Hypohydration can unknowingly occur in military personnel during exertion, especially in the heat. Such dehydration can impair cognitive and physical performance. Some portion of exercise-induced dehydration may be offset by changing the perceptual biases of soldiers when filling and drinking from canteens. Using comparably sized water bottles, we investigated this perceptual bias with 50 Army and Marine ROTC students by showing that those given short, wide, clear water bottles poured and drank more water than those given taller bottles that held the same volume. Even although those given short, wide water bottles poured 38% more water, they did not perceive themselves as having poured or drunk more. The implications for decreasing dehydration in the field and in garrison are discussed.