FASTA to FASTQ Converter

How to convert FASTA to FASTQ?

The easiest way to convert FASTA to FASTQ format online is to use this ProteinIQ FASTA to FASTQ converter. To convert FASTA to FASTQ, paste FASTA records or upload a .fasta, .fa, .fas, .seq, or .txt file, choose a quality score profile, and run the converter. ProteinIQ preserves FASTA headers, generates synthetic Phred+33 quality strings, skips records with invalid sequence characters, and returns a .fastq file.

For one FASTA record:

>read_1 sample
ACGTNACGTN

The FASTQ output contains the same identifier and sequence, plus a + separator and one quality character per sequence letter:

@read_1 sample
ACGTNACGTN
+
IIIIIIIIII

For multiple FASTA records, each sequence becomes a separate FASTQ record:

@read_1
ACGTACGT
+
IIIIIIII
@read_2
TTGGAACC
+
IIIIIIII

FASTA does not store real base-call confidence, so every quality string generated by this converter is synthetic. Use the output for reference sequences, designed sequences, examples, and pipeline tests. If the original data came from sequencing and real quality scores matter, use the original FASTQ file instead.

Results

The converter produces FASTQ-formatted output that can be copied to clipboard or downloaded as a .fastq file. A summary file is also returned so skipped records and generated score settings are auditable.

How FASTA to FASTQ conversion works

FASTA records contain a header line that starts with > followed by one or more sequence lines. FASTQ records contain four logical sections:

• Header: Starts with @ and contains the FASTA header without the leading >.
• Sequence: Contains the converted sequence in uppercase.
• Separator: A + line.
• Quality: Contains one ASCII quality character per sequence letter.

ProteinIQ reads each FASTA record, preserves the full header description, validates the sequence, generates a quality string with the selected profile, then writes one FASTQ record for each valid input record. Records with invalid characters are skipped instead of being silently modified.

The converter uses Phred+33 encoding. Each numeric Phred score is converted to a single ASCII character by adding 33 to the score.

FASTA to FASTQ quality score options

Quality score selection controls how downstream FASTQ-aware tools interpret confidence. The sequence text does not change, but quality-aware filters, trimmers, aligners, and variant workflows may behave differently.

FASTQ output rules

FASTQ output is strict because sequence and quality lengths must match exactly.

• Header line: Each converted record starts with @, followed by the FASTA header without the leading >.
• Sequence line: The sequence is written in uppercase.
• Separator line: The third line is +.
• Quality line: The quality string has exactly one Phred+33 character per sequence letter.
• Synthetic scores: Generated quality scores do not recover real sequencing confidence.
• Skipped records: Invalid records are omitted from the FASTQ file and counted in the conversion summary.

Which FASTQ converter should I use?

Use FASTA to FASTQ when the sequence is already in FASTA format and a downstream tool requires FASTQ input. Use a different converter when the source file already has quality scores or needs sequence cleanup first.

FAQ

How do I convert a FASTA file to FASTQ format?

Upload the .fasta, .fa, or .fas file, choose a score generation method, and run the converter. The result can be copied or downloaded as a .fastq file.

Does FASTA to FASTQ recover real quality scores?

No. FASTA does not contain per-base quality scores. This converter generates synthetic quality strings so FASTA sequences can be used with tools that require FASTQ input.

Which quality score method should I choose?

Use High quality (Q40) for reference sequences, designed sequences, and compatibility testing. Use Declining quality (Q40 to Q10) or Random scores (Q0-Q40) when testing quality trimming, filtering, or quality-aware algorithms.

Can I convert multiple FASTA records at once?

Yes. Paste or upload a multi-record FASTA file. Each valid FASTA record becomes one FASTQ record, and skipped records are listed in the conversion summary.

Should I use FASTA to FASTQ or TXT to FASTA?

Use FASTA to FASTQ when the input already has FASTA headers that start with >. Use TXT to FASTA for raw sequence text copied from notes, reports, or plain text files.