Read length for records.
- Read GC percent for context.
- Reverse by hand for reverse complement.
Formula
- seq
- Input strand letters ACGT.
- comp
- Paired output strand.
- GC
- Share of G plus C.
DNA alphabet A, C, G, T up to 5000 letters. At any other letter there is no pairing to show. Ambiguity codes like N sit outside this strict page.
Worked example
Example: complement of ATGCCGTA
Defaults: ATGCCGTA. Only A, C, G, T accepted up to 5000 letters.
- Inputs: ATGCCGTA (8 letters).
- Pair: A-T, T-A, G-C, C-G gives TACGGCAT.
- Length: 8 letters.
- GC: G,C,G,C is 4 of 8, or 50 % GC.
- Lowercase accepted and spaces ignored. Tool shows the same.
Reference table
| Input | Complement | GC |
|---|---|---|
| ATGCCGTA | TACGGCAT | 50 % GC |
| GGCCAAATTT | CCGGTTTAAA | 40 % GC |
| AAAA | TTTT | 0 % GC |
| GGGG | CCCC | 100 % GC |
| ACGT | TGCA | 50 % GC |
Limits
- ACGT only. N and U excluded.
- Forward complement. No auto reverse.
- 5000 letters max.
- No melting temp model.
Common mistakes
- Including U from RNA.
- Pasting FASTA headers with greater than signs.
- Reading complement as reverse complement.
- Counting spaces in length.
- Using GC alone for melting temp.
Try it yourself
Quick checks. Answers sit tucked under each question.
GGCCAAATTT?
Pairs give CCGGTTTAAA at 10 letters with 40 % GC.
acgtACGT in mixed case?
Pairs give TGCATGCA at 8 letters with 50 % GC.
Questions
How is complement figured?
Swap A-T and G-C down the strand. Here TACGGCAT.
How is GC percent figured?
Count of G plus C over length. Here 50 % GC.
Does U work here?
No. Uracil belongs to RNA. This page takes DNA letters only.
Are lowercase and spaces fine?
Yes. Case folds and spaces strip before pairing.
Is this reverse complement?
No. This is the complementary strand, same direction naive. Reverse complement needs reversing after.
Does GC give melting temp?
Primer melting needs nearest neighbor math, not just GC share.