Water Intake Calculator

Why generic hydration advice falls short

You've probably heard the 'eight glasses a day' rule, but that guideline was never based on individual body size, activity level or climate — it's a rough average that doesn't fit most people particularly well. A 50 kg office worker in a mild climate has very different hydration needs from a 95 kg construction worker in a hot, humid environment. This calculator starts from your body weight and adjusts for activity and climate to give a more personalised daily target.

The science behind the estimate

A commonly used clinical starting point is roughly 30–35 millilitres of water per kilogram of body weight per day for a sedentary adult in a temperate climate. We add extra for moderate or high activity levels, since sweat losses during exercise need to be replaced on top of baseline needs, and add further for hot climates where passive sweat loss increases even without exercise. This mirrors the general approach used by sports dietitians when setting hydration guidelines for athletes and active individuals.

Water from food and other drinks

This target refers to total fluid intake, and roughly 20% of most people's daily water needs are actually met through food, particularly fruits, vegetables and soups. Tea, coffee and other beverages also contribute, though water remains the simplest, calorie-free way to meet the bulk of your target. You don't need to hit the number exactly every single day — thirst, urine colour (pale yellow is a good sign) and energy levels are useful everyday indicators alongside this estimate.

When to adjust your target upward

Illness with fever, vomiting or diarrhoea, pregnancy and breastfeeding, high altitude, and long endurance exercise all increase fluid needs beyond this baseline calculation. If you fall into any of these categories, treat this calculator's result as a floor rather than a ceiling, and consider discussing specific hydration needs with a healthcare provider, especially during pregnancy or illness.