How do you know what number in the fridge represents the ideal temperature?
There is really no way to know what the numbers on the dial in your fridge represent. In fact, I’ve seen refrigerators that didn’t have numbers, just “<-- Colder | Warmer -->“. The actual temperature achieved by any given setting in any given fridge depends on its make, model, age, how much food it contains, what kind of food it contains, when was the door last opened, when did you last vacuum underneath, … Really the only way to tell what temperature it is in your fridge is to invest in a refrigerator thermometer. (Or two – put one on or near the top shelf, and one in the meat drawer, if you have one.
