You come home after a weekend away, or wake up after a stormy night, and the clock on the oven is blinking. The freezer is cold again and everything inside looks frozen solid. But was it like that the whole time? There is no way to tell by looking at a bag of peas. Unless, a few weeks earlier, you had placed one small object on top of a cup of ice.
How does the coin trick work?
The idea has circulated for years, especially before hurricane and winter storm seasons, and it takes about a minute to set up:
- Fill a cup or small container with water and freeze it solid.
- Place a coin, typically a quarter, on top of the ice.
- Leave the cup in your freezer permanently.
If the power goes out while you are away, check the coin when you get back. If it is still sitting on top, the ice never melted. If it has sunk to the middle or the bottom, the freezer warmed up enough for the ice to melt and refreeze once the power returned, which means your food probably thawed too. Without the coin, refrozen food can look perfectly normal.
Why do food safety experts add a rule?
Here is the part the viral version usually skips. When fact-checkers at Snopes looked into the trick, they pointed out a limit: ice floats on its own meltwater, so the coin may stay near the top even after a significant partial thaw. In other words, a coin at the bottom is a clear warning, but a coin on top does not guarantee everything is safe.
That is why experts recommend pairing it with a second check. According to the FDA, a full freezer keeps food safe for about 48 hours without power, and a half-full one for about 24 hours, as long as the door stays closed. An unopened refrigerator only keeps food cold for about four hours.
Food that still contains ice crystals, or is at 40°F or below, can be refrozen or cooked, the FDA says, and you should never rely on appearance or smell to decide. When in doubt, the rule stays the same: throw it out.
Is there an even better version?
Yes. The N.C. Cooperative Extension at NC State University suggests a variation. Freeze a partly filled water bottle lying on its side, then stand it upright once it is frozen. If you later find the ice has moved to the bottom of the bottle, the freezer thawed. It works on the same principle as the coin, with an even easier-to-read result.
The most reliable option remains an inexpensive appliance thermometer kept inside the freezer. It should read 0°F or below. If it reads 40°F or below when the power comes back, the FDA says the food is safe and can be refrozen.
How to set it up this weekend
- Use a clear cup, so you can see where the coin sits without touching it.
- Keep the freezer full: a full freezer holds its temperature about twice as long as a half-empty one. Bags of ice or frozen water bottles fill the gaps.
- Add a thermometer and note the reading before any storm.
- Keep the door closed during an outage. Every opening lets cold air escape.
While you are rethinking your kitchen habits, a few minutes are also enough to get your microwave clean without scrubbing, and to check the right thermostat setting for fall.
The bottom line
A coin on a cup of ice will not replace a thermometer, but it costs nothing and answers the one question a blinking oven clock cannot: did my freezer thaw while I was gone? Few people know it exists. After the next storm, you may be glad you did.
