Count the water that is actually in the system
Use a measured fill or inside water-column calculation with displacement and connected water accounted for once. A gallon label on the aquarium is not that measurement.
What the container result means
Full containers plus the final partial amount add up to the selected water change. The trip count rounds up only when there is water left for a partial container. Zero percent returns zero water and zero trips; it does not recommend skipping needed care.
Prepare the route and replacement water
Choose the amount using your care plan. Follow the cleaning and water-change sequence, including equipment isolation, and use the exact conditioner instructions when applicable. This calculator gives no dose, frequency or water-quality verdict.
Compare siphons, containers and hose routes before deciding what is practical to carry.