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How your daily water target is calculated

This calculator estimates a sensible daily fluid goal from your body weight, then adjusts it for the things that genuinely change how much water you lose: physical activity, climate, and pregnancy or breastfeeding. The core figure comes from a widely used rule of thumb — roughly 35 millilitres of water per kilogram of body weight. From that baseline the tool adds fixed amounts for the factors that increase your needs. Everything runs in your browser; nothing you enter is uploaded or stored anywhere.

The exact formula, step by step

Here is precisely what the tool adds up. If you enter weight in pounds it first converts to kilograms (×0.4536), then:

Sedentary activity, temperate or cold climate, and no special conditions add nothing — they are the baseline. The result is shown three ways: total litres per day, the equivalent number of 250 ml glasses (rounded up), and a breakdown so you can see exactly where each part of the number came from.

A worked example

Take a 70 kg person who exercises a few times a week (moderate), lives in a temperate climate, with no special conditions. The base is 70 × 35 = 2,450 ml. Moderate activity adds 500 ml, for a total of 2,950 ml — about 2.9 litres, or 12 glasses of 250 ml. Now make that same person pregnant and living somewhere hot: add 300 ml for climate and 300 ml for pregnancy, reaching 3,550 ml, roughly 3.6 litres or 15 glasses. The breakdown panel spells out each line so the jump is transparent rather than a mystery number.

The reminder schedule

Once you have a target, the tool can spread it across your waking hours. Enter your wake time and bedtime, and it divides your total intake evenly across the hours you are awake, generating an hourly schedule with a rough per-hour amount. For the 2,950 ml example above over a 16-hour day, that is about 184 ml per hour — a small, steady sip rather than gulping a litre at once, which your kidneys handle far better.

Practical tips

• Count all fluids and watery foods. Tea, coffee, milk, and juicy fruits and vegetables all contribute to hydration. Plain water does not have to supply the entire target.
• Spread it out. The body can only absorb so much at a time, so steady intake across the day beats a few large boluses. That is the logic behind the hourly schedule.
• Stop drinking heavily right before bed. Front-load earlier in the day so you are not woken by a full bladder; the schedule's last entry is intentionally a "before bed" marker rather than a large amount.
• Use urine colour as a real-time check. Pale straw means well hydrated; dark yellow means drink more. It is a more responsive signal than any formula.
• Adjust on the day. A hard workout, a hot afternoon, or illness raises your needs beyond the standing estimate. Treat the number as a starting point, not a ceiling.

Important limits and common mistakes

• This is an estimate, not medical advice. The 35 ml/kg rule and the fixed add-ons are reasonable general guidance, not a clinical prescription. If you have kidney, heart, or liver conditions, or you are pregnant, your doctor may set a very different target — follow that instead.
• More is not always better. Drinking far beyond your needs can dilute blood sodium (hyponatraemia), which is dangerous. The goal is adequate, steady hydration, not maximising intake.
• Thirst lags behind dehydration. By the time you feel thirsty you may already be mildly dehydrated, so a schedule helps you stay ahead rather than reacting late.
• Don't double-count caffeine fears. Moderate coffee and tea hydrate you on balance despite their mild diuretic effect; you do not need to "cancel them out" with extra water.
• Weight units matter. Make sure the kg/lbs selector matches what you typed — entering pounds while the unit says kg roughly doubles the result.

Used as a guide, this calculator turns a vague "drink more water" into a concrete daily number and an easy hourly rhythm — and because it runs entirely in your browser, your personal details never leave your device.