The second item is half off. Your cart is not.

You came for one $40 item. A sign says “Buy one, get one 50% off.” Suddenly, the $20 item beside it seems hard to leave behind.

At checkout, the two-item total is $50 before tax. The promotion did save $10 on those two items. But your original plan would have cost $40. You are spending $10 more than you came to spend.

That is the part a big “50%” cannot tell you. The headline describes a discount on one eligible item—not necessarily the saving on your cart, and certainly not your saving against buying only what you needed.

The comparison card above starts with this $40 + $20 example. Try your own two prices, or keep reading the fixed examples and offer assumptions below without using the card.

First, find the item that actually gets discounted

In the examples below, imagine a fictional offer that takes 50% off the less expensive of two eligible items. Some real offers have different rules: identical items only, exclusions, a code, or a limit per order. Read the terms on the offer you actually see.

The last row is not a bad calculation. Half of $20 is still $10, but $10 is a much smaller fraction of a $100 pair. These are invented pre-tax examples, not prices from a store.

For the two $40 items in the table, the shortcut is simple: together they cost $80. Half off one saves $20. Then $20 ÷ $80 = 25% off the pair. The “50%” has not vanished; it was applied to only half of what you bought.

That 25% shortcut assumes cent rounding does not change the result. At penny prices it can: under the card’s stated half-cent-up rule, two $0.01 items at 50% off yield a $0.01 discount on a $0.02 pair—50% of that tiny pair. A real store may round differently.

Now compare the cart you wanted, not just the cart you built

There are two honest comparisons, and they answer different questions:

1. Offer versus buying both at full price. In the $40 + $20 example, the pair falls from $60 to $50. You saved $10 on that pair.
2. Offer versus buying only what you planned. If you needed only the $40 item, the $50 pair costs $10 extra. You did not save money against your original purchase.

The second item may still be worth $10 to you. It might be a gift or something you were going to buy soon. The point is to make that decision deliberately, not to let the banner make it for you.

This also explains why a different offer can beat a louder one. If you genuinely need two $40 items, a hypothetical 30% off the entire $80 cart would bring it to $56. Our “second item half off” example brings it to $60. Promotions are often mutually exclusive, so compare the actual final totals rather than assuming you can stack them.

Keep the two checkout totals side by side

Write down the prices before you add an item just to qualify. Identify which item the promotion discounts and by how much. Add the discounted prices, then compare that total with the items you intended to buy. Finally, include shipping, taxes and any other checkout charges that apply to you.

Keep these three lines in your own notes so you can check again when the cart changes:

• Planned items at checkout: the final total for only the items you intended to buy.
• Offer cart at checkout: the final total after the promotion, including any extra item needed to qualify.
• Extra spending: offer-cart total minus planned-items total. A positive number means the offer costs more than your original plan; it is not a saving against that plan.

Use the charges shown for each cart separately: adding an item can change shipping or eligibility. The comparison card above excludes those charges, so its pair total is not a prediction of a real checkout bill.

You can reproduce the $40 + $20 example in our percentage calculator: the Discount tab shows that 50% off $20 saves $10 and leaves a $10 second item. The Ratio tab shows that $10 is 16.67% of the original $60 pair. The calculator works in your browser. Its Copy page link button copies the page address only: it does not include your inputs or calculation result. Someone opening that link will need to enter the example numbers themselves. If you want to keep your comparison, write the two totals and assumptions in your own notes. Copying the address bar manually is a different action and can preserve query parameters already in the URL; do not put private spending details there.

The FTC’s online-shopping guidance similarly recommends checking the full cost, the terms of the advertised deal and return rules for sale items. Its advice does not turn our hypothetical offer into a retailer policy. If you may return one item, check how that store recalculates a discounted pair before you buy.

A useful question at the end is not “How big is the percentage?” It is: “What will I pay for the things I was already going to buy?”