How long can a human survive without water?

Under average conditions, a human can survive approximately 3–5 days without water, though this varies significantly based on temperature, humidity, physical activity, and individual health. In extreme heat or during heavy exertion, survival time can drop to as little as a few hours. The human body is about 60% water by weight, and the brain is approximately 75% water. Even mild dehydration (2% body weight loss) measurably impairs cognitive function, mood, and reaction time. At 5% loss, serious symptoms appear including rapid heartbeat and overheating. At 10% loss, organ failure begins. At 15% or more, death is likely. The kidneys can process about 0.8–1.0 liters of water per hour, and the average adult needs 2–3 liters daily to replace losses through urine, sweat, breathing, and other processes.