What a struck-through price is actually allowed to be

There is no thirty-day rule here

If you have read European advice about discounts, you have met the thirty-day rule: the crossed-out price has to be the lowest the shop charged in the previous thirty days. It is a real rule, and it does not apply in the United States.

There is no federal rule fixing a reference window. Which does not mean anything goes.

What the rule here actually says

The Federal Trade Commission’s guides against deceptive pricing say a former price has to be a bona fide one: a price the item was actually offered at, openly and for a reasonably substantial period — not a figure invented so that something could be struck through it.

• Actually offered. Not a price list nobody ever paid, not a number borrowed from a different seller.
• Openly. On the shelf, on the page, where a customer could have bought at it.
• For a reasonably substantial period. A price that existed for one afternoon in August is not a former price, it is a prop.

This is a standard, not a stopwatch. Nobody will tell you from the page whether the shop met it — which is why the checks below are worth more than the law is.

So what do you look at instead

Without a fixed window, the crossed-out number carries very little information on its own. Two numbers carry more.

• The typical price. On Amazon.com this is often shown next to the deal: the median of what customers actually paid over the last ninety days. It is not a list price and nobody sets it for marketing reasons.
• The price history. A chart of what the item cost week by week tells you whether today is low, ordinary, or a quiet rise with a bigger strike-through on top.

A discount measured against a list price nobody paid and a discount measured against the median of what people did pay are not the same claim, and the second one is the one worth reading.

The thirty seconds that settle it

• Find the typical or median price if the page shows one, and compare today against that, not against the strike-through.
• Open the price history and look at the last three months, not the last three days.
• If the only reference is a list price, treat the percentage as decoration until something else backs it.