Free MAT File Viewer: Read and Plot .mat Files Online, Without MATLAB

Where your .mat files come from

The viewer finds the signals in the file for you, in any of these layouts:

MATLAB save

Workspaces saved with save or from the Workspace browser: vectors, matrices, structs (also nested) and struct arrays of signals. A time vector such as t, time or tout becomes the x axis of the signals of the same length.

Simulink To Workspace formats and logged outputs

Structure with time, Structure and Array formats, plus tout and yout. Each block’s signals become one table with their labels as column names, multi-element signals included.

Simulink simulation output and Simulation Data Inspector exports

The output of sim saved as it is, for example save('run.mat', 'out'), in v7 or v7.3: the logged signals (logsout), the outputs (yout, named after their Outport blocks), the states (xout), tout and every To Workspace variable open as the same tables you get when you save them on their own, under their own names. A bus becomes one channel per element. Exports from the Simulation Data Inspector open with every run’s signals, each run in tables of its own.

Simulink To Workspace Timeseries format and structs of timeseries

MATLAB timeseries objects on their own, in structs (nested ones too), in a tscollection or in a logged Simulink Dataset, from v7 and v7.3 files alike. Every timeseries becomes a channel with its name and unit, multi-column ones as name(1), name(2). Signals that share a time vector are offered as one table, and signals logged at different rates are plotted together, each on its own time axis.

MATLAB tables and timetables

Numbers, text, categories, dates and durations become columns with their units. A timetable’s row times are the x axis, also when it was made with a sample rate or a time step.

MATLAB datetime, duration, string and categorical variables

On their own, in structs, in tables and timetables: dates and times show in the time zone saved with them, as in MATLAB, and become a time axis you can plot against; durations show as MATLAB writes them and plot in seconds; strings and categories show as text in the table. Missing dates, strings and categories stay empty.

Cell arrays: runs, num2cell and readcell

Vectors kept in a cell array, such as runs = {run1, run2, run3}, become signals named runs{1}, runs{2} and so on: those of one length share a table, other lengths get tables of their own, and a matrix in a cell is split into its columns. Structs, tables and timetables in cells open as they do anywhere else. A cell of single numbers, as num2cell gives it, is one column, with an empty cell as a missing value; a cell from readcell with a header row is a table named by that row, and a column called Time is its x axis. A file that holds only single values opens as a table of their names and values.

dSPACE-style measurement files

Files laid out like dSPACE measurement exports, with X and Y channels. Every raster (for example a 1 ms and a 10 ms task) becomes its own table with its own time axis, and channel names and units are kept.

Data-logger exports with a time matrix and a value matrix

Files that keep a recording as P_Time next to P_Value (or P_Data), one column per channel, as many data-acquisition and Java programs write them: the channels are plotted against their time in one table, which opens first. When every channel has its own time column, each keeps its own time axis.

CANape- and INCA-style exports

Files where every channel is a vector, with one or more time vectors such as t_eng or time_brake. Signals are paired with the matching time vector by name, and matrices with a list of channel names get those names as column headers.

Python

Files written by scipy.io.savemat (v4, v6, v7) and hdf5storage (v7.3) open the same way as files from MATLAB.

What isn’t supported yet

• containers.Map, dictionary and your own classes. These objects (and calendarDuration values) can only be read by MATLAB itself today. The viewer counts them in the notes, names them when you ask, and opens everything else in the file.
• Arrays of tables, and Dataset elements that are not timeseries. A table or timetable array, a Simulink Dataset element that holds something other than timeseries, or a signal that logs a matrix at every step in a v7.3 file or a Simulation Data Inspector export, is listed in the notes instead of being plotted.
• Function handles. Skipped, with a note.
• Very large sparse matrices. Sparse matrices with more than a million elements are listed in the notes instead of being expanded.
• Editing and saving. The viewer reads .mat files; it never changes them.

Tip: to open a containers.Map or an object of your own class here, convert it before saving: k = keys(m); v = values(m) gives a map’s contents, s = struct(obj) an object’s properties.

Built for big files

Long test runs and high-rate loggers easily produce .mat files of hundreds of megabytes. The viewer opens them in seconds, right in your browser. In our tests in Chrome on a recent laptop, a file with a time vector and four channels of 10 million samples each opened in:

• about 1 second saved with -v6 (400 MB);
• about 2 seconds saved with -v7 (330 MB, compressed);
• under 3 seconds saved with -v7.3 (330 MB).

A one-sample spike in 10 million samples is still visible in the overview, and zooming in reloads the exact values of the visible range instantly. Slower machines take longer, and very large files need enough free memory.

Private by design: nothing is uploaded

Measurement data is often confidential: unreleased prototypes, customer projects, supplier test results. MegaRows reads your .mat file in your browser, on your own device: the file and its name are never sent anywhere, and you can check that in the Network tab of your browser’s developer tools while it opens. Read the privacy statement for details.

Need to plot CSV, Excel or text logs as well? The same viewer opens them: see the CSV plotter.

Other ways to open a .mat file

Python, GNU Octave, MATLAB Online and a free VS Code extension from MathWorks can open .mat files too, each with its own limits. Our guide How to open a .mat file without MATLAB compares them with this viewer, with tested Python code, a quick way to tell which version a file is, and how to convert a .mat file to CSV.