Space exploration is moving towards establishing a lunar colony, but success depends heavily on resource management, particularly water. While frozen water exists in permanently shadowed lunar regions, current estimates suggest it may not be sufficient for large-scale civilizations. Researchers analyzed the water needs of a lunar city, considering personal use, food production, rocket fuel, and breathing. Even with a generous baseline of 1 billion tons of water, a population of one million would exhaust these reserves in 40 to 100 years, depending on water recycling efficiency. The International Space Station's recycling system, even with recent improvements, is not efficient enough for long-term lunar settlement.A future moon colony's water usage will be higher than on the ISS due to factors like laundry and toilets adapted to lunar gravity. A city of 100,000 people might sustain itself for a thousand years with current recycling, but a city of one million would face depletion much faster. The sustainable long-term lunar population might be limited to a few hundred thousand due to water constraints. Future advancements in water recycling technology, such as a tenfold increase in efficiency, could extend the viability of larger populations.Other potential solutions include vertical farming, reduced meat consumption, or importing water from asteroids, though these have their own challenges. Ultimately, careful planning, governance, regulation, and thoughtful stewardship of lunar water resources are crucial for the growth of any significant human presence on the moon. A "first come, first served" approach to water extraction is not sustainable for a growing lunar civilization. Evaluating these options well in advance is essential.