Find out exactly how much water you should drink per day based on your body, activity, and lifestyle.
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Enter your body weight, age, biological sex, activity level, typical climate, and any special conditions (pregnancy or breastfeeding). Click Calculate and you'll instantly see a personalized daily water recommendation in liters, ounces, and cups — along with a visual breakdown of what's driving your number.
Water is the most essential nutrient your body needs, and most people are mildly dehydrated without realizing it. A 2% drop in body water can impair cognitive performance, reduce physical endurance by up to 10%, and cause headaches, fatigue, and poor concentration.
The old "8 glasses a day" rule is an oversimplification. A 55 kg woman who works at a desk in a cool office needs far less water than a 95 kg male construction worker in summer heat. A breastfeeding mother needs an extra ~700 mL/day just to produce milk. A marathoner training twice daily may need over 5 liters total.
Getting hydration right has tangible effects: better skin appearance, improved kidney function, healthier digestion, and regulated body temperature. Athletes who track hydration carefully can see measurable improvements in performance and recovery. Even for non-athletes, optimal hydration reduces kidney stone risk and helps maintain a healthy metabolism.
This calculator gives you a science-backed starting point — adjust based on how you feel, how much you sweat, and what your doctor advises.
Our formula uses a weight-based baseline modified by activity, climate, age, sex, and special conditions:
Formula: Daily Water (mL) = (35 × weight_kg × activity_factor) + climate_adj + sex_adj + age_adj + condition_adj
This aligns with guidance from the EFSA (European Food Safety Authority) and the U.S. National Academies of Medicine adequate intake values.
No — the "8×8" rule (eight 8-oz glasses) is a rough average that doesn't account for body size, activity, climate, or health conditions. A small sedentary person in a cool climate may only need 1.5–2 liters, while an active athlete in summer heat may need 4+ liters. Use a personalized calculator like this one for a more accurate estimate.
Research shows that drinking water before meals can reduce calorie intake, and staying hydrated supports metabolic function. However, water alone is not a weight-loss solution. It works best as part of a calorie-controlled diet and active lifestyle. Our Calorie Deficit Calculator can help you pair hydration with a solid nutrition strategy.
Yes — hyponatremia (water intoxication) occurs when you drink so much water that sodium levels in your blood drop dangerously low. This is rare for average people but can happen to endurance athletes who drink excessively during long races. The upper range shown in this calculator (~4 liters for most) is well within safe limits for healthy adults.
Generally yes — illness involving fever, vomiting, or diarrhea increases fluid loss significantly. Fever raises your metabolic rate and causes sweating, which demands extra fluid replacement. When sick, aim to drink more than your baseline and consider electrolyte-containing drinks if you're losing fluids rapidly. Always follow your doctor's guidance.