The short answer
For most healthy people, thirst is a reliable everyday guide. Drink water with your meals and between meals, and let your thirst tell you when to drink more. That covers the day for most people.
There are official daily figures, too — and they are higher than most people expect, because they count all the water you take in, including the water in your food. The next section explains them.
The official numbers: total water
The U.S. National Academies of Sciences, Engineering, and Medicine set an Adequate Intake for total water. “Total” means water from food and all drinks combined — not just glasses of plain water.
Men
3.7 liters a day
Total water from food and all drinks. The drinks part alone is about 13 cups a day.
Women
2.7 liters a day
Total water from food and all drinks. The drinks part alone is about 9 cups a day.
Where that water comes from
About 20% of daily water usually comes from food. The other roughly 80% comes from drinks. That is how the totals above work out to about 13 cups of drinks a day for men and 9 cups a day for women.
And “drinks” really does mean all of them. Milk, juice, tea, and coffee count toward the day’s fluid — it does not have to be plain water.
Source: U.S. National Academies figures, via Mayo Clinic.
What about “8 glasses a day”?
The famous rule says to drink eight 8-ounce glasses of water a day:
8 glasses × 8 ounces = 64 ounces ≈ 1.9 liters
That is easy to remember, and it is a reasonable goal for many people. But it is not based on a scientific study — it is a handy rule of thumb, not a finding from research.
Notice, too, that 1.9 liters is less than the total-water figures above. That is because the official figures also count the water in your food and in drinks other than water.
Source: Mayo Clinic.
When you need more fluid
Some days and some situations call for more than usual. You need more fluid when:
- You exercise
- It is hot
- You have a fever
- You are vomiting or have diarrhea
- You are pregnant
- You are breastfeeding
Source: Mayo Clinic.
Can you drink too much?
More is not always better. It is uncommon, but drinking far more water than the body can handle can dilute the sodium in the blood, which can be dangerous. Endurance athletes — people who exercise hard for long stretches and drink very large amounts — are the usual case.
For most people in normal daily life, this is not a worry. It is one more reason thirst, not force, is the everyday guide.
Source: Mayo Clinic.
One important exception
Some people — for example, people with certain heart or kidney conditions — are given a fluid limit by their doctor.
If a clinician has given you a limit, follow that limit — not this page. Your doctor’s instruction for you comes first, always.
A simple, healthy swap
The CDC points out that choosing water over sugary drinks is one of the simplest healthy swaps you can make.
Water also does quiet, everyday work in the body: it helps the body keep a normal temperature, cushion the joints, and get rid of waste.
Source: CDC.
Unit converter: cups, ounces, milliliters, liters
Guidance comes in different units depending on where you read it. Type an amount in any box and the other boxes fill in. This tool only changes units — like inches to centimeters — it does not say how much anyone should drink.
1 cup = 8 fluid ounces = about 237 milliliters. The converter runs right here in the page.
Remember: General information only — not medical advice. How much you need depends on your body, your health, and your day. For your situation, talk to a doctor or pharmacist.
Sources
- Mayo Clinic — “Water: How much should you drink every day?”
- CDC (Centers for Disease Control and Prevention) — drinking water and healthy beverages guidance
- U.S. National Academies of Sciences, Engineering, and Medicine — Dietary Reference Intakes for water
Facts checked November 2026.