Pick a unit that matches what you use
Start with two products that serve the same purpose. For trash bags of the same size and type, use one bag. For similar liquids sold in different bottle sizes, use one fluid ounce. Check the fine print on the shelf tag. If one label shows price per fluid ounce and the other shows price per quart, calculate both prices in the same unit first.
Divide the price by the quantity
Suppose a pack of 20 bags costs $5 and a pack of 40 matching bags costs $8. The first pack costs 25 cents per bag because 5 ÷ 20 = 0.25. The second costs 20 cents per bag because 8 ÷ 40 = 0.20. The larger pack costs 5 cents less per bag and $3 more at checkout. These prices are examples.
Check that the comparison holds
Read the actual count or volume on each package. Use a sale price only if you qualify for it. If a discount requires buying two packs, divide the total you would pay by the total quantity in those packs. For an online order, count any shipping charge that changes between the two options.
Decide how much to buy
Choose the lower unit price when both products meet your needs and you expect to use the quantity. If you need only a few, the smaller pack can still cost less today. Compare the cost per unit and the amount you would pay at checkout.