When you have the flu, you need roughly 50 to 100 percent more fluid than usual. For most adults, that means aiming for about 80 to 100 ounces (2.5 to 3 liters) of total fluids per day instead of the typical 64 ounces. Fever, sweating, vomiting, and diarrhea all drain your body’s water supply faster than normal, so the exact amount depends on how severe your symptoms are.
Why the Flu Drains Fluid So Fast
Fever is the biggest driver of fluid loss during the flu. For every degree your temperature rises above 98.6°F, your body loses an extra 2.5 mL per kilogram of body weight per day through your skin and breathing. For a 155-pound (70 kg) adult running a 102°F fever, that’s roughly an extra 20 ounces of water lost daily just from the fever alone, on top of what you’d normally lose.
Sweating compounds the problem, especially during the cycles of chills and fever breaks that are typical with influenza. If you’re also dealing with vomiting or diarrhea, fluid losses can escalate quickly. Your body uses water to mount an immune response, produce mucus, and regulate temperature, so demand rises at the same time supply drops.
A Practical Daily Target for Adults
A good starting point is to take your normal water intake and add at least 3 to 4 extra cups (24 to 32 ounces) spread throughout the day. If you have a high fever, significant sweating, or gastrointestinal symptoms, push that closer to 5 or 6 extra cups. Rather than forcing large amounts at once, take small sips every 10 to 15 minutes. This is especially important if nausea makes it hard to keep fluids down.
The simplest way to track whether you’re drinking enough is your urine. Pale yellow means you’re well hydrated. Dark yellow or amber-colored urine, especially in small amounts, signals that you need to drink more right away. If you’re urinating very infrequently or your urine is consistently dark and strong-smelling, you’re already significantly dehydrated.
What to Drink Besides Water
Plain water works, but it isn’t always the best option when you’re losing fluids rapidly. When you sweat or have diarrhea, you also lose sodium and potassium, and water alone doesn’t replace those. Oral rehydration solutions are designed to optimize fluid absorption through a specific balance of sodium and glucose that helps your intestines pull water in more efficiently. Sports drinks, clear broths, diluted juice, and ice pops all count toward your fluid intake and offer some electrolytes.
A study comparing half-strength apple juice to a standard electrolyte solution found that children tolerated the juice well and stayed hydrated, suggesting that palatable options are perfectly reasonable when the alternative is not drinking enough. The priority is getting fluids in, so if plain water doesn’t appeal to you while you’re sick, choose something you’ll actually sip consistently.
Chicken soup earns its reputation here. Broth provides sodium, the warm liquid promotes hydration, and the heat helps get mucus flowing. Herbal teas and warm water with honey are other solid choices.
Warm Drinks Offer Extra Symptom Relief
Hot beverages provide a measurable comfort advantage over cold or room-temperature ones. A study on hot drinks during cold and flu found that warm liquids provided immediate and sustained relief from runny nose, cough, sneezing, sore throat, chills, and tiredness. The same drink served at room temperature only helped with runny nose, cough, and sneezing. The hot version didn’t actually change nasal airflow when measured objectively, but the subjective improvement was significant. When you feel miserable, that perception of relief matters.
How Much Children Should Drink
Children dehydrate faster than adults because they have a higher surface-area-to-weight ratio and smaller fluid reserves. For mild to moderate dehydration, the recommended oral rehydration volume is 50 to 100 mL per kilogram of body weight, given over 2 to 4 hours. For a 44-pound (20 kg) child, that translates to roughly 34 to 67 ounces over a few hours during active rehydration.
For day-to-day maintenance while sick, a common formula breaks down like this: 100 mL per kilogram for the first 10 kg of weight, 50 mL per kilogram for the next 10 kg, and 20 mL per kilogram for each kilogram after that. So a 30 kg child (about 66 pounds) would need roughly 1,700 mL, or about 57 ounces, as a daily baseline before accounting for fever or vomiting losses. Small, frequent sips work better than large volumes for kids who feel nauseated. Oral rehydration solutions are the first-line approach for children with mild to moderate dehydration.
When to Be Careful About Overhydrating
Most healthy people can safely drink extra fluids during the flu without any risk. But if you have heart failure or kidney disease, pushing fluids aggressively can cause problems. People with heart failure are sometimes restricted to less than 2 liters (about 67 ounces) per day, and those with severe heart failure or low sodium levels may need to stay below 1 to 1.5 liters daily. If you have either condition and come down with the flu, your fluid targets need to be individualized, not based on general advice.
Signs You’re Not Drinking Enough
Mild dehydration shows up as thirst, dry lips, slightly darker urine, and fatigue beyond what the flu itself causes. Moderate dehydration brings a dry mouth, dizziness when standing, reduced urine output, and headache. In children, watch for fewer wet diapers, no tears when crying, and unusual irritability or sleepiness.
Severe dehydration is a medical emergency. The warning signs include very dark urine in tiny amounts (or none at all), rapid heartbeat, confusion, sunken eyes, and skin that stays tented when you pinch it. Vomiting so persistent that you can’t keep any fluids down for several hours also puts you at serious risk, because oral rehydration becomes impossible. At that point, you need professional help to get fluids replaced.