Reverse Complement Calculator
Paste a DNA or RNA sequence to instantly get its reverse complement. IUPAC ambiguity codes, FASTA format, and letter case are all preserved.
What is a reverse complement?
DNA is a double helix made of two antiparallel strands. If you know the sequence of one strand, the reverse complement gives you the other strand, read 5' to 3'. It's computed in two steps.
- Reverse: read the sequence backwards, from the last letter to the first.
- Complement: swap each base for its pairing base (AāT, CāG; RNA's U pairs with A).
For example, the reverse complement of 5'-ATGC-3' is 5'-GCAT-3'. This tool performs both steps at once, or you can view the Reverse-only or Complement-only result separately.
When would you use this?
- Designing a reverse primer for PCR and checking its sequence
- Cloning work or restriction site checks that require the opposite strand
- Lining up two sequences by orientation before an alignment
- Coursework or lab exercises that ask you to verify a reverse complement
IUPAC ambiguity code support
Ambiguity codes are converted according to the standard IUPAC rules below. Letter case is preserved exactly as entered.
| Code | Meaning | Complement |
|---|---|---|
| A | Adenine | T |
| T / U | Thymine / Uracil | A |
| C | Cytosine | G |
| G | Guanine | C |
| R | A or G | Y |
| Y | C or T | R |
| S | C or G | S (same) |
| W | A or T | W (same) |
| K | G or T | M |
| M | A or C | K |
| B | C, G, or T | V |
| V | A, C, or G | B |
| D | A, G, or T | H |
| H | A, C, or T | D |
| N | Any base | N (same) |
FASTA format support
Paste an input containing FASTA headers (lines starting with >) and the header is kept unchanged while only the sequence body is reverse-complemented and re-wrapped at 60 characters per line. Multi-sequence (multi-FASTA) input is handled record by record.
How to use it
- Paste your DNA/RNA sequence (or FASTA block) into the input box above.
- Choose Reverse only, Complement only, or Reverse Complement from the tabs if needed.
- Copy the result directly, or download it as a FASTA file.
Frequently asked questions
What's the difference between reverse and complement?
Reverse only flips the order of the sequence. Complement only swaps each base for its pairing base, keeping the order unchanged. Reverse complement applies both.
Can I enter RNA sequences (with U)?
Yes. U is treated the same as T and pairs with A. The output is returned in DNA notation (A/T/G/C).
Can my input include spaces, line breaks, or numbers?
Any character that isn't a recognized nucleotide letter (spaces, digits, symbols) is left unchanged. For plain text without a FASTA header, the whole string is reversed as-is.
Is my sequence stored or sent anywhere?
No. This tool makes no server requests ā everything is computed in your browser with JavaScript, and no sequence data is ever transmitted to or stored on BioOnSun's servers.
Can I change the 60-character line wrap width?
The current version wraps at 60 characters per line, following common FASTA convention. If you need a different width, let us know via the contact page.