Estimated Reading time Wordpress without Plugin

functions.php

<?php

function reading_time() {
   $content = get_post_field( 'post_content', $post->ID );
   $word_count = str_word_count( strip_tags( $content ) );
   $reading_time_minutes = ceil($word_count / 200);

   if ($reading_time_minutes == 1) {
      $reading_time_label = 'minute';
   } else {
      $reading_time_label = 'minutes';
   }

   return '<p>Reading time: <span class="reading-time">' . $reading_time_minutes . '</span> ' . $reading_time_label . '</p>';
}
// call this function to display on your content, example: content.php - echo reading_time();

?>