CalcKitYour body loses water all day — through breathing, sweating, and digestion — and even mild dehydration of just 1–2% of body weight can bring on headaches, fatigue, and poor concentration. Yet the most repeated advice on the internet, "drink eight glasses a day," was never designed around your body. It is a population average that can be off by 30–50% for your size. A weight-based formula fixes that: it gives you a personal daily water target in under a minute, using nothing but your bathroom scale.
The 30–35 ml per kg formula
The most commonly used personal hydration guideline is: drink 30–35 milliliters of water per kilogram of body weight per day. Body weight is the best single predictor of individual water needs because blood volume, metabolic rate, and muscle mass all scale with size — a 100 kg person genuinely needs more water than a 50 kg person, and this formula captures that automatically. Use 30 ml/kg if you are mostly sedentary in a cool climate, and 35 ml/kg if you are active, work outdoors, or live in a hot climate. To turn milliliters into liters, divide by 1,000; for standard 250 ml glasses, divide the milliliters by 250. Try your own numbers in our free Water Intake Calculator, which runs the same formula plus exercise adjustments instantly.
Worked example
Take someone weighing 70 kg with a desk job in a mild climate: 70 × 30 = 2,100 ml = 2.1 liters, or about eight and a half standard glasses. Now take the same person on a day they play football for an hour in the heat: start with the 35 ml/kg rate (2,450 ml) and add roughly 500–1,000 ml for the hour of exercise, giving a target of around 3–3.5 liters. That is nearly 70% more than the sedentary baseline — exactly the kind of difference the flat "8 glasses" rule misses entirely.
Why weight beats the 8-glasses rule
The 8-glasses rule (about 2 liters) has no strong scientific basis as a universal prescription. Large population guidelines, like those from the US National Academies or Europe's EFSA, publish averages for a "typical" adult — but averages hide enormous individual variation. Weight is the most practical single variable you can measure yourself: lean muscle tissue is about 75% water and turns over fluid constantly, bigger bodies run more metabolic activity, and blood volume scales with size. A target anchored to your weight is a personal baseline; a target anchored to a population average is a guess.
Adjust for exercise, heat, and illness
The formula gives you a resting baseline — real life pushes it higher. Add roughly 300–500 ml for every 30 minutes of exercise, and drink before you feel thirsty, since thirst is a lagging signal. Hot or humid climates add several hundred more milliliters of daily need, especially in Gulf-style heat where sweat losses soar. Fever, vomiting, and diarrhea all increase fluid requirements too. Keep in mind that roughly a fifth of your total water comes from food — fruit, vegetables, soups, and yogurt all count — so your drinking-water target sits slightly below your total intake need. And while coffee and tea do contribute to hydration for regular drinkers, plain water remains the best default.
Signs you are drinking too little
Your body gives early warnings before real dehydration sets in. The simplest daily check is urine color: pale straw yellow means you are on track; consistently dark yellow or amber means drink more. Other early signs include a dry mouth, a headache that appears in the afternoon, feeling unusually tired or foggy, and skin that snaps back slowly when pinched. If you notice several of these on the same day, add an extra glass or two and see whether the symptoms fade — that feedback loop is more reliable than any single formula.
Can you drink too much water?
Yes — more is not always better. Drinking many liters beyond thirst in a short time can dilute blood sodium to dangerous levels, a condition called hyponatremia, which is why marathon runners and challenge drinkers sometimes collapse. Treat your target as a sensible goal spread across the day, not a race: a glass with each meal plus a few in between gets most people to their number. One important exception — if you have kidney disease, heart failure, or are on a doctor-ordered fluid restriction, ignore generic formulas entirely and follow your healthcare provider's advice. This guide is general information, not medical advice.
Your 60-second hydration check
Do this once and you will have a target that fits your body for years. First, weigh yourself in kilograms. Second, multiply by 30 (sedentary day) or 35 (active day or hot climate) to get your milliliters. Third, add 300–500 ml per half hour of exercise planned that day. Finally, sanity-check with the urine-color test for a week and nudge the number up or down until it matches how you feel. That is the whole method — personal, evidence-based, and it takes less than a minute. Want it done for you? Our Water Intake Calculator gives you the answer in liters and glasses instantly.