Joeri van Veen

The reading progress bar is a great user experience on longreads. Especially if it accurately depicts the reading progress in the article text, and nothing else. This is standard on single blog posts and enabled by default.
Customization:
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Location top of screen, bottom of screen or below sticky elements.
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Select post types you wish the bar to appear, or individual posts.
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Choose color of the reading progress bar.
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Have the bar start at 0% even when part of the article is visible.
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Behaviour:
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The reading progress bar has smooth initializing since part of the text may already be visible, after that a lightweight update-function ensures quick response while scrolling.
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The bar can attach itself to multiple (sticky) elements that you define as an admin, the first visible element will be used.
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When there is no (longer a) visible element to attach to, the bar displays at the top.
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Use <code>dir=rtl</code> on your html tag to have the bar display correctly for right-to-left languages.
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</ul>
Estimated reading time (beta)
Since 1.6.0 this plugin has rudimentary estimated reading time functionality, for when your theme does not support it out of the box. 
There are some potential issues, some of which cannot be fixed in a plugin. If it does not work for you, switch it off. It will have no effect on the plugin then.
This is my 6th WordPress plugin but my first one freely available to everybody. I hope you enjoy using it as much as I enjoy building it!
Regards, 
Joeri

WP Reading Progress

Light weight fully customizable reading progress bar. Sticks to top, bottom or sticky menu, with fallback for small screens. Includes ert (beta).

Md Asadujjaman Sabuj

MinuteRead adds an estimated reading time label to your WordPress posts — helping readers decide whether to dive in or save it for later.
After activating the plugin, reading time appears automatically before every post. Everything is configurable from Settings > MinuteRead.
Features:
<ul>
<li>Automatically displays reading time on all single posts</li>
<li>Choose to show reading time before or after the content</li>
<li>Configurable words-per-minute reading speed (default: 200, range: 50–1000)</li>
<li>Custom label text — change “Estimated Reading Time” to anything you like</li>
<li>Custom time format — use “%d min”, “%d minutes”, or any text with %d as placeholder</li>
<li>Enable / disable without deactivating the plugin</li>
<li><code>[minuteread_time]</code> shortcode — place reading time anywhere</li>
<li>Works with non-Latin scripts (Bengali, Arabic, CJK, etc.)</li>
<li>Lightweight: no jQuery, no external requests</li>
<li>Developer-friendly: <code>minuteread_reading_time_output</code>, <code>minuteread_word_count</code>, <code>minuteread_output_html</code> filters</li>
<li>Translation-ready</li>
</ul>
Shortcode usage:
Place <code>[minuteread_time]</code> in any post, page, or widget to display the reading time for the current post.
<h3>Why Use This Plugin?</h3>
<ul>
<li>Improve user engagement</li>
<li>Reduce bounce rate</li>
<li>Help readers estimate time commitment</li>
</ul>

MinuteRead — Reading Time for Posts

Show estimated reading time on every post. Customize label, format, position, and reading speed from the settings page.

WP Reading Progress vs MinuteRead — Reading Time for Posts