How much water should you actually drink a day?

Where does the 35ml per kg baseline come from?

It's a commonly cited general hydration guideline for healthy adults — roughly 35ml of water per kilogram of body weight per day, before accounting for exercise, climate, or individual health factors. It's a reasonable starting estimate, not a clinically precise requirement; actual needs vary by individual, and this figure includes water from all sources, not just plain drinking water.

Does this include water from food and other drinks?

The baseline figure represents total daily water needs from all sources combined, including food — fruits and vegetables in particular are often 80-95% water — and other beverages, not just plain water you deliberately drink. Most people get roughly 20% of total water intake from food, so the number shown here is a reasonable target for total fluid intake, not a mandate to drink that exact amount as plain water alone.

Should I drink more if I have a medical condition?

This is a general wellness estimate, not medical guidance — certain conditions (kidney disease, heart failure, and others) require fluid intake to be managed specifically by a doctor, sometimes with a lower limit than a general calculator would suggest. If you have a relevant medical condition or take medication affecting fluid balance, follow your doctor's specific guidance over this general estimate.

Worked example: a 70kg person exercising 30 minutes a day in a hot climate: base need is 2,450ml (70 × 35ml), plus 350ml for exercise, plus 245ml (10%) for hot climate — a daily target of about 3,045ml, or roughly 3 liters.

Where the number comes from

Bodyweight-based estimates typically use around 30-40ml per kg per day. At 35ml/kg a 70kg adult lands near 2,450ml — about 2.45 litres. That is a starting point, not a requirement, and it already includes water from all sources rather than just what you drink.

The familiar "eight glasses a day" has no strong evidential basis. It is a reasonable ballpark for many adults and a poor fit for anyone notably larger, smaller, more active or in a different climate.

Food and other drinks count

Roughly 20-30% of most people's water intake comes from food. Fruit and vegetables are largely water — cucumber, watermelon, tomatoes and lettuce are all above 90%. Soup, yoghurt and rice contribute meaningfully too.

Tea and coffee count as well. The idea that caffeine is so dehydrating it produces a net loss doesn't hold at normal intakes; the fluid outweighs the mild diuretic effect. Alcohol is genuinely different and does increase fluid loss.

What raises the requirement

• Exercise — add roughly 350-700ml per hour, more in heat. Weighing yourself before and after a session is the accurate method: each kilogram lost is about a litre of fluid.
• Heat and humidity, and altitude above about 2,500m
• Pregnancy and breastfeeding — breastfeeding in particular adds substantially
• Fever, vomiting or diarrhoea
• High-protein or high-fibre diets, which need more water to process

Thirst, urine colour, and overdoing it

For most healthy people thirst is a reliable guide, and pale straw-coloured urine is the simplest practical check — dark yellow suggests drinking more, completely clear suggests you can ease off. Thirst becomes less reliable with age, which is why older adults are more prone to dehydration.

Drinking too much is possible and occasionally dangerous. Hyponatraemia — sodium diluted below safe levels — has occurred in endurance athletes who drink heavily without replacing electrolytes. If you are sweating for several hours, sodium matters as much as volume.

Anyone with kidney disease, heart failure, or on diuretics should follow medical advice rather than a general formula; fluid restriction is sometimes the correct treatment. For activity-adjusted energy needs, see the TDEE Calculator.