source: https://carlosbecker.com/posts/jekyll-reading-time-without-plugins/
We can estimate the reading time with the measure called Words per Minute (WPM). According to Wikipedia, an average person can read 180 words per minute in a computer monitor. So, we can include this in our blog post layout with one of these hibrid snippets between HTML (to style with CSS later) and Liquid:
<span class="reading-time" title="Estimated read time">
{% assign words = content | number_of_words %}
{% if words < 360 %}
1 min
{% else %}
{{ words | divided_by:180 }} mins
{% endif %}
</span>
or, if you want to be more accurate:
{% assign words = content | number_of_words %}
{% if words < 270 %}
1 minute
{% else %}
{{ words | divided_by:135 }} minutes
{% endif %}
You can call it with
{% include read_time.html %}