A refrigerator works efficiently when the shelves are stocked but air can still flow around every container. As a practical guideline, fill about three-quarters of the shelves without blocking the back wall or the air outlet. In an overpacked refrigerator, cold air cannot circulate: containers in front cool while those in back stay warm, and the thermostat does not detect the difference.
A refrigerator does not cool a container directly; it circulates air cooled by the compressor, and the food transfers its heat to that moving air. So what matters is not the number of shelves, but whether air can pass between the containers. If a shelf is packed front to back and piled high, air hits a wall: a few containers in front cool while those in back stay warm for hours. Because the thermostat measures only the spot where it is located, it does not detect this difference; some food can remain outside a safe temperature range even when the motor is working properly. A running refrigerator does not mean that every container inside is cold.
The back wall is the most critical area. In many models, cold air blows out through a vent there, while water that condenses on the back surface drains through a hole below. A pot pushed against the back or a bag stuck to the wall interferes with these 2 functions: air cannot flow out and water cannot drain. As a result, the whole refrigerator warms up and a layer of ice may start to build on the back wall. Leaving about a palm’s width of space between the back edge of the shelf and the wall eliminates this problem. Containers on the shelves should also sit a finger’s width forward, rather than against the wall.
An empty refrigerator is not ideal either, for a different reason. With little thermal mass inside, cold air spills out whenever the door opens, room-temperature air flows in, and the motor has to start over each time. Stocked shelves retain their chill and act as a buffer that slows the temperature change when the door opens. So in a home where little cooking is done, placing water bottles on the shelves helps stabilize the temperature and reduces the energy cost of opening the door. Put the bottles on a lower shelf, not in the door.
After a large grocery run, the refrigerator can suddenly be packed to the brim, which is when it has to work hardest. When dozens of room-temperature packages go in at once, the motor runs continuously for a long time, and the food already inside warms up in the meantime. Splitting the load in two makes it easier: put perishable items away first, and leave dry, shelf-stable items on the counter while the refrigerator recovers. Keeping the door closed for a while after putting everything away also helps it recover faster. Put anything you plan to cook that day at the front for easy access.
You can tell whether the refrigerator is stocked properly with a simple check instead of a guess. Place a thermometer in a glass of water in the middle of the back shelf and leave it for a few hours; if the reading is outside the 0–4°C / 32–40°F range, the problem is often the way the refrigerator is stocked, not the thermostat. Repeat the check on the front shelf: a large difference between the two spots means the air is not circulating. Spacing out the items, clearing the back wall, and moving large containers forward measurably reduces the difference. Repeating this check 1 or 2 times a year is enough.