Water Intake Calculator

The old rule of "8 glasses a day" is a rough guideline that does not account for body size, activity, or environment. In reality, water needs vary significantly from person to person. A 50 kg sedentary office worker in a cool climate needs far less than a 90 kg athlete training in the heat.

This calculator uses a base rate of 35 ml per kilogram of body weight, which aligns with guidelines from the European Food Safety Authority (EFSA). It then adjusts for physical activity, climate, and special conditions like pregnancy.

Keep in mind that about 20% of your daily water intake typically comes from food, especially fruits, vegetables, soups, and other water-rich foods. The remaining 80% should come from beverages — water, tea, coffee, and other drinks all count.

Mild dehydration (1–3% body weight loss): Thirst, darker urine, dry mouth, slight headache, reduced concentration. Most people operate in this range without realizing it.

Moderate dehydration (3–5%): Strong thirst, very dark urine, dizziness, rapid heartbeat, fatigue. Exercise performance drops significantly at this level.

Severe dehydration (5%+): Confusion, rapid breathing, fainting, no urine output. This is a medical emergency requiring immediate attention.

The simplest check is urine color. Pale straw to light yellow means you are well hydrated. Dark yellow or amber means you need to drink more. Clear and colorless may indicate overhydration.

During exercise, you lose water through sweat at a rate of 0.5 to 2 liters per hour depending on intensity, temperature, and individual sweat rate. This is on top of your normal daily needs.

A good practice is to drink 400–600 ml of water 2–3 hours before exercise, sip 150–250 ml every 15–20 minutes during exercise, and drink 500–700 ml for every 0.5 kg of body weight lost after exercise.

For workouts longer than 60 minutes, especially in heat, consider adding electrolytes. Plain water is sufficient for shorter sessions.

Does coffee dehydrate you? No. While caffeine has a mild diuretic effect, the water in coffee more than compensates. Moderate coffee consumption (3–4 cups) contributes positively to daily hydration.

Can you drink too much water? Yes. Overhydration (hyponatremia) occurs when you drink so much water that blood sodium levels drop dangerously low. This is rare but can happen during prolonged exercise. Drink to thirst rather than forcing excessive amounts.

Does sparkling water count? Yes. Sparkling water hydrates just as effectively as still water. The carbonation does not affect absorption.

Should I drink water before or after meals? Either is fine. Drinking water with meals does not impair digestion, contrary to a popular myth. Some people find that a glass before meals helps with portion control.