What does "cruise altitude" primarily depend on?

Cruise altitude primarily depends on the gross weight of the airplane. When an aircraft is loaded with more weight, it typically requires a lower cruise altitude to maintain optimal performance and efficiency. Heavier aircraft have reduced climb capabilities and may not be able to achieve higher altitudes as effectively. Therefore, the operating weight influences the aircraft's aerodynamics, engine performance, and fuel efficiency, guiding the flight crew in selecting an appropriate altitude for cruising.

While factors such as weather conditions, flight path restrictions, and aircraft type also have roles in determining the cruise altitude, the gross weight is the most decisive factor, as it directly affects how the aircraft performs in flight.