How Much Water Should Women Actually Drink Per Day?

The National Academies’ guideline for adult women is about 9 cups (74 fl oz) of water a day from drinks, or 91 fl oz counting the water in food. Every drink counts toward it, not just plain water.

You’ve probably heard a different number your whole life: eight glasses a day, 64 ounces. Here’s why that isn’t the official advice, what the guideline actually means, and how to adjust it for the day you’re having.

How much water should a woman drink a day?

The National Academies set the Adequate Intake for women 19 and older at 2.7 liters (about 91 fl oz) of total water a day, counting food as well as drinks. About 2.2 liters (74 fl oz, roughly 9 cups) of that is expected to come from beverages. The other fifth or so comes from food; the National Academies estimate about 20% of the water people take in comes that way.

So the often-quoted 91 fl oz isn’t how much to drink. About 74 fl oz is.

For context, US women drink an average of 43 oz of plain water a day (CDC), but coffee, tea, milk and other drinks make up much of the rest. See it laid out in cups on our women’s hydration statistics page.

Is 8 glasses of water a day a real rule?

No. “Eight 8-oz glasses a day” was never official advice. A 2002 review in the American Journal of Physiology found no scientific studies supporting it. The review traced the idea to a 1945 recommendation that also noted most of that water is already contained in prepared food, a detail that got lost along the way. Our study of Google’s hydration answers found the 8-glasses rule still presented as fact on page-one results.

Sixty-four ounces isn’t a bad day, but it’s below the 74 oz from drinks the National Academies use for women.

Why “half your body weight in ounces” is a myth

The other number you’ll see everywhere is “drink half your body weight in ounces.” No study or health body sets water needs this way. It ignores the roughly 20% of water that comes from food, and it ignores activity, climate and health; the National Academies set intakes by age and sex, not weight. Our research found it repeated as fact on page-one Google results too.

Body size does play some part in what you need, which is why Hydrate factors in your weight alongside activity and health. But treat any weight-based figure from a calculator as a rough starting point, not a rule.

These are typical intakes, not minimums

An Adequate Intake is what healthy people typically take in, not a line you fail by missing. Some women do well on less, and some need more. For most healthy people, the National Academies concluded, thirst plus drinks with meals is a reasonable guide.

What increases your water needs?

Exercise and activity You lose water through sweat when you move, so active days need more. How much depends on how hard, how long and how hot it is. Dehydration of 2% of body weight or more consistently reduces endurance performance, according to the National Athletic Trainers’ Association, so it’s worth drinking around longer workouts. See our workout hydration guide for more.

Heat Hot, humid weather raises sweat losses a lot. The National Academies note that very active people in hot weather can need 6 liters (about 200 fl oz) or more of total water a day.

Pregnancy and breastfeeding During pregnancy the guideline rises to about 78 fl oz (10 cups) a day from drinks. While breastfeeding it’s about 105 fl oz (13 cups) from drinks, roughly 31 oz more than other women. More in our guides to water during pregnancy and water while breastfeeding.

Your menstrual cycle Many women notice they feel different, more bloated or more thirsty, at certain points in their cycle, especially the week before their period. It’s worth paying attention to your own pattern. We cover it in depth in the menstrual cycle and hydration.

Does coffee dehydrate you?

Not at normal amounts. The National Academies found that caffeinated drinks count toward your daily water intake like other beverages, so you don’t need to “make up” for your morning coffee with extra water. Keep caffeine within the FDA’s 400 mg a day for most adults, about two to three 12-oz cups of coffee (FDA). More in does coffee count as water?

Alcohol is different: it isn’t a good way to hydrate, and it’s worth alternating with water if you’re drinking.

Signs you’re not drinking enough

• Urine color: pale straw yellow is a good sign; dark yellow or amber means drink up
• Thirst: for most healthy people it’s a reasonable cue, not a sign you’re already in trouble. The idea that “if you’re thirsty, you’re already dehydrated” is a myth
• Headache or a slump in the afternoon: not always water, but an easy thing to rule out
• Dry mouth or lips

Why apps often get this wrong

Most hydration apps show you a static daily goal and call it done. But your needs aren’t static.

A day with a spin class in summer heat calls for more water than a rest day in autumn. Using one number for both means you’re likely under-drinking on the hard days and nagged for nothing on the easy ones.

A smarter approach starts from the guideline, tracks your activity (via HealthKit), and adjusts your daily target as your day changes.

Your starting point

A simple way to set today’s goal:

1. Start with the National Academies’ 74 fl oz (about 9 cups) from drinks
2. Count every drink: coffee, tea, milk and sparkling water all go toward it
3. Add more if you’re working out, especially for longer or harder sessions
4. Add more if it’s hot or humid
5. Use the pregnancy (78 fl oz) or breastfeeding (105 fl oz) figure if either applies
6. Let thirst and urine color tell you if you’re off

That’s your goal for today. Tomorrow might be different, and that’s the point.

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Hydrate calculates your personalized daily goal using your weight, activity data from HealthKit, and your cycle phase. Join the waitlist to be first when we launch.