Does my file get uploaded anywhere?

No. The file is read with the browser FileReader/streams API and processed on your machine. It never leaves your computer and no network request is made with its contents.

Can it handle large or gzipped files?

Yes. The file is streamed and parsed in chunks rather than loaded whole, and .gz files are decompressed in the browser, so large files do not freeze the tab.

Which quality encoding is assumed?

Phred+33 (Sanger / Illumina 1.8+), where Q = ASCII code − 33. This is standard for modern Illumina data.

FASTQ quality check

Drop in a .fastq or .fastq.gz file to see read count, read-length distribution, GC content and per-base quality — computed entirely in your browser.

Your file never leaves your computer. It is read and analysed entirely in your browser — nothing is uploaded and no network request is made with its contents.

Drop a FASTQ file here or click to choose — .fastq, .fq or .fastq.gz

How it works

Formula

A FASTQ record is four lines: @header, sequence, +, and a Phred+33 quality string (Q = ASCII code − 33). The tool aggregates read count, length min/max/mean, length-weighted GC%, and per-base mean quality.

Worked example

For a read ACGT with quality IIII: each "I" is ASCII 73, so Q = 73 − 33 = 40. The read is 4 nt long and 50% GC, contributing four Q40 bases to the per-base quality profile.

When to use it

A fast first look at a sequencing file before you commit to a full pipeline: confirm the read count and length, spot a quality drop-off along the read, or flag unexpected GC content.

Sensible defaults

There are no defaults — drop in your own .fastq or .fastq.gz file. Nothing is uploaded; parsing happens locally in your browser.

FAQ

Does my file get uploaded anywhere?: No. The file is read with the browser FileReader/streams API and processed on your machine. It never leaves your computer and no network request is made with its contents.
Can it handle large or gzipped files?: Yes. The file is streamed and parsed in chunks rather than loaded whole, and .gz files are decompressed in the browser, so large files do not freeze the tab.
Which quality encoding is assumed?: Phred+33 (Sanger / Illumina 1.8+), where Q = ASCII code − 33. This is standard for modern Illumina data.

FASTQ quality check