free · in-browser · no upload
Drop your FASTQ file. We'll convert it to JSON right here in your browser, your file never leaves your device.
Select your file here to get started
or drop your file here.
Accepts .fastq, .fq, .txt
How it works
Click the dropzone above or drag a FASTQ from your desktop. Files of any size, there's no upload, so there's no upload limit.
The conversion runs entirely in this tab using JavaScript and WebAssembly. Your file never touches our servers, we don't have any.
Get your JSON the moment the conversion finishes. Convert another, or close the tab.
Your file is never uploaded. The entire conversion runs in your browser using WebAssembly. We can't see what you convert because we have no server to see it.
Server converters cap free users at 1-2 GB and gate larger files behind a paid plan. Since nothing uploads, our limit is whatever your browser can handle.
No account required. No watermark on the output. No queue. Drop a file, get a converted file.
Why convert FASTQ → JSON
FASTQ is the standard short-read sequencing output (Illumina, MGI, PacBio HiFi). Every record carries a Phred quality string we usually want as a column in our downstream dataframe. This parses each 4-line record into { id, description, sequence, quality } so you can compute base-call quality stats, filter on average quality, or chunk for batching.
You have a 500 MB FASTQ from an Illumina run and want to compute the per-read mean Phred quality before deciding what to trim. Convert here, then pandas + a string-to-quality decoder gets you a histogram in two lines.
"FASTQ has N non-empty lines but records require exactly 4 lines each."
The file is truncated or contains extra blank lines inside records. FASTQ does NOT allow blank lines inside records. Validate the file with seqkit or fastqc first.
Formats involved
FASTQ is a file format we support converting. Detailed format information is being added, for now, drop your file in the converter above and you'll get the conversion you came for.
How to open
Most operating systems open this format with a default application; if not, search for a free reader/viewer for the format.
JSON is a lightweight text format for structured data, nested objects, arrays, strings, numbers, booleans. It's the lingua franca of web APIs, configuration files, and data interchange between programs. Human-readable when formatted, machine-parseable in every programming language, and roughly half the size of equivalent XML.
How to open
Any text editor reads JSON. Browsers display .json files in a formatted tree view. VS Code and similar editors highlight syntax.
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FAQ
Yes. No signup, no watermark, no daily file count limit. Every conversion runs in your browser, your file never touches our servers because there are no servers.
Nowhere. The conversion runs entirely in your browser using JavaScript and WebAssembly. Your file is never uploaded to our servers. We don't have any servers handling files, there's nothing for us to log, store, or accidentally leak.
Whatever your browser can hold in memory. Practically, this means a few hundred MB on most computers, significantly larger than the 1-2 GB caps that server-upload converters charge for. Very large files (multi-GB) may require closing other browser tabs first.
File interchange. API responses, configuration files, structured data interchange. The most common reason to convert is compatibility, JSON works in places where FASTQ doesn't, or vice versa.
Most operating systems open this format with a default application; if not, search for a free reader/viewer for the format.
Once the page is loaded, the conversion itself runs entirely offline. The first time you use a tool, your browser downloads the conversion library (a one-time cache). If you reload while offline, the page won't load, but you can install the site as a Progressive Web App for full offline use.
Single file at a time for now. Batch conversion is on the roadmap, for now, drop one file, download the result, then convert the next.
