A multipack has two quantity layers: the amount in each inner package and the number of packages in the bundle. Ignoring either layer produces the wrong unit price. Treat the full bundle as one purchasable package, then normalize its complete quantity.
Multiply before dividing
A four-pack of 250 millilitres contains 1,000 millilitres, or one litre. If it costs €10, its unit price is €10 per litre. Dividing €10 by 250 mL would incorrectly treat the other three bottles as if they did not exist.
Compare equivalent total quantities
A larger bundle may have a lower unit price but require more cash and storage. UnitPrice identifies mathematical package value; it does not claim that buying extra quantity is the better household decision. Check shelf life, expected use, and available space separately.
Apply promotions to the actual paid price
If a promotion changes the checkout price, enter that final package price only when you actually qualify. Loyalty rules, subscriptions, deposits, coupons, and minimum purchase conditions can make a shelf comparison misleading. Record the condition beside the result if the decision matters.