How do I increase the cell width of the Jupyter/ipython notebook in my browser?

If you don't want to change your default settings, and you only want to change the width of the current notebook you're working on, you can enter the following into a cell:

from IPython.display import display, HTML
display(HTML("<style>.container { width:100% !important; }</style>"))

To get this to work with jupyter (version 4.0.6) I created ~/.jupyter/custom/custom.css containing:

/* Make the notebook cells take almost all available width */
.container {
    width: 99% !important;
}   

/* Prevent the edit cell highlight box from getting clipped;
 * important so that it also works when cell is in edit mode*/
div.cell.selected {
    border-left-width: 1px !important;
}

It's time to use jupyterlab

Finally, a much-needed upgrade has come to notebooks. Jupyterlab uses the full width of your window like any other full-fledged native IDE by default.

All you have to do is:

pip install jupyterlab
# if you use conda
conda install -c conda-forge jupyterlab
# to run 
jupyter lab    # instead of jupyter notebook

Here is a screenshot from blog.Jupyter.org

That div.cell solution didn't actually work on my IPython, however luckily someone suggested a working solution for new IPythons:

Create a file ~/.ipython/profile_default/static/custom/custom.css (iPython) or ~/.jupyter/custom/custom.css (Jupyter) with content

.container { width:100% !important; }

Then restart iPython/Jupyter notebooks. Note that this will affect all notebooks.