Python: How to increase/reduce the fontsize of x and y tick labels?

One shouldn't use set_yticklabels to change the fontsize, since this will also set the labels (i.e. it will replace any automatic formatter by a FixedFormatter), which is usually undesired. The easiest is to set the respective tick_params:

ax.tick_params(axis="x", labelsize=8)ax.tick_params(axis="y", labelsize=20)

or

ax.tick_params(labelsize=8)

in case both axes shall have the same size.

Of course using the rcParams as in @tmdavison's answer is possible as well.

You can set the fontsize directly in the call to set_xticklabels and set_yticklabels (as noted in previous answers). This will only affect one Axes at a time.

ax.set_xticklabels(x_ticks, rotation=0, fontsize=8)ax.set_yticklabels(y_ticks, rotation=0, fontsize=8)

Note this method should only be used if you are fixing the positions of the ticks first (e.g. using ax.set_xticks). If you are not changing the tick positions from the default ones, you can just change the font size of the tick labels without changing the text using ax.tick_params

ax.tick_params(axis='x', labelsize=8)ax.tick_params(axis='y', labelsize=8)

or

ax.tick_params(axis='both', labelsize=8)

You can also set the ticklabel font size globally (i.e. for all figures/subplots in a script) using rcParams:

import matplotlib.pyplot as pltplt.rc('xtick',labelsize=8)plt.rc('ytick',labelsize=8)

Or, equivalently:

plt.rcParams['xtick.labelsize']=8plt.rcParams['ytick.labelsize']=8

Finally, if this is a setting that you would like to be set for all your matplotlib plots, you could also set these two rcParams in your matplotlibrc file:

xtick.labelsize      : 8 # fontsize of the x tick labelsytick.labelsize      : 8 # fontsize of the y tick labels

Use the keyword size instead of fontsize.