I can not count the number of characters

Hello! I really need your help!
I created a single record template. In this template, I have a field in which I derive the approximate time for reading the article using a shortcode.

Here is the php code for counting characters:

function reading_time() {
  $content = get_post_field( 'post_content', $post->ID );
  $word_count = str_word_count( strip_tags( $content ) );
  $readingtime = ceil($word_count / 1);
  if ($readingtime == 1) {
    $timer = " минуту";
  } elseif ($readingtime == 2 || $readingtime == 3 || $readingtime == 4) {
    $timer = " минуты";
  } else {
    $timer = " минут";
  }
  $totalreadingtime = 'Время чтения: ' . $readingtime . $timer;
  return $totalreadingtime;
}
add_shortcode( 'readingtimes', 'reading_time');

The problem is that he incorrectly counts the number of characters, shows that I have 13 characters in total (I set the division by 1 specifically to verify this).

Perhaps the fact is that I have several text editors, a list with icons, etc.

How to make it work?

I would be very grateful for the help!