To show “% discount” in a facet, we’ll first create the facet:

We’re setting the Data source to Post Type just as a placeholder.

Next, we add an indexer hook to calculates the percent discount and modify the index value. Add the following into your (child) theme’s functions.php:

How to use custom PHP code?
PHP code can be added to your (child) theme's functions.php file. Alternatively, you can use the Custom Hooks add-on, or a code snippets plugin. More info
add_filter( 'facetwp_index_row', function( $params, $class ) {
  if ( 'percent_discount' == $params['facet_name'] ) {
    $post_id = (int) $params['post_id'];

    if ( 'product' == get_post_type( $post_id ) ) {
      $product = wc_get_product( $post_id );
      $discount = 0;

      if ( $product ) {
        $regular_price = $product->get_regular_price();
        $sale_price = $product->get_sale_price();
        $discount = round( 100 * ( $regular_price - $sale_price ) / $regular_price );
      }

      $params['facet_value'] = $discount;
      $params['facet_display_value'] = $discount;
    }
  }
  return $params;
}, 10, 2 );

Make sure to re-index afterwards.

See also