Set the refrigerator to 37°F and the freezer to 0°F. Those are the targets that balance food safety, food quality, and energy use, per FDA and USDA Food Safety and Inspection Service guidance.
FDA Food Code requires cold holding at 40°F or below and frozen storage at 0°F. Setting the refrigerator to exactly 40°F leaves no buffer — every door opening, every warm bag of groceries put in, every power-cycling defrost cycle pushes temperatures briefly higher. A 37°F setting keeps the effective temperature below the 40°F safety line through normal daily use.
Verify rather than assume. The factory thermostat scales on most refrigerators are labeled 1 through 10 or Cold-to-Coldest, not in actual degrees. Put a cheap $5 fridge-freezer thermometer on the middle shelf, wait 24 hours, and read the actual temperature. Calibration drift on 10-plus-year-old thermostat dials is common; what reads as "5 of 10" can deliver 32°F or 42°F depending on the unit.
Avoid settings colder than 35°F in the refrigerator. Vegetables, greens, and dairy held at 33 to 34°F for days lose texture, and the compressor runs longer cycles for no food-safety benefit. Freezers below -5°F burn more electricity without extending storage life meaningfully.
When door-gasket seals age, set points drift. If you've had to turn the dial colder to get the same feel, inspect the gasket before adjusting further.