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How Much Water Should I Drink?

From bodyweight and activity, counting food and coffee.

Inputs
kg
0.5hours/day
Climate
To drink
2.16 L
Total water needed
2.7 L
From food (about 20%)
0.54 L
In glasses (250 mL)
9
Added for exercise
0.25 L
total ≈ 35 mL per kg + 500 mL per hour of exercise

An estimate, not a prescription. Thirst is a reliable guide for most healthy adults, and urine colour is a better daily check than any calculation — pale straw is the target, and clear means you are ahead of what you need.

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Roughly 35 mL per kilogram of bodyweight, plus about 500 mL per hour of exercise. Around a fifth arrives in food, so a 70 kg person needs about 2.5 L total and roughly 2 L of it from drinks.

How to use Water Intake Calculator

  1. Enter your weight. The baseline scales with body mass.
  2. Add your exercise. Roughly half a litre per hour of activity.
  3. Set the climate. Heat and humidity raise losses substantially.

About estimating daily water intake

Daily water requirement varies with body size, activity, climate, diet and health, which is why every general figure is a starting estimate rather than a target. The most durable rule of thumb is around 30 to 40 millilitres per kilogram of bodyweight for a sedentary adult in a temperate climate, with roughly half a litre added per hour of exercise and more again in heat. The part most calculators leave out is that food supplies a substantial share. A mixed diet contributes around twenty percent of total water intake, and considerably more for someone eating a lot of fruit and vegetables — many of which are over ninety percent water. Prescribing the full requirement as fluid to drink therefore over-prescribes by roughly a fifth, which is one reason the familiar eight-glasses advice feels like a struggle. That advice appears to descend from a 1945 recommendation which specified about 2.5 litres a day and then noted that most of it comes from prepared food; the second half of the sentence is what got lost. It is also worth knowing the limit exists in both directions. Drinking far beyond what the kidneys can excrete dilutes blood sodium and causes hyponatraemia, which is genuinely dangerous and has killed endurance athletes who over-drank during events. For a healthy adult, thirst is a well-calibrated signal and urine colour is a better daily check than arithmetic.

Frequently asked questions

Is eight glasses a day correct?
It is a reasonable ballpark for an average adult and was never based on research. The figure appears to trace back to a 1945 recommendation that also said most of it comes from food — a sentence that got dropped somewhere along the way.
Does coffee count?
Yes. The diuretic effect of caffeine at normal intakes is far smaller than the volume of the drink, so tea and coffee contribute net water. The idea that they dehydrate you comes from studies using large caffeine doses in people who never drink it.
Can you drink too much water?
Yes, and it is dangerous. Drinking far more than the kidneys can excrete dilutes blood sodium, which causes hyponatraemia — occasionally fatal, and documented in endurance athletes who over-drink during long events. Several litres above need, quickly, is the risk zone.
How do I know if I am drinking enough?
Urine colour is the practical check: pale straw is about right, dark amber suggests more is needed, and consistently clear means you are drinking more than necessary. It is more informative than any formula because it reflects your actual state.
Should I drink more when unwell?
Usually yes. Fever, vomiting and diarrhoea all increase losses substantially, and rehydration solutions are more effective than plain water when salts are being lost too. This calculation assumes an ordinary healthy day.

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