set custom post excerpt length

#wordpress is show too much text the the_excerpt, so we need to customize it for showing shorted excerpt paragraph. copy this code and paste into your current active theme’s functions.php file

function my_custom_excerpt_length($length){
    return 100;
}
add_filter('excerpt_length', 'my_custom_excerpt_length', 999);

set different post excerpt length for mobile devices

wp_is_mobile is builtin function of #wordpress which return true when it found mobile text in #user_agent of your device

function my_custom_excerpt_length($length){
    return wp_is_mobile() == true ? 50 : 100;
}
add_filter('excerpt_length', 'my_custom_excerpt_length', 999);