How to round Woocommerce cart item (product) subtotal and calculate cart subtotal and total from item subtotal?

Question

I have been conducting extensive research on rounding at the cart item subtotal level, but I have not been able to find a solution.

What I want to do is to round the product subtotal after discout applied. Then caculate cart total and total by adding all product subtotal together.

I have tried different hooks such as:

1. woocommerce_before_calculate_totals - This hook only allows me to modify the product price.
2. woocommerce_cart_product_subtotal (filter) - This hook only affects the display of the cart item subtotal; it does not alter the calculation of the cart subtotal and total.
3. woocommerce_calculate_totals - This hook only allows me to modify calculations based on the product price and quantity, and it is not directly related to the cart item subtotal.

If you have any suggestions or solutions, I would greatly appreciate your assistance. Thank you!

Answer 1

After some trials, I came up with this:

add_action('woocommerce_before_calculate_totals', 'my_woocommerce_calculate_totals', 10, 1);

function my_woocommerce_calculate_totals($cart) {
if (is_admin() && !defined('DOING_AJAX')) {
    return;
}

// Loop over $cart items
foreach ($cart->get_cart() as $cart_item_key => $cart_item) {
    // Get line specific data
    $itemQuantity = $cart_item['quantity'];
    $productObject = $cart_item['data']; // Get the product object
    $productPrice = $productObject->get_price();
    $regular_price = $productObject ->get_regular_price();
    
    // Calculate new price per item
    $discount = ($regular_price - $productPrice) / $regular_price;
    $new_price = (round($productPrice * $itemQuantity) / $itemQuantity) / (1 - $discount);

    // Set new price
    $productObject->set_price($new_price);
}
}

Example: Product regular price $750, quantity 5, discount 15%Off.

Normal calculation: Item subtotal = $750 x 0.85 x 5 = $3187.5

Modified calculation:

New Price = round($750 x 0.85 x 5) / 5 / 0.85 = 750.1176471.

Item subtotal = $750.1176471 x 0.85 x 5 = $3188.

The theory is to calculate item subtotals and round them to the nearest integers. Then re-calculate the new price from the rounded item subtotals. I did some tests and this function shows correct rounded subtotals and totals. However, the product price will be modified.

Any comment on this solution? Will it cause any problems? Thank you very much in advance.