Daily Water Intake Calculator
Your daily water from EFSA and IOM reference intakes, plus exercise — and a glass counter.
Both references for you
| Reference | Total water | From drinks | From food |
|---|
Measure your sweat rate (weigh-in)
Weigh yourself just before and just after a typical session, with the same minimal clothing, after towelling dry. Note what you drank and any urine you passed in between.
Today’s glasses
Kept only in this browser and starts again at midnight. Nothing is sent anywhere.
For general information only, not medical advice. Consult a qualified health professional for advice about your situation.
About the Daily Water Intake Calculator
Official reference values for water are adequate intakes of total water — from drinks and from food — based on what healthy people actually drink. The two most used come from EFSA (the European Food Safety Authority, 2010) and the US/Canadian IOM (2005, now the National Academies). They differ: EFSA suggests 2.0 L a day for women and 2.5 L for men, the IOM 2.7 L and 3.7 L. The IOM values are the median intakes in a US survey, while EFSA combined observed intakes with a desirable urine concentration.
This calculator shows both for your age, sex, pregnancy or breastfeeding, separates the part that usually comes from food, and adds the water you lose as sweat during exercise — from an athlete average for your sport, or from your own weigh-in. An optional glass counter keeps today’s tally in your browser. If a doctor has given you a fluid limit, follow that instead.
How to use it
- Choose your age group and sex, and pregnant or breastfeeding if that applies. Body weight is needed only for babies under 6 months.
- If you exercise today, enter the minutes and pick the sweat rate of a similar sport — or tick in your own measured rate.
- Read how much to drink today and how much water food usually supplies. Switch between the EFSA and IOM references; the table shows both.
- To measure your sweat rate, weigh yourself before and after a session, enter what you drank, and press Work out my sweat rate, then Use it above.
- Use Add a glass through the day. The tally is kept in millilitres, so you can switch glass size at any time; it is stored only in this browser and starts again at midnight.
Examples
EFSA: 2.0 L total water, about 1.6 L from drinks (≈ 6½ glasses of 250 mL) · IOM: 2.7 L total, 2.2 L from drinks
EFSA 2.5 L total (≈ 2.0 L from drinks) + 1.28 L sweat at the endurance-athlete average = about 3.3 L to drink
EFSA 2.3 L total water (+300 mL) · IOM 3.0 L total, 2.3 L from drinks
Sweat rate 1.30 L per hour; body weight −1.1% (under the 2% ACSM limit)
The reference intakes
Adequate intakes of total water (drinks plus the water in food), litres a day:
- Adults: EFSA women 2.0, men 2.5 (the same for older adults); IOM women 2.7, men 3.7 (from age 19).
- Pregnancy: EFSA 2.3 (+300 mL); IOM 3.0. Breastfeeding: EFSA 2.7 (+700 mL); IOM 3.8.
- Teenagers: EFSA treats 14 and over as adults; IOM 14–18 years: boys 3.3, girls 2.3. 9–13 years: EFSA boys 2.1, girls 1.9; IOM boys 2.4, girls 2.1.
- Children: 4–8 years EFSA 1.6, IOM 1.7; 2–3 years 1.3 on both; in the second year of life EFSA 1.1–1.2 (IOM 1.3 for 1–3 years); 7–12 months EFSA 0.8–1.0, IOM 0.8.
- Babies 0–6 months: all of it from breast milk — EFSA 100–190 mL per kg a day, IOM 0.7 L a day.
In the US survey behind the IOM values, food supplied about 19% of adults’ total water (Table S-1); the IOM tables give the drinks/food split directly. EFSA publishes totals only, so for EFSA the split here uses the same IOM food shares — an estimate, marked with an asterisk. Tea, coffee, milk and juice all count as drinks; the IOM notes that the diuretic effect of caffeine appears to be transient.
Exercise, heat and sweat
Both references assume moderate temperatures and moderate activity. During exercise the extra water you need is essentially the sweat you lose, and sweat rates vary several-fold between people. As a starting point the calculator offers the average whole-body sweating rates measured in 1,303 athletes (Barnes et al.): endurance sports 1.28 ± 0.57 L per hour, American football 1.51 ± 0.70, basketball 0.95 ± 0.42, soccer 0.94 ± 0.38 and baseball 0.83 ± 0.34.
The ACSM recommends customised plans because of this variation and suggests estimating your own rate by weighing before and after exercise. Corrected for what you drink and pass, as sweat-testing studies do: sweat (L) = weight before − weight after (kg) + fluid drunk (L) − urine passed (L), taking 1 kg as 1 L. Aim not to lose more than 2% of your body weight during a session — and not to gain weight, which means you drank more than you sweated. In hot climates daily sweat losses often exceed several litres: in studies summarised in the IOM report (chapter 4), active soldiers without air conditioning lost on average 4.9 L of sweat a day in the desert and 2.3 L in the tropics, and EFSA notes that losses under extreme heat and exercise can reach about 8 L a day. So measure your sweat rate in the heat rather than relying on an average.
The sport averages come from athletes, so for babies and children under 9 the calculator adds nothing for exercise: offer them drinks regularly when they are active or it is hot. For 9–13-year-olds a rate measured with the weigh-in is a better guide than an average.
When these amounts do not apply
- Fluid restriction: people with heart failure, kidney failure or on dialysis, advanced liver disease or low blood sodium may be given a daily fluid limit. Follow it; you can track against it with the glass counter’s own-goal option.
- Too much, too fast: healthy kidneys can excrete only about 0.7–1.0 L of water an hour (IOM 2005; EFSA 2010). Drinking far more than that — water-drinking challenges, or drinking well beyond your sweat losses during long events — can cause dangerously low blood sodium.
- Babies: the amounts for under-6-month-olds describe what breast milk provides, not water to give. Ask your doctor or health worker.
- Fever, diarrhoea and vomiting increase losses; follow medical advice and use oral rehydration solution where advised.
Sources
- EFSA NDA Panel, Scientific Opinion on Dietary Reference Values for water, EFSA Journal 8(3):1459, as summarised in EFSA, Dietary reference values for nutrients: Summary report (2017), section 3.5 and Table 2
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, Tables S-1 and S-2, and chapter 4, Water — sweat losses in hot climates
- American College of Sports Medicine; Sawka MN et al. Exercise and fluid replacement. Med Sci Sports Exerc 39:377-90
- Barnes KA et al. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes. J Sports Sci 37:2356-66
Limitations
- Adequate intakes are based on observed intakes of healthy groups, not on a measured requirement for you; many healthy people drink less or more.
- Sport sweat rates are averages for athletes; your own rate can be half or double, which is why the weigh-in is offered.
- Hot weather is not converted into a number, because the extra depends on your activity and sweat rate; measure it with the weigh-in.
- No exercise extra is worked out for babies and children under 9: the sweat rates offered are athletes’ averages.
- The glass counter is a simple tally kept in this browser’s storage; clearing site data or using another device starts it again.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
How much water should I drink a day?
For adults, EFSA suggests 2.0 L of total water a day for women and 2.5 L for men, of which roughly four-fifths usually comes from drinks — about 1.6 and 2.0 L. The IOM’s figures are higher (2.7 and 3.7 L total, 2.2 and 3.0 L from drinks). Add the sweat you lose when you exercise.
Is 8 glasses of water a day right?
It is a handy rule of thumb. Eight 250 mL glasses is 2 L of drinks: about EFSA’s total-water figure for women, and less than the 2.2 L from drinks in the IOM’s. Tea, coffee, milk and other drinks count, and food supplies some water too.
How much more water do I need when I exercise?
Roughly what you sweat. Athletes average about 0.8–1.5 L an hour depending on the sport, but individuals vary widely. Weigh yourself before and after a session: each kilogram lost, plus what you drank, is about a litre of sweat.
Do tea and coffee count towards water intake?
Yes. The IOM concluded that the diuretic effect of caffeinated drinks appears to be transient and that they contribute to total water intake.
Can you drink too much water?
Yes. Healthy kidneys can excrete only about 0.7–1.0 L of water an hour. Drinking much more than that, or far more than you sweat during long exercise, can lower blood sodium dangerously (hyponatraemia).
How much water should I drink when pregnant or breastfeeding?
EFSA adds 300 mL a day in pregnancy (2.3 L total water) and 700 mL while breastfeeding (2.7 L). The IOM gives 3.0 L and 3.8 L total, of which 2.3 L and 3.1 L from drinks.