Wordpress - Get featured image URL by page id
Did you try anything? Its always helpful to share what you have tried.
$url = wp_get_attachment_url( get_post_thumbnail_id($post_id) );
Or if you want to get the image by image size.
$src = wp_get_attachment_image_src( get_post_thumbnail_id($post_id), 'thumbnail_size' );
$url = $src[0];
http://codex.wordpress.org/Function_Reference/get_post_thumbnail_id
http://codex.wordpress.org/Function_Reference/wp_get_attachment_url
http://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src
As of Wordpress 4.4.0, we can use get_the_post_thumbnail_url():
$url = get_the_post_thumbnail_url( $post_id, 'thumbnail' );