Drop an FCS file, inspect its metadata, keywords, channels, and compensation matrix.
FCS 2.0 / 3.0 / 3.1 / 3.2 from BD, Cytek, Sony, Beckman Coulter and others.
🔒 Your file never leaves your computer. The viewer runs entirely in your browser.
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An FCS file is more than a column of numbers — its TEXT segment carries dozens of metadata keywords that describe how the data was acquired: instrument model, channel detectors, voltages, fluorochrome assignments, spillover matrix, acquisition date, operator. Reading this metadata before opening a file in your analysis tool catches problems early.
Typical reasons people inspect FCS metadata directly:
$CYT), acquisition date, total events, parameter count, data type, byte order$PnN (detector), $PnS (stain), bit width, range, amplifier type$SPILL or $SPILLOVER keyword is present, with off-diagonal values color-coded for quick scanningFor full event-level analysis (gating, MFI, spectral unmixing, batch processing), the FlowVision desktop app reads the same files and adds the analysis workflow on top of this metadata.
No. The viewer runs entirely in your browser via JavaScript. Your file is read into local memory and parsed on the client — it is never sent to any server. Open your browser's Network tab while you drop a file to verify: there are no upload requests.
File summary (FCS version, cytometer, date, total events, parameter count, datatype, endianness), the full channels table ($PnN detector, $PnS stain, $PnB bits, $PnR range, $PnE amplifier), the spillover / compensation matrix if present ($SPILL or $SPILLOVER keyword), and every keyword in the TEXT segment with a search filter.
FCS 2.0, 3.0, 3.1 and 3.2 — the same coverage as the FlowVision desktop app. All standard datatypes (Float32, Float64, Integer 8/16/32/64), both little-endian and big-endian, list mode.
$SPILL (FCS 3.1+) and $SPILLOVER (FCS 3.0) are TEXT keywords that store the instrument's spillover matrix — the linear contribution each fluorochrome makes to each detector. Diagonal values are normalised to 1.0 (each fluorochrome's primary channel). Off-diagonal values are spillover; the inverse of this matrix is what compensation actually applies. The viewer parses and displays this matrix with off-diagonal values above 0.15 highlighted, so you can quickly spot panels with tight spillover.
Yes — click "Export metadata as JSON" at the bottom of the result panel. You get a single JSON file containing all keywords, channels, parsed spillover matrix, datatype info — everything except the event data itself. Useful for sharing file structure with collaborators (or with a developer debugging an instrument export issue) without sharing the actual data.
Yes — FCS is an instrument-independent standard. Files from BD (FACSDiva, Chorus, FACSDiscover S8), Cytek (SpectroFlo on Aurora and Northern Lights), Sony (ID7000, FACSChorus), Beckman Coulter (CytoFLEX, DxFLEX, Kaluza) and Stratedigm all use the same FCS format. The viewer uses the same parser as the FlowVision desktop app, tested on production files from each vendor.
The viewer only reads the file header, but the browser still has to allocate the full file in RAM first. The tool refuses files larger than 1.5 GB (browsers crash above that) and warns for files larger than 500 MB. For typical files of 50–500 MB the viewer is instant. For multi-GB Symphony spectral files, use the FlowVision desktop app which uses memory-mapped streaming.