How does the water intake calculator work?
- Choose your sex, and whether you are pregnant or breastfeeding, which adds a little.
- Pick your units: liters, US cups or fluid ounces.
- Read your daily range. It shows total water from drinks and food, and roughly how much of that comes from drinks.
- Optional: check your sweat rate. Weigh yourself before and after a workout, add what you drank, and the calculator shows how much you sweated and what share of your body weight you lost.
Nothing you enter is stored.
How much water do you need a day?
Official figures for healthy adults fall in a fairly narrow band. They count total water, from drinks and from the water in food.
| Women | Men | |
|---|---|---|
| European Food Safety Authority, adequate intake | 2.0 liters | 2.5 liters |
| US Institute of Medicine, average for well-hydrated adults | 2.7 liters | 3.7 liters |
The European Food Safety Authority states that "a daily intake of 2.0 litres is considered adequate for women and 2.5 litres for men." Its full scientific opinion notes these figures include drinks and food and apply to moderate temperatures and moderate activity.
In the US, the Institute of Medicine found that women who appear to be adequately hydrated consume "approximately 2.7 liters (91 ounces) of total water" a day, and men "approximately 3.7 liters (125 ounces)". The calculator shows the span between the two as your range.
Do you really need eight glasses of water a day?
There is no official eight-glass rule. The chair of the US panel put it plainly: "We don't offer any rule of thumb based on how many glasses of water people should drink each day because our hydration needs can be met through a variety of sources in addition to drinking water."
About 80 percent of the water you take in comes from drinks and "the other 20 percent is derived from food." Tea, coffee, milk and juice count toward the drinks share, as does the water in fruit, vegetables, soups and cooked grains.
Is thirst a good guide to how much to drink?
For most healthy people, yes. The US report concluded that "The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide." The numbers above describe what well-hydrated people take in, not a target you need to measure out glass by glass.
Thirst is a less reliable signal during long or hard exercise, especially in the heat, which is where measuring your sweat helps.
How much more water do you need when you exercise or it is hot?
More, by an amount that varies a lot between people. The US report notes that "those who are very physically active or who live in hot climates may need to consume more water."
The National Athletic Trainers' Association says adult sweat rates during exercise "can range from 0.5 to 4.0 L/h", which is why a single number does not work. Its advice is to measure your own:
- Weigh yourself just before exercise, in as little clothing as possible.
- Note how much you drink during the session.
- Weigh yourself again straight after.
- Sweat lost equals weight lost plus what you drank, counting each kilogram as about a liter.
During exercise, the association's goal is to "not allow more than a 2% body mass loss", because performance drops beyond that point. Replacing what you lose is the aim, not drinking well beyond it. The calculator flags a loss of 2% or more.
Do pregnancy and breastfeeding change how much water you need?
Yes, a little. The European Food Safety Authority's opinion adds about 300 mL a day in pregnancy, in line with the extra energy needed, and about 700 mL a day while breastfeeding. The calculator adds these when you select them.
Who should not use these numbers?
- People told to limit fluids, for example because of heart failure or kidney disease. Follow your doctor's advice instead.
- Children and teenagers, whose reference values are different.
- Endurance athletes in long events, who need a personal plan based on their own sweat rate and the conditions.
For what to eat alongside it, see the anti-inflammatory plate and how much added sugar and salt you are eating. If you drink coffee to stay alert, the caffeine calculator shows how much is left at bedtime. The full guide to nutrition covers the rest.
