Woocommerce: Show % saved

functions.php

<?php

/**
 * Add save percent next to sale item prices, 
 * wrapped in a span + strong for CSS
 * targeting with .saving{} and .saving strong{}
 */
add_filter( 'woocommerce_sale_price_html', 'woocommerce_sales_price_saving', 10, 2 );
function woocommerce_sales_price_saving( $price, $product ) {
  $percentage = round( ( ( $product->regular_price - $product->sale_price ) / $product->regular_price ) * 100 );
  return $price . sprintf( __('<span class="saving">Save <strong>%s</strong></span>', 'woocommerce' ), $percentage . '%' );
}