Free Hydration Calculator
Estimate how much water to drink each day based on your routine
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About hydration
We start with 35 ml of total water per kilogram of body weight. We then add water for exercise: 500 ml per hour for light activity, 750 ml for moderate activity and 1 litre for intense activity. A hot day adds 500 ml.
About 25% of daily hydration usually comes from food, so the main result shows the remaining 75% as drinks.
Water, unsweetened tea, black coffee and herbal tea can usually be consumed during a fasting window. Because you do not get water from food while fasting, keep a bottle nearby and drink regularly rather than waiting until you are very thirsty.
- Dark, strong-smelling urine
- Dry mouth or headache
- Unusual fatigue or difficulty concentrating
- Dizziness when standing up
Confusion, an unusually fast heartbeat or no urine for many hours need prompt medical attention.
Water needs vary with diet, climate, altitude, health, medication and sweat loss. This tool is a starting point, not a medical prescription. Avoid forcing fluids; thirst and pale-yellow urine are useful everyday cues.
Medical disclaimer. For healthy adults only. Do not use this calculator to manage hydration during pregnancy, breastfeeding, kidney or heart disease, diabetes, or when following a medical fluid restriction. Ask a healthcare professional for individual advice.
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