How much water should I drink?

How much water do you need?

You areA womanA manExercise a day, in minutesThe weatherMild or coolHot, or you sweat a lot

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How the calculator works

Europe's food safety experts set an adequate intake of total water of 2.0 litres a day for women and 2.5 litres for men. That counts water from drinks and from food, in a moderate climate with moderate activity ¹. In US surveys, about a fifth of the water people took in came from food ², so the calculator takes 80% of the European figure as what to drink: about 1.6 litres (54 oz) for women and 2.0 litres (68 oz) for men. It then adds 500 ml (17 oz) for every hour of exercise and, if you choose, 500 ml for a hot day. Nothing you enter leaves your browser.

	Women	Men
Europe: total water, drinks and food 1	2.0 L	2.5 L
US: total water, drinks and food, young adults 2	2.7 L	3.7 L
US: of which from drinks 2	2.2 L	3.0 L
This calculator's starting point for drinks	1.6 L (54 oz)	2.0 L (68 oz)

Neither figure is a strict minimum. Europe's is based largely on what healthy people actually take in ¹, and the US experts found that, day to day, thirst and drinks with meals keep most people well hydrated ².

Exercise and hot weather

Sweat varies hugely from person to person. Distance runners often sweat about 0.7 litres an hour (women) to 1.0 litre an hour (men), and people working in desert heat 0.3 to 1.2 litres an hour ². Sports scientists suggest weighing yourself before and after exercise to learn your own sweat rate, and aiming not to lose more than 2% of your body weight. They also warn that drinking more than you sweat is the main cause of dangerously low blood sodium during long events ³. That's why the calculator adds a cautious 500 ml an hour, below most measured sweat rates, and a cautious 500 ml for a hot day: let thirst guide the rest.

Coffee and tea count

You don't need to count coffee as a loss. In 50 men who usually drank coffee, four cups a day for three days kept them as well hydrated as the same amount of water ⁴. Plain tea and black coffee count, and neither breaks your fast. See does tea break a fast?.

Water while you fast

When insulin falls during a fast, your kidneys let go of more sodium and the water that travels with it ^5,6. Even losing just over 1% of your body weight in water brought more headaches, fatigue and low mood in one study ⁷. If you get a dull headache while fasting, drink a large glass of water first: for most people, a water-deprivation headache eases within 30 minutes ⁸. Pale yellow urine is a simple sign you're drinking enough.

Fasting Tracker has a daily water goal, 2,000 ml or 64 oz to start, which you can change to suit you. Log a glass from the app, a widget or your Apple Watch and watch the day fill up.

Sources

1. EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA). Scientific Opinion on Dietary Reference Values for water. EFSA J. 2010;8(3):1459. doi.org/10.2903/j.efsa.2010.1459
2. Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: The National Academies Press; 2005. doi.org/10.17226/10925
3. American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-90. doi.org/10.1249/mss.0b013e31802ca597
4. Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014;9(1):e84154. doi.org/10.1371/journal.pone.0084154
5. DeFronzo RA, Cooke CR, Andres R, et al. The effect of insulin on renal handling of sodium, potassium, calcium, and phosphate in man. J Clin Invest. 1975;55(4):845-55. doi.org/10.1172/JCI107996
6. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
7. Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. J Nutr. 2012;142(2):382-8. doi.org/10.3945/jn.111.142000
8. Blau JN, Kell CA, Sperling JM. Water-deprivation headache: a new headache with two variants. Headache. 2004;44(1):79-83. doi.org/10.1111/j.1526-4610.2004.04014.x