Rehydrating after exercise, illness, or a day of not drinking enough typically requires 16 to 24 ounces of fluid for every pound of body weight you’ve lost. That’s roughly 1 to 1.5 liters for every kilogram. If you’re not weighing yourself before and after activity, a practical starting point is drinking about 6 to 8 cups over the next few hours while monitoring your urine color. Pale, straw-colored urine means you’re back on track.
Why You Need More Than What You Lost
Your body doesn’t hold onto every ounce of fluid you drink. Even after you stop sweating, you continue losing water through urine and breathing. That’s why rehydration guidelines call for 150% of what you lost, not just a one-to-one replacement. If you sweated out two pounds during a run, you’d want to take in 32 to 48 ounces of fluid afterward, not just 32.
The simplest way to estimate how much you’ve lost is to weigh yourself before and after physical activity, wearing minimal clothing. The difference is almost entirely water. Without a scale, pay attention to thirst, dry mouth, fatigue, and darker urine. These are signals that your body is already running a meaningful deficit.
Plain Water vs. Drinks With Electrolytes
Drinking plain water works, but your body retains significantly more fluid when sodium is involved. In a study published in the International Journal of Sport Nutrition and Exercise Metabolism, participants who drank a sodium-containing solution after exercise retained about 70% of the fluid they consumed, compared to just 50% for those who drank plain water. The plain-water group also produced nearly twice as much urine. In practical terms, if you chug a liter of water after a hard workout, roughly half of it may pass through you without contributing much to rehydration.
This doesn’t mean you need a special drink every time. Eating a meal alongside your water provides sodium naturally and improves retention. A handful of pretzels, some soup, or any salted food paired with water gets you most of the benefit. Sports drinks and oral rehydration solutions are most useful when you can’t eat, when you’ve lost a lot of sweat (more than 2% of your body weight), or when you’re recovering from vomiting or diarrhea.
Milk, both skim and whole, actually outperforms plain water for hydration at rest. Its natural combination of sodium, protein, and carbohydrates slows gastric emptying and improves fluid retention. Oral rehydration solutions like Pedialyte perform similarly well because of their higher sodium content (around 45 mmol/L or more). Standard sports drinks fall somewhere between water and these options, and the electrolyte levels in most commercial sports drinks aren’t consistently high enough on their own to improve retention over water.
How Fast to Drink
Spacing your fluids out over one to two hours is more effective than drinking everything at once. When you flood your stomach with a large volume of water quickly, your kidneys ramp up urine production to maintain balance, and you lose much of what you just drank. Sipping steadily gives your gut time to absorb the fluid and your kidneys time to adjust.
There’s also an upper safety limit. Your kidneys can process roughly 0.8 to 1 liter of water per hour under normal conditions. Drinking well beyond that rate, especially without food or electrolytes, risks diluting the sodium in your blood to dangerous levels. This condition, called hyponatremia, is rare in everyday life but has occurred in marathon runners and people who force-hydrate during illness. Someone eating very little who drinks 3 or more liters of water in a day can overwhelm their kidneys’ ability to excrete the excess, because there isn’t enough dietary solute to carry the water out.
Rehydrating Children
Children dehydrate faster than adults because of their smaller fluid reserves and higher surface-area-to-weight ratio. For mild dehydration from a stomach bug or a hot day, the general approach is small, frequent sips rather than large volumes. Pediatric oral rehydration solutions are designed with a 1:1 ratio of sodium to glucose, which optimizes absorption through the gut wall. The WHO-recommended formula uses 75 milliequivalents per liter of both sodium and glucose, a balance that pulls water efficiently into the bloodstream.
For children with up to 5% dehydration (which looks like a dry mouth, fewer wet diapers, and mild listlessness), the fluid deficit is typically replaced over 24 hours. More significant dehydration, around 10%, is replaced in stages: half in the first 24 hours, the other half over the following day. Pushing fluids too quickly in young children can trigger vomiting, which sets the process back.
Your Baseline Matters
Rehydration isn’t just about catching up after a deficit. It’s easier to rehydrate when your daily intake is already adequate. The average healthy adult needs roughly 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day, with the higher end applying to men and people who are physically active. About 20% of that comes from food, particularly fruits, vegetables, soups, and yogurt. The remaining 80% comes from what you drink.
If you’re consistently under-drinking, you start each day or each workout already in a mild deficit, which means rehydration takes longer and requires more volume. Building a habit of steady fluid intake throughout the day, rather than trying to catch up in one sitting, keeps your baseline closer to where it should be.
How to Know You’re Rehydrated
Urine color is the most reliable self-check. A pale yellow, almost straw-like color with little odor indicates good hydration. Darker yellow or amber means you still have a deficit to close. Completely clear urine, on the other hand, can signal you’re drinking more than you need.
Other signs that rehydration is working: your thirst subsides, your mouth feels moist, your energy returns, and any headache you had starts to fade. If you’ve been exercising, your heart rate should settle back to its normal resting level more quickly once you’re adequately rehydrated. For most people recovering from a moderate deficit (say, a tough workout in the heat), the process takes one to two hours of steady sipping with some food. More severe dehydration from prolonged illness can take 24 to 48 hours of consistent fluid and electrolyte intake to fully resolve.