ReadingTime is a free WordPress plugin, developed by the Whiletrue.it stuff to estimate and show the post reading time.

The estimation is based on the “speed” value stored in the settings menu.The value for speed should be chosen between a span of 150 words per minute (slightly slower than average) to 250 words per minute (slightly higher than average).

The calculation is automatic; a different value (in seconds) for a single post can be passed using a Custom Field named “readingtime”.

Download the ReadingTime plugin from the official WordPress website, clicking on the button: