A water-deprivation headache should go away within an hour or two after you drink 16 to 32 ounces of water. More prolonged or severe dehydration requires more fluids and lying down for several hours until the pain dissipates.

While there is no "magic number," the general medical consensus is to drink 8 to 16 ounces (250–500ml) of water immediately if you suspect a dehydration-triggered headache, followed by consistent sips over the next few hours. Research suggests that increasing daily water intake by roughly 1.5 liters can significantly reduce the duration and intensity of chronic headaches in those who are prone to them. It is important to avoid "chugging" massive amounts of water at once, which can lead to electrolyte imbalances; instead, aim for steady hydration throughout the day. Dehydration causes the brain to temporarily contract or shrink due to fluid loss, pulling away from the skull and causing pain. For immediate relief, combining water with a small amount of electrolytes (like those found in sports drinks or a pinch of salt/lemon) can help the body absorb the fluid more effectively. If the headache persists after rehydrating, it may be triggered by stress, lack of sleep, or other non-dehydration factors.