Calculate Reading Time & Store in Meta Field

reading_time.php

function calculate_reading_time() {
    if (is_single()) {
        global $post;

        // Adjust the words-per-minute rate according to your preference
        $words_per_minute = 200;

        // Get the post content
        $content = get_post_field('post_content', $post->ID);

        // Strip shortcodes and HTML tags from the content
        $stripped_content = strip_tags($content);
        $word_count = str_word_count($stripped_content);

        // Calculate reading time in minutes
        $reading_time_minutes = ceil($word_count / $words_per_minute);

        // Save reading time as post meta, remove minutes if you need int value.
        update_post_meta($post->ID, 'reading_time', $reading_time_minutes." minutes");
    }
}
add_action('wp', 'calculate_reading_time');