Discount percentage on the WooCommerce "on sale" badge

percentage_woocommerce_sale_flash.php

<?php

add_filter( 'woocommerce_sale_flash', 'percentage_woocommerce_sale_flash', 10, 3 );
function percentage_woocommerce_sale_flash( $html, $post, $product ) {
	if ( $html!='' ) {
		$perc = round( 100 - ( $product->sale_price * 100 / $product->regular_price ) );
		if ( $perc>0 ) $html = '<span class="onsale">-'.$perc.'%</span>';
	}
	return $html;
}