How much water should you drink?
This calculator estimates your recommended daily water intake from your body weight and activity level. Hydration needs scale with body mass, so a weight-based estimate is a practical starting point for most healthy adults — more reliable than the one-size-fits-all “eight glasses a day” rule.
How it works
The tool converts your weight to kilograms if needed, then applies a common hydration guideline of about 33 ml of water per kilogram of body weight, and adds a fixed allowance for exercise:
daily water (litres) = (weight in kg × 33 ÷ 1000) + activity allowance
The activity allowance is 0 litres for low/sedentary, 0.5 litres for moderate exercise, and 1.0 litre for high/intense exercise. The result is shown in litres, then converted to 250 ml cups and US fluid ounces (1 litre ≈ 33.814 US fl oz).
| Activity level | Added allowance |
|---|---|
| Low / sedentary | 0 L |
| Moderate exercise | +0.5 L |
| High / intense exercise | +1.0 L |
Example
A 70 kg adult with moderate activity: baseline = 70 × 33 ÷ 1000 = 2.31 L, plus 0.5 L for exercise = 2.81 litres per day. That is about 11 cups (250 ml each) or roughly 95 US fl oz.
Why weight-based is better than a fixed rule
The “eight glasses” rule (about 2 litres) was popularized as an easy-to-remember target, but it ignores body size entirely. A 55 kg sedentary adult needs substantially less fluid than a 90 kg person who runs each morning. Weight-based estimates scale the baseline linearly with body mass, producing a personalized starting point rather than a blanket figure.
Body weight also roughly correlates with lean muscle mass, which holds more water than fat tissue. People with higher muscle mass have greater total body water and somewhat higher fluid turnover, making the weight-based approach a reasonable first approximation even without knowing body composition.
Factors that shift the target up
The calculator covers the two most controllable variables — weight and activity — but several other factors increase fluid needs:
Heat and humidity: Sweating in hot or humid weather increases fluid losses significantly. In a temperate climate a sedentary person may be comfortable at the baseline estimate; in a hot, humid climate the same person may need 20–30% more. If you live or work in warm conditions, treat the calculator’s output as a floor rather than a ceiling.
High-protein or high-fibre diets: Protein metabolism produces urea, which must be excreted in urine, so high-protein diets increase the kidneys’ water demand. High dietary fibre absorbs water in the gut; both warrant slightly higher fluid intake.
Altitude: At elevations above roughly 2,500 metres, respiration rate increases, raising fluid loss through breathing. Altitude acclimatization typically increases urine output temporarily as well.
Illness, fever, vomiting, or diarrhoea: These situations can dramatically increase fluid losses and may require medical guidance on oral rehydration rather than plain water — particularly for children or elderly adults.
Breastfeeding: Lactating adults produce milk from maternal fluid stores, increasing daily fluid needs substantially above the weight-based baseline.
How food contributes
Not all of your daily fluid target needs to come from plain water. Roughly 20% of typical daily fluid intake comes from food — fruits and vegetables are mostly water, and even cooked grains and legumes contribute. Coffee and tea count toward hydration despite mild diuretic effects. The calculator outputs total fluid needs; drinking plain water equal to about 80% of that figure is a reasonable starting point, with the rest coming from food and other beverages.
Reading the signs of hydration
Rather than obsessing over a precise millilitre target, many health professionals suggest using urine colour as a real-time guide:
- Pale yellow (like straw) → well hydrated
- Dark yellow or amber → drink more
- Colourless → possibly over-hydrated (rare but worth noting)
The weight-based figure gives you a reasonable daily target; urine colour tells you how you are doing throughout the day.
Your real needs also depend on climate, diet, illness and medical advice, so use this as a starting point. Everything is calculated in your browser — nothing is uploaded.