The full cruise ship passenger capacity is based on double occupancy (2 guests per cabin). The cabin's capacity varies according to stateroom's type, category, and also by line and ship. A standard passenger ship cabin usually accommodates 2 or 3 guests, but there are some smaller and bigger exceptions.

Cruise ship capacity is primarily defined by "Double Occupancy," which assumes two passengers per cabin. When you see a ship’s official capacity (e.g., 4,000 passengers), this number typically refers to this baseline. However, "Total Capacity" can be much higher because many cabins feature pull-down bunks or sofa beds for third and fourth guests, especially on family-oriented lines like Royal Caribbean or Disney. Beyond passenger count, a ship's size is measured by Gross Tonnage (GT), a calculation of total internal volume where one GT equals 100 cubic feet of space. A vital metric for guests is the Passenger Space Ratio, calculated by dividing the Gross Tonnage by the number of passengers. A ratio above 40 indicates a spacious ship, while a ratio under 25 suggests a more crowded environment. Safety limits are strictly governed by the "Lifeboat Capacity," ensuring there is a seat for every soul on board, including crew.