Hong Life CalculatorsUnderstanding the daily water intake calculator — concept and principle
How much you should drink depends on your size, how active you are and how hot it is. This calculator gives a daily total water target based on body weight with extra for exercise and hot weather, then the part to drink and the number of 250 mL glasses.
How it is calculated
- Total water per day, food included (L) = weight (kg) × 0.033 + exercise minutes ÷ 30 × 0.35 + hot × 0.5
- From drinks (L) = total × 0.8, because food usually supplies about 20%
- Glasses of drinks = from drinks ÷ 0.25, rounded
Weight is in kilograms, exercise in minutes per day, and hot is 1 for hot weather or 0 otherwise.
The principle
At rest, the body loses water through urine, breathing, skin and stool, and these losses scale roughly with body size, giving the rule of thumb of about 30–35 mL per kilogram a day. Exercise adds sweat losses, which commonly run around 0.5–1 L per hour in moderate conditions, so about 350 mL per 30 minutes is a middle estimate. Heat raises sweating even at rest, so half a litre is added. Official reference values are expressed differently: the European Food Safety Authority sets adequate total water intake at 2.0 L for women and 2.5 L for men a day, including water from food, which typically supplies about a fifth of the total. This calculator's weight-based total is also food-inclusive, but it tends to be higher and more generous for larger or active people.
Worked example
- Default inputs: 75 kg, 45 minutes of exercise, normal weather (0).
- Base = 75 × 0.033 = 2.475 L.
- Exercise = 45 ÷ 30 × 0.35 = 0.525 L.
- Heat = 0 × 0.5 = 0 L.
- Total water = 2.475 + 0.525 = 3 L a day, food included.
- From drinks = 3 × 0.8 = 2.4 L, and 2.4 ÷ 0.25 = 9.6, about 10 glasses of 250 mL.
| Weight (kg) | Total water per day, food included (L) |
|---|---|
| 18.8 | 1.14 |
| 37.5 | 1.76 |
| 56.2 | 2.38 |
| 75 | 3 |
| 93.8 | 3.62 |
| 112 | 4.24 |
| 150 | 5.47 |
How to read the result
Treat the figure as a guide, not a quota. Pale-yellow urine and rarely feeling thirsty are good everyday signs of adequate hydration. Coffee, tea, milk and soups all count toward fluid. If you sweat heavily or train for long periods, measure your sweat rate for a more precise plan. Drinking far beyond your needs brings no benefit and, in extreme cases during long events, can lower blood sodium.
Common mistakes
- Counting only plain water and forgetting other drinks and water-rich foods.
- Forcing large volumes at once instead of spreading drinks through the day.
- Ignoring medical fluid limits for heart or kidney conditions.
Frequently asked questions
Do coffee and tea dehydrate you?
At usual intakes, no; the mild diuretic effect is smaller than the fluid they provide.
Can you drink too much water?
Yes. Drinking very large amounts quickly, especially during long endurance events, can cause hyponatraemia, a dangerous fall in blood sodium.
Sources: EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA Journal, 2010. · Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2005.