WooCommerce - Show savings in sale flash

functions.php

<?php
/**
 * Get the savings in an onsale product.
 *
 * @param WC_Product $product The product that is onsale.
 * @param string     $in      Should the savings be absolute or in percentage.
 * @return string
 */
function mas_wc_get_savings_on_sale( $product, $in = 'amount' ) {
    if ( ! ( $product instanceof WC_Product ) ) {
        return ''; 
    }
    if ( ! $product->is_on_sale() ) {
        return '';
    }

    if ( $product->is_type( 'variable' ) ) {
        $var_regular_price    = array();
        $var_sale_price       = array();
        $var_diff_price       = array();
        $available_variations = $product->get_available_variations();

        foreach ( $available_variations as $key => $available_variation ) {
            $variation_id     = $available_variation['variation_id']; // Getting the variable id of just the 1st product. You can loop $available_variations to get info about each variation.
            $variable_product = new WC_Product_Variation( $variation_id );

            $variable_product_regular_price = $variable_product->get_regular_price();
            $variable_product_sale_price    = $variable_product->get_sale_price();

            if ( ! empty( $variable_product_regular_price ) ) {
                $var_regular_price[] = $variable_product_regular_price;
            } else {
                $var_regular_price[] = 0;
            }
            if ( ! empty( $variable_product_sale_price ) ) {
                $var_sale_price[] = $variable_product_sale_price;
            } else {
                $var_sale_price[] = 0;
            }
        }

        foreach ( $var_regular_price as $key => $reg_price ) {
            if ( isset( $var_sale_price[ $key ] ) && 0 !== $var_sale_price[ $key ] ) {
                $var_diff_price[] = $reg_price - $var_sale_price[ $key ];
            } else {
                $var_diff_price[] = 0;
            }
        }

        $best_key = array_search( max( $var_diff_price ), $var_diff_price, true );

        $regular_price = $var_regular_price[ $best_key ];
        $sale_price    = $var_sale_price[ $best_key ];
    } else {
        $regular_price = $product->get_regular_price();
        $sale_price    = $product->get_sale_price();
    }

    $regular_price = wc_get_price_to_display(
        $product,
        array(
            'qty'   => 1,
            'price' => $regular_price,
        )
    );
    $sale_price    = wc_get_price_to_display(
        $product,
        array(
            'qty'   => 1,
            'price' => $sale_price,
        )
    );

    if ( 'amount' === $in ) {
        $savings = wc_price( $regular_price - $sale_price );
    } elseif ( 'percentage' === $in ) {
        $savings = '<span class="percentage">' . round( ( ( $regular_price - $sale_price ) / $regular_price ) * 100 ) . '%</span>';
    }

    return $savings;
}

/**
 * Function for overriding the default sale flash with sale amount or percentage.
 */
function mas_wc_get_sale_flash( $html, $post, $product ) {
    $savings = mas_wc_get_savings_on_sale( $product, 'percentage' );
    if ( ! empty( $savings ) ) {
        /* translators: %s - Saved amount or percentage from the sale. */
        $html = '<span class="onsale">' . sprintf( esc_html__( 'Save %s', 'your-theme-text-domain' ), $savings ) . '</span>';
    }

    return apply_filters( 'mas_wc_get_sale_flash', $html, $post, $product );
}

add_filter( 'woocommerce_sale_flash', 'mas_wc_get_sale_flash', 20, 3 );