For mild to moderate dehydration, most adults need to drink about 1 to 1.5 liters (roughly 32 to 48 ounces) of fluid over the first few hours, then continue sipping steadily until symptoms resolve. The key is drinking at a pace your body can absorb, not gulping it all at once. How much you ultimately need depends on how dehydrated you are, your body size, and what caused the fluid loss in the first place.
How Much to Drink and How Fast
Your kidneys can safely process about 32 ounces (one liter) of water per hour. Drinking significantly more than that, especially plain water, can dilute the sodium in your blood to dangerous levels. So the practical ceiling is roughly a liter per hour, and most people don’t need to push anywhere near that limit.
A good starting point for mild dehydration is to drink about 8 to 16 ounces (one to two glasses) right away, then continue taking small sips every 10 to 15 minutes for the next several hours. If you’re dealing with moderate dehydration from vomiting, diarrhea, or heavy exercise, aim for closer to 1 to 1.5 liters in the first two hours, broken into frequent small amounts rather than large gulps. Small, steady sips are easier on the stomach and give your intestines time to actually absorb the fluid.
You may notice improvement quickly. Signs of dehydration like dry mouth and dizziness can start to ease within five to ten minutes of drinking. Mild to moderate cases typically resolve fully within a day, while more significant dehydration can take two to three days of consistent fluid intake to fully recover from.
Water Alone Isn’t Always Enough
Plain water works fine if you’re mildly dehydrated from not drinking enough during the day. But if you’ve lost fluids through sweating, vomiting, or diarrhea, you’ve also lost sodium and other electrolytes. Drinking only plain water in that situation can actually slow rehydration because your gut absorbs water more efficiently when sodium and a small amount of sugar are present together.
This is why oral rehydration solutions exist. The World Health Organization’s formula uses equal parts sodium and glucose (75 millimoles per liter of each), which activates a transport system in the small intestine that pulls water into the bloodstream faster than water alone. Commercial rehydration drinks available at pharmacies and grocery stores use a slightly different ratio (about one part sodium to three parts glucose) but still work well for most situations.
If you don’t have a commercial product on hand, you can approximate the effect by adding a pinch of salt and a small spoonful of sugar to a glass of water. Sports drinks are another option, though many contain more sugar than is ideal for rehydration. Broth, coconut water, and diluted fruit juice also provide some electrolytes alongside the fluid.
How to Tell How Dehydrated You Are
The amount you need to drink depends partly on how far behind you are on fluids. Your body gives you reliable signals to gauge severity.
Mild dehydration (under 5% of body weight lost as fluid) shows up as thirst, slightly darker urine, a dry or sticky mouth, and mild fatigue. Your heart rate and skin still look normal. This is the most common level, and steady drinking over a few hours fixes it.
Moderate dehydration (5 to 9% fluid loss) brings more noticeable symptoms: noticeably reduced urine output, sunken-looking eyes, a slightly elevated heart rate, and skin that doesn’t snap back immediately when you pinch it on the back of your hand. At this level, an oral rehydration solution is more effective than plain water, and you’ll want to take in fluid consistently over several hours.
Severe dehydration (10% or more fluid loss) is a medical emergency. Signs include confusion or extreme drowsiness, very rapid heartbeat, faint pulse, cool or mottled skin, and little to no urine output. If you or someone else shows these signs, oral fluids alone won’t be enough. This level typically requires IV fluids in a hospital setting. The same is true if someone can’t keep fluids down due to persistent vomiting.
Rehydrating Children
Children dehydrate faster than adults because they have a higher surface-area-to-body-weight ratio and are more vulnerable to fluid losses from fever, vomiting, or diarrhea. The approach is similar (small, frequent sips of an electrolyte solution) but the volumes are scaled to body size.
A practical guideline for children: offer a teaspoon to a tablespoon of oral rehydration solution every few minutes rather than a full cup at once. For infants, continue breastfeeding or formula alongside rehydration fluids. The overall goal is roughly 50 to 100 milliliters of fluid per kilogram of body weight over the first three to four hours for mild to moderate dehydration. For a 10-kilogram toddler (about 22 pounds), that works out to roughly 500 mL to 1 liter, or about 2 to 4 cups, spread over several hours.
Watch for wet diapers or trips to the bathroom as a sign that rehydration is working. If a child refuses fluids, becomes unusually sleepy, or has no tears when crying, those are signs to seek medical care promptly.
What to Avoid During Rehydration
Drinking too much too fast is a real risk, not just an abstract warning. Consuming more than about a gallon (3 to 4 liters) in an hour or two can cause water intoxication, a condition where blood sodium drops low enough to cause confusion, seizures, and in extreme cases, death. This is rare, but it happens most often to endurance athletes or people who panic-drink large volumes after feeling dehydrated.
Caffeinated drinks and alcohol both increase urine output, which works against your rehydration effort. They’re not completely off-limits, but they shouldn’t be your primary fluid source when you’re trying to catch up. Sugary sodas and undiluted fruit juices can also slow absorption in the gut and sometimes worsen diarrhea.
The most reliable approach: keep a water bottle nearby, add electrolytes if you’ve been sweating or sick, drink steadily rather than in large bursts, and use your urine color as a guide. Pale yellow means you’re on track. Clear may mean you’re overdoing it. Dark amber means you still have catching up to do.