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How much water do you really need?
The old "eight glasses a day" advice is a rough rule of thumb, not a personalised target. Your real needs depend mainly on your body size and how much you sweat, because bigger bodies and harder workouts both lose more fluid.
Water keeps your blood volume up, lubricates joints, regulates temperature and helps your kidneys flush waste. Even mild dehydration — losing just 1–2% of body water — can sap concentration, mood and physical performance, which is why a target tuned to you beats a one-size-fits-all number.
How this calculator works
We start with a baseline of 35 ml of water per kilogram of body weight, the figure commonly used by clinicians, then add about 350 ml for every 30 minutes of exercise to replace what you sweat out. The result is the fluid you should aim to drink; food typically supplies another 20% on top.
For context, health authorities suggest a total daily fluid intake (drinks plus food) of roughly 2.7 litres for women and 3.7 litres for men — but those are population averages, and your personal number will sit above or below them depending on your size and activity.
What increases your needs
- Heat and humidity. You sweat more, so you need more.
- Altitude. Higher elevations increase fluid loss through breathing and urination.
- Illness. Fever, vomiting or diarrhoea raise requirements sharply.
- Pregnancy and breastfeeding. Both add to daily fluid needs.
- High-protein or high-fibre diets. Both require extra water to process.
Easy ways to hit your target
Keep a refillable bottle within reach, drink a glass with every meal, and have one when you wake up and before bed. If plain water bores you, add fruit, herbs or a splash of juice. Coffee and tea count toward your total too — the mild diuretic effect of caffeine doesn't cancel out the fluid they provide.
Helpful tools
Keep water cold all day and track how much you drink.
View options →Get reminders and log your intake automatically.
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Frequently asked questions
Can you drink too much water?
Yes, though it's rare. Drinking extreme amounts in a short time can dilute blood sodium (hyponatraemia), which is dangerous. Spread your intake through the day rather than forcing large volumes at once.
Do coffee and tea count toward my water intake?
Yes. Despite caffeine's mild diuretic effect, caffeinated drinks still provide a net fluid gain and count toward your daily total.
Should I drink more on workout days?
Yes — that's why this calculator adds fluid for exercise. Drink before, during and after training, and more in hot conditions.
Does this include the water in food?
No. The result is the fluid you should drink. Food usually supplies around another 20% of your total daily water.