How Much Water Should You Drink a Day?

You've probably heard "drink eight glasses a day." It's a decent nudge to sip more, but it isn't where the evidence actually points. Most healthy adults need roughly 2.7 litres (women) to 3.7 litres (men) of total water daily — and about a fifth of that arrives in your food. Your real target moves with your body size, how hard you sweat, and the weather. Here's how to find yours.

How much water do you actually need?

The most-cited benchmark comes from the U.S. National Academies of Sciences, Engineering, and Medicine: an Adequate Intake of about 3.7 litres (125 oz) of total water a day for men and 2.7 litres (91 oz) for women. "Total water" is the key phrase — it counts everything you drink plus the water in food, which supplies roughly 20%. Take out food and you land near 13 cups of fluid a day for men and 9 for women. Pregnancy nudges the target to about 3 litres, and breastfeeding to around 3.8.

Is the '8 glasses a day' rule actually real?

Not really — and that's the part most guides skip. There's no strong evidence behind the exact 8×8 figure (eight 8-oz glasses). It's a memorable rule of thumb, not a clinical guideline. Health authorities give ranges rather than a single number precisely because a 60-kg office worker in a mild climate and a 90-kg runner in July have very different needs. Use 8 glasses as a floor to aim past if it helps you remember — just don't treat it as a finish line.

What changes how much water you need?

• Heat and humidity — you lose more through sweat, sometimes a litre or more per hour of hard effort.
• Exercise — drink before, during, and after; long or intense sessions may also need electrolytes.
• Pregnancy and breastfeeding — needs rise to support blood volume and milk production.
• Illness with fever, vomiting, or diarrhoea — replace the extra fluids you're losing.
• Altitude above about 2,500 m — faster breathing and more urination increase losses.
• Body size and a high-protein or high-fibre diet — larger bodies and those diets ask for a bit more.

Do coffee, tea, and food count?

Yes to all three. Coffee and tea are mildly diuretic, but at normal intakes the fluid they contains still leaves you net-hydrated, so your morning cup counts — you can sanity-check your caffeine with the caffeine intake calculator. Milk, juice, and most foods count too: soups, yoghurt, and produce like cucumber and watermelon are more than 80% water. Only heavy alcohol works against you. If you're also watching intake, pairing hydration with a realistic target from the calorie calculator keeps both honest.

How do you know if you're drinking enough?

Skip the math and read your body. Pale, straw-coloured urine and rarely feeling thirsty are the two most practical green lights. Dark yellow urine, a dry mouth, headache, or afternoon fatigue often mean you're behind. For most healthy people with easy access to water, thirst is a reliable guide. The main exception is older adults, whose thirst signal dulls with age — they do better drinking on a light schedule rather than waiting to feel it.

Can you drink too much water?

Rarely, but yes. Drinking far more than your kidneys can excrete dilutes blood sodium and causes hyponatremia — seen most often in endurance athletes who down litres of plain water during long races. Symptoms include nausea, headache, confusion, and, in severe cases, seizures. The lesson isn't to fear water; it's to drink to thirst and need rather than forcing a fixed number of litres, and to add electrolytes during very long, sweaty efforts.