Daily water intake in litres, glasses and fl oz from body weight, exercise and climate, compared with EFSA and US adequate intakes.
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The estimate comes to about 2.75 L of fluid a day, roughly 11 glasses of 250 mL: 2.45 L for your body weight, 0.3 L for 30 minutes of exercise.
30–35 mL per kg per day is the clinical rule of thumb; the Nutrients 2020 review cites 30 mL/kg for older adults.
0.6 L/h is the midpoint of the ACSM 2007 suggestion of 0.4–0.8 L/h; heavy sweaters in heat need more.
IPCS reference values rise from about 2 L/day in normal conditions to 2.8–3.4 L/day at 32 °C (WHO/SDE/WSH/03.02, Table 1); hot adds 1.0 L, warm 0.5 L.
Glasses are rounded up. EFSA and NASEM figures count water from food too (about 20–30 % of the total), so they are context, not the same quantity.
The estimate adds three parts: a baseline of 35 mL per kg of body weight (30 mL/kg is often used for adults over 65), 0.6 L for each hour of exercise, which is the midpoint of the American College of Sports Medicine's 0.4–0.8 L per hour, and 0.5 or 1.0 L for a warm or hot climate. The total is shown in litres, 250 mL glasses and US fluid ounces.
People use it to set a daily drinking goal. A 70 kg adult who exercises for 30 minutes in a temperate climate gets 2.45 + 0.3 = 2.75 L, about 11 glasses. The European Food Safety Authority's adequate intake is 2.5 L for men and 2.0 L for women, and the US National Academies' is 3.7 L and 2.7 L; both count water from food as well as drinks.
These are population figures for healthy adults. Sweat rates vary widely, and heart failure, kidney disease and some medicines call for a fluid limit set by a clinician.
A common estimate is 35 mL of fluid per kilogram of body weight, plus 0.6 L for each hour of exercise and 0.5 to 1.0 L in hot weather. For a 70 kg adult who exercises 30 minutes in mild weather, that comes to 2.45 + 0.3 = 2.75 L, about eleven 250 mL glasses.
The rule is meant for healthy adults. Heart failure, kidney disease and some medicines call for a fluid limit set by a clinician, and that limit replaces any formula.
Take a 180 lb adult who trains for an hour on a hot day. 180 lb is 81.65 kg, so the baseline is 81.65 × 35 = 2,858 mL. Adding 0.6 L and 1.0 L gives 4.46 L, about 151 US fl oz.
The default is the 70 kg adult above. The older baseline example is 60 kg at 30 mL/kg with no exercise in mild weather. The warm exercise example is 55 kg at 35 mL/kg with 90 minutes of exercise in warm weather.
| Scenario | Modeled liters per day | Whole 250 mL glasses |
|---|---|---|
| Default | 2.75 | 11 |
| Older baseline example | 1.80 | 8 |
| Warm exercise example | 3.32 | 14 |
Liters are rounded to two decimals with halves going to the even digit, so the warm example's 3.325 L appears as 3.32. Glasses are always rounded up: 1.8 L is 7.2 glasses, counted as 8, and 3.325 L is 13.3, counted as 14.
A 250 mL glass holds 8.45 US fl oz, a little more than an 8 oz cup.
The European Food Safety Authority sets adequate intakes of total water at 2.5 L a day for men and 2.0 L for women. The US National Academies set 3.7 L and 2.7 L, of which about 3.0 L and 2.2 L typically come from drinks.
Both figures count water from food, which usually supplies 20 to 30 % of the total. A drinking estimate and an adequate intake are therefore different quantities, and needs rise with heat, exercise, pregnancy and breastfeeding.
Coffee and tea count. The National Academies concluded in 2005 that caffeinated drinks contribute to total water intake much as other drinks do.
The "eight glasses a day" rule, eight 8 oz glasses or 1.9 L, is a rule of thumb. A 2002 review by Heinz Valtin found no scientific evidence behind it.
Weigh yourself before and after a session: each kilogram lost is about 1 L of sweat. The ACSM aims to keep the loss under 2 % of body weight, which is 1.4 kg for a 70 kg adult.
Drinking well beyond your sweat losses carries its own risk, exercise-associated hyponatremia. More than about 1 L an hour can dilute blood sodium.
When a formula gives more than about 6 L a day, drink to replace measured sweat loss instead of aiming for the formula total.
For the energy side of a workout, use the calories burned calculator. To plan the rest of your diet, the protein intake calculator works from body weight in the same way.
Checked against: 70 × 35 / 1000 + 0.6 × 30/60 = 2.75 L; 2.75 / 0.25 = 11 glasses
Checked against: 180 × 0.45359237 × 35 / 1000 + 0.6 + 1.0 = 4.457632 L = 150.73 US fl oz
Checked against: 60 × 30 / 1000 = 1.8 L; 1.8 / 0.25 = 7.2, rounded up to 8
Checked against: 55 × 35 / 1000 + 0.6 × 1.5 + 0.5 = 3.325 L
Reference values count total water from drinks and food. EFSA sets adequate intakes of 2.5 L a day for men and 2.0 L for women; the US National Academies set 3.7 L and 2.7 L, of which about 3.0 L and 2.2 L typically come from drinks. These are population averages for healthy adults in temperate conditions, and needs rise with heat, exercise, pregnancy and breastfeeding.
It is a rule of thumb, not a guideline. Eight 8 oz glasses make 1.9 L, and a 2002 review by Heinz Valtin in the American Journal of Physiology found no scientific evidence behind the rule. The figure falls between EFSA's adequate intakes of total water for women (2.0 L) and men (2.5 L), which already include the 20–30 % that usually comes from food.
The American College of Sports Medicine's 2007 position stand gives about 0.4 to 0.8 L per hour as a starting point, adjusted to sweat rate, with the aim of keeping body weight loss under 2 %. Weighing before and after a session shows the loss: each kilogram lost is about 1 litre of sweat. Drinking well beyond sweat losses risks exercise-associated hyponatraemia.
Yes. The US National Academies' 2005 reference intakes for water concluded that caffeinated drinks contribute to total water intake much as non-caffeinated drinks do, because moderate caffeine does not cause a net loss of body water over the day. Food counts too: it supplies about 20 % of total water intake in the US diet.
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
This calculator includes 4 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.