Do I need MATLAB to open a .mat file?

No. Files saved by MATLAB's save command - with the default version 7, older version 6, or -v7.3 - open here directly, as do .mat files written by Python's scipy and Octave. MATLAB 4 files, from before 1997, are not read, and nor are objects MATLAB keeps in a hidden part of the file (string arrays, tables, datetimes), which are listed but not shown.

Can it open very large HDF5 files?

Yes. An HDF5 file (including NetCDF-4 and MATLAB 7.3) is read in place: only the variables and slices you look at are loaded, so a file of many gigabytes opens as quickly as a small one. MATLAB files up to version 7 and classic NetCDF files are read whole, up to 1.5 GB. Views of more than a million values are thinned evenly for display.

Which compression does it read?

All the usual ones: HDF5's own deflate (gzip), shuffle and SZIP, MATLAB's compression, and the add-on HDF5 filters used by h5py, hdf5plugin, PyTables and many instruments - Blosc, Blosc2, bitshuffle, LZ4, LZF, Zstandard, bzip2, ZFP, JPEG, BitGroom and BitRound. A decoder is downloaded only when a file needs it.

Are my data files uploaded?

No. The file is opened and read in your browser, and nothing is sent to a server.

Stays on your computer Needs a desktop or laptop browserHow this works

MAT, HDF5 & NetCDF viewer - free, in your browser

Open MATLAB .mat files, HDF5 (.h5, NeXus) and NetCDF (.nc) files without MATLAB or Python: browse every variable, see any slice as a table, chart or heat map, and save it as CSV. Nothing is uploaded

Drag & drop a .mat, .h5 or .nc file here, or browse

MATLAB level 5 and 7.3 · HDF5, NeXus, HDF-EOS5 · NetCDF classic and NetCDF-4 · large HDF5 files read in parts

About this MAT, HDF5 and NetCDF viewer

Scientific data often arrives in a format only MATLAB or Python can open: a .mat file from a colleague, an HDF5 file from an instrument or simulation, a NetCDF file of climate or ocean data. This viewer opens all three in the browser, so a file can be checked or its numbers taken out without installing anything. It is free to use, with no account and no upload limit, and it runs entirely in your browser: nothing is uploaded, and while the page stays open, a feature you have already used keeps working offline.

MATLAB files saved up to version 7 are read by this site's own reader, written from MathWorks' published format; version 7.3 files are HDF5 inside. HDF5 and NetCDF-4 files are read with h5wasm, the HDF5 library compiled for the browser by the US National Institute of Standards and Technology (NIST), and only the parts needed are read, so even very large files open quickly. Classic NetCDF files are read with netcdfjs.

What you can do

Browse every variable, group, struct and cell in a .mat, .h5 or .nc file, with its shape, type and attributes
See any array as a table, with a slider for each extra dimension of a 3-D or larger array
Draw one-dimensional data as a line chart and two-dimensional data as a heat map
Save the slice you are looking at as a CSV file for Excel or another program
Read NetCDF scale factors, offsets and fill values, so stored integers show as real values
Read datasets compressed with Blosc, Zstandard, LZ4, bitshuffle, LZF, bzip2 and ZFP as well as gzip
Open MATLAB structs, struct arrays, cell arrays, sparse and complex matrices, and text

How to open a MAT, HDF5 or NetCDF file

Drop the file onto the page. The variables appear on the left.
Choose a variable to see its values as a table, or switch to the chart.
For an array with three or more dimensions, pick the rows and columns and move the sliders through the rest; save what you see as CSV.

Questions

Do I need MATLAB to open a .mat file?: No. Files saved by MATLAB's save command - with the default version 7, older version 6, or -v7.3 - open here directly, as do .mat files written by Python's scipy and Octave. MATLAB 4 files, from before 1997, are not read, and nor are objects MATLAB keeps in a hidden part of the file (string arrays, tables, datetimes), which are listed but not shown.
Can it open very large HDF5 files?: Yes. An HDF5 file (including NetCDF-4 and MATLAB 7.3) is read in place: only the variables and slices you look at are loaded, so a file of many gigabytes opens as quickly as a small one. MATLAB files up to version 7 and classic NetCDF files are read whole, up to 1.5 GB. Views of more than a million values are thinned evenly for display.
Which compression does it read?: All the usual ones: HDF5's own deflate (gzip), shuffle and SZIP, MATLAB's compression, and the add-on HDF5 filters used by h5py, hdf5plugin, PyTables and many instruments - Blosc, Blosc2, bitshuffle, LZ4, LZF, Zstandard, bzip2, ZFP, JPEG, BitGroom and BitRound. A decoder is downloaded only when a file needs it.
Are my data files uploaded?: No. The file is opened and read in your browser, and nothing is sent to a server.

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