WooCommerce add "Save x%" next to sale prices

functions.php

<?php
// Add save percent next to sale item prices.
add_filter( 'woocommerce_sale_price_html', 'woocommerce_custom_sales_price', 10, 2 );
function woocommerce_custom_sales_price( $price, $product ) {
	$percentage = round( ( ( $product->regular_price - $product->sale_price ) / $product->regular_price ) * 100 );
	return $price . sprintf( __(' Save %s', 'woocommerce' ), $percentage . '%' );
}
?>