<p>This paper proposes an analytic method for calculating the collision area to evaluate collision risk between ships in both static and dynamic encounter situations. The collision area is defined as the region that the own ship will intrude into in the future if it maintains its current speed, given a specific safety domain surrounding a target ship. For static situations, assuming the target ship maintains its course and speed, we derive exact boundaries of the collision area using both implicit function representations and parametric representations. This method accommodates various domain shapes, including circles, symmetric ellipses, and asymmetric ellipses. Furthermore, we extend the framework to dynamic situations. We demonstrate calculations for scenarios where the target ship alters its course and speed, and where the ship domain expands over time to account for prediction uncertainties. Finally, we clarify the theoretical relationship between the collision area and conventional indicators, such as the Dangerous Area of Collision (DAC) and the Obstacle Zone by Target (OZT), showing that the collision courses in OZT correspond to the tangent lines to the collision area. This study offers a robust and versatile analytical foundation for collision avoidance in Maritime Autonomous Surface Ships (MASS).</p>