Water intake calculator: how much water to drink a day

How much water should you drink a day

There is no strong evidence for any specific daily fluid target in healthy adults, and the eight-glasses rule that everyone repeats has no clear origin. This calculator uses roughly 35 ml per kilogram of body weight, a common clinical heuristic, with additions for exercise and heat. It is an estimate of a starting point, not a requirement to hit.

Thirst is a well-calibrated signal in healthy adults under 65, and it responds to the things a formula cannot know: what you ate, how humid it is, what you drank yesterday. Urine the colour of pale straw is a better check than any arithmetic on this page.

What counts toward daily water intake

Everything you drink, including tea and coffee. The diuretic effect of caffeine is real and smaller than the fluid the drink contains, so a cup of coffee is a net gain. Food supplies roughly a fifth of total water intake — more if you eat a lot of fruit, vegetables and soup — which is why the result separates total water from what you actually have to drink.

When daily water intake genuinely matters

• Endurance exercise in heat, where losses genuinely outrun thirst.
• Illness with fever, vomiting or diarrhoea.
• Older age. The thirst signal weakens, which is why dehydration is far more common in the elderly.
• Kidney stones, where a higher intake is one of the few interventions with good evidence.

Drinking too much water is a real risk

Exercise-associated hyponatraemia — dangerously diluted blood sodium — is rare but serious, and it is caused by drinking far beyond thirst during prolonged exercise. It has killed marathon runners. If you are going hard for several hours, replace sodium as well as water rather than only drinking more.

There is more on where the eight-glasses figure came from in the water article.