Daily water intake calculator
Enter your weight, how long you are training today and the climate you train in. The calculator splits your daily water need into three lines — what your body needs at rest, the sweat you have to replace after training, and what the heat adds — then shows you how to spread it over the day.
About 30 seconds
How the estimate is built
Baseline = 35 mL per kg of body weight per day, the D-A-CH reference value for adults aged 19 to 50, never below the EFSA adequate intake of 2.5 L a day for men and 2.0 L for women. This is total water: drinks, the water in your food, and the water your metabolism produces.
Training = predicted sweating rate × session length. The rates come from the ACSM's published table for running at 8.5 to 15 km/h; the calculator reads the 10 km/h row, a moderate continuous endurance session, and interpolates it for your body weight.
Heat = the gap between the warm row (28 °C) and the temperate row (18 °C) of that same table, about 0.1 L per hour of training. It only covers the extra sweat during the session: at rest in the heat you lose more too, and that part is not quantified here.
For a 70 kg adult, the baseline need is about 2.5 L of total water a day. A 60-minute session adds roughly 0.8 L of sweat to replace, for a total of about 3.3 L.
Sources: EFSA, Scientific Opinion on Dietary Reference Values for water (EFSA Journal, 2010); D-A-CH / DGE reference values for water intake; American College of Sports Medicine, Position Stand "Exercise and Fluid Replacement" (Med Sci Sports Exerc, 2007), Table 4.
This is an estimate, not medical advice. Sweat rates vary fourfold between two people doing the same session, so treat the figure as a starting point. Thirst and the colour of your urine — pale straw rather than dark — remain the practical guides, and weighing yourself before and after a session is the only way to know your own sweat rate.
Your breakdown, line by line
| Line | Volume |
|---|---|
| Baseline at rest (35 mL/kg, floored at the EFSA adequate intake) | — L |
| Sweat replaced after training (— L/h × — h) | — L |
| Extra sweat from the heat (28 °C instead of 18 °C) | — L |
| Total water per day | — L |
| Of which to actually drink (food and metabolism cover the rest) | — L |
Sweating rates at 70 kg
| Session | Temperate (18 °C) | Hot (28 °C) |
|---|---|---|
| Easy run, 8.5 km/h | 0.65 L/h | 0.75 L/h |
| Steady run, 10 km/h | 0.79 L/h | 0.89 L/h |
| Fast run, 12.5 km/h | 1.02 L/h | 1.12 L/h |
| Very fast run, 15 km/h | 1.25 L/h | 1.36 L/h |
How to spread it over the day
Before: 350 to 490 mL sipped slowly, at least four hours before the session — the ACSM recommends 5 to 7 mL per kg of body weight. If your urine is dark, or you have not passed any, add a little more two hours out.
During: aim for around 0.79 L per hour, which is your estimated sweating rate. The goal is simply to finish within 2% of your starting body weight. Drinking far more than you sweat is a real risk, not a bonus.
After: if you have to be fully recovered fast — a second session the same day, a competition tomorrow — drink about 1.5 L for every kilogram lost during the session. Over a normal day, replacing what you lost is enough.
The rest of the time: spread it across your meals and between them rather than in two big glasses. Roughly a third of your total water comes from food, so the amount you actually have to drink is always lower than the total above.
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Frequently asked questions
Here are some common questions about the daily water intake calculator.
EFSA sets the adequate intake of total water at 2.5 L a day for adult men and 2.0 L for adult women, in a moderate climate with moderate activity. The D-A-CH reference values say the same thing per kilogram: about 35 mL per kg per day, so roughly 2.5 L at 70 kg. Remember that this is total water, and about a third of it already comes from your food.
Enough to replace what you sweat. The ACSM's predicted sweating rates for running sit between 0.5 and 1.0 L per hour for most adults in a temperate climate, and higher in the heat. A one-hour session at 70 kg costs about 0.8 L. Weighing yourself before and after is the only way to know your own rate: each kilogram lost is roughly one litre of sweat.
Yes, though less than most people assume for a single session. In the ACSM table, going from 18 °C to 28 °C adds about 0.1 L per hour of running. Humidity, direct sun, full kit and long sessions push that much higher — American football players in full pads have been recorded losing more than 8 L in a day.
Yes. Drinking far more than you sweat, especially during a long event, dilutes the sodium in your blood and can cause exercise-associated hyponatraemia, which is dangerous. That is why the guidance is to match your losses rather than to drink as much as possible. Thirst and pale-straw urine are reliable everyday signals; this calculator is an estimate, not medical advice.
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