To rehydrate from mild to moderate dehydration, most adults need to drink about 1 to 1.5 liters of fluid (roughly 4 to 6 cups) over two to three hours. The exact amount depends on how much fluid you’ve lost, but a reliable rule of thumb is to drink 1.5 times whatever you lost. If you sweated through an hour of exercise and lost a pound, that’s about 16 ounces of water gone, so you’d aim for 24 ounces to fully recover.
How to Estimate What You’ve Lost
The simplest way to gauge dehydration is by weighing yourself before and after a bout of sweating, illness, or heat exposure. Every pound lost represents roughly 16 ounces (about 500 mL) of fluid. Replacing 150% of that loss is the standard recommendation because your body continues to lose fluid through urine and breathing even as you drink.
If you don’t have a scale handy, three signals can help you assess where you stand: body weight change of more than 1%, urine color at or darker than apple juice, and noticeable thirst. When two or all three of those overlap, you’re meaningfully dehydrated. Once you lose 2% of your body weight in fluid, which is about 3 pounds for a 150-pound person, both physical performance and mental sharpness start to decline measurably.
How Fast You Should Drink
Sipping steadily over a couple of hours works better than chugging a large volume at once. Your small intestine absorbs water by following sodium and other solutes across its lining. When sodium gets pulled into intestinal cells (especially alongside glucose), it creates an osmotic pull that drags water along with it. This process is efficient but not instant, so flooding your stomach with a liter all at once mostly just makes you urinate more.
A practical pace is about 200 to 300 mL (roughly one cup) every 15 to 20 minutes until you’ve hit your target. Healthy kidneys can filter around 800 mL to 1 liter per hour, so drinking faster than that provides no benefit and, in extreme cases, can dilute your blood sodium to dangerous levels, a condition called hyponatremia.
Water Alone vs. Water With Electrolytes
For everyday dehydration from exercise, hot weather, or not drinking enough during the day, plain water is usually fine as long as you’re eating regular meals. Food supplies the sodium and potassium your body needs to hold onto that water.
When dehydration is more significant, from prolonged sweating, vomiting, diarrhea, or a hangover, adding electrolytes speeds things up considerably. The World Health Organization’s oral rehydration formula uses a combination of sodium (60 to 90 milliequivalents per liter), potassium (15 to 25 milliequivalents per liter), and a small amount of glucose (under 20 grams per liter). The glucose is the key ingredient most people don’t realize matters. Sodium hitches a ride into your intestinal cells alongside glucose through a specific transport channel, and water follows. That’s why a pinch of salt and a splash of juice or a spoonful of sugar in water can rehydrate you faster than water alone.
Commercial electrolyte drinks, rehydration packets, and even diluted fruit juice with a pinch of salt all take advantage of this mechanism. You don’t need to match the WHO formula precisely. The principle is simple: a little salt, a little sugar, and water.
How Long Full Rehydration Takes
Your blood volume starts recovering quickly. Research shows that plasma volume can bounce back within one hour when you drink a fluid containing electrolytes and carbohydrates. With plain water, recovery tends to take closer to three hours because the fluid isn’t retained as efficiently.
Deeper tissue hydration takes longer. Even after your blood volume normalizes, muscles and organs continue pulling in fluid for several hours. Full cellular rehydration after significant fluid loss can take 6 to 12 hours, which is why you often still feel off the morning after heavy exercise or illness even if you drank plenty before bed. Spreading your fluid intake across the rest of the day, rather than trying to rehydrate all at once, gives your tissues time to absorb what they need.
Rehydration Amounts for Common Scenarios
- After a workout (1 hour, moderate intensity): 16 to 24 ounces (500 to 750 mL) over the next hour. Weigh yourself before and after if you want precision.
- After a stomach bug or food poisoning: Small, frequent sips totaling 1 to 2 liters over 2 to 4 hours. An electrolyte solution is strongly preferred here because vomiting and diarrhea deplete sodium and potassium quickly.
- After a night of drinking alcohol: About 1 liter before bed and another liter the next morning. Alcohol suppresses the hormone that tells your kidneys to retain water, so you lose more fluid than you’d expect from the volume of drinks consumed.
- After spending hours in heat: 1 to 1.5 liters over 1 to 2 hours, with electrolytes if you were sweating heavily. Sweat rates in hot conditions can reach 1 to 2 liters per hour for some people.
Signs You’re Rehydrated
The most reliable indicator is urine color. You’re aiming for pale yellow, roughly the shade of light lemonade. Clear urine actually suggests you’re drinking more than you need, and dark yellow or amber means you have more catching up to do. Thirst fading, a return of normal energy, and the disappearance of a dehydration headache are other good signals.
If you’ve been rehydrating for several hours and your urine stays dark, you feel dizzy, or your heart rate remains elevated, you’re likely dealing with more than mild dehydration. Severe fluid loss, beyond 5% of body weight, sometimes requires intravenous fluids because the gut can’t absorb fast enough to keep up with what the body needs.