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Hydration
Water Intake Calculator: How Much Water to Drink a Day
Work out how much water to drink a day from your body weight and exercise. Individual needs also vary with climate, diet, and health.
A common daily guide is 30–35 ml of water per kilogram of body weight, plus about 350 ml per 30 minutes of exercise, counting all drinks and food moisture.
How much water to drink a day
There is no single correct figure, which is why the familiar “eight glasses a day” advice survives despite having no clear scientific origin. Daily fluid needs vary from person to person, but body weight and activity are the two biggest levers, and they're what this calculator uses. It applies roughly 33 millilitres of water per kilogram of body weight and adds about 350 millilitres for every 30 minutes of exercise. The result is a sensible baseline target rather than a strict prescription.
Importantly, your target includes all fluids, not just plain water. Tea, coffee, milk, and the water in food all count, and food alone can supply around a fifth of many people's daily fluid. That's why the number may look larger than the amount of water you actually pour into a glass.
Adjusting for real life
Hot or humid weather, altitude, illness with fever, pregnancy, breastfeeding, and hard training all raise your needs, sometimes substantially. On those days, drink more than the baseline. In cool weather with little activity, you'll naturally need less.
Rather than fixating on hitting an exact figure, use simple feedback: pale-yellow urine and only occasional thirst usually mean you're well hydrated. Sip through the day instead of drinking large amounts at once, and if you exercise hard or sweat heavily, consider replacing electrolytes as well as fluid. If you run or train regularly, our running pace calculator can help you plan sessions.
Baseline water intake by weight
| Body weight | Baseline water | ≈ Cups (240 ml) |
|---|---|---|
| 50 kg (110 lb) | 1.7 litres | 7 cups |
| 60 kg (132 lb) | 2.0 litres | 8 cups |
| 70 kg (154 lb) | 2.3 litres | 10 cups |
| 80 kg (176 lb) | 2.6 litres | 11 cups |
| 90 kg (198 lb) | 3.0 litres | 12 cups |
| 100 kg (220 lb) | 3.3 litres | 14 cups |
Baseline before exercise (≈33 ml/kg). Add about 0.35 litres per 30 minutes of activity; food and all drinks count toward the total.
Per-kilogram estimates reflect common clinical guidance; total-fluid advice follows EFSA and U.S. National Academies reference intakes. Individual needs vary — this is an estimate only.
Frequently asked questions
How much water should I drink a day?
A common guide is about 30–35 ml per kilogram of body weight, plus extra for exercise. This calculator uses roughly 33 ml per kg and adds about 350 ml for every 30 minutes of activity. Health authorities also note that food and other drinks contribute to your total fluid.
Is eight glasses of water a day the right amount?
It is a rule of thumb rather than a finding. A 2002 review in the American Journal of Physiology looked for the evidence behind the "8 x 8" rule and could not identify a scientific origin for it. Eight 8-ounce glasses comes to about 1.9 litres, which is in a reasonable range for many adults but too little for a larger or very active person and more than a small, sedentary one needs. Body weight and activity are better starting points, which is what the calculator above uses.
Does coffee and tea count toward hydration?
Yes. Despite their reputation, moderate coffee and tea count toward daily fluid intake, and food contributes roughly 20% of the total for many people. The figure here is a target for total water; you do not need to drink all of it as plain water.
Can you drink too much water?
Rarely, but yes — drinking far more than you need in a short time can dilute blood sodium, a condition called hyponatremia. Spread intake through the day and let thirst and urine colour guide you rather than forcing large volumes at once.
How do I know if I am hydrated?
Pale-yellow urine and infrequent thirst are good everyday signs. Needs rise in hot weather, at altitude, during illness, and with hard exercise, so treat the number as a baseline to adjust upward when conditions demand.
How many ounces (or gallons) is the recommendation?
Handy conversions: 2.0 litres is about 68 fl oz (0.53 US gallons), 2.7 litres is about 91 fl oz (0.71 gal), and 3.7 litres is about 125 fl oz (0.98 gal) — so the often-quoted “gallon a day” roughly matches the adequate-intake level for men, counting all beverages and food moisture.
Sources
- National Academies (IOM) — Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate — Adequate Intake basis for total water
- MedlinePlus (NIH/NLM) — Water in Diet — 2.7-3.7 L/day; varies by weight, activity
- American College of Sports Medicine — Position Stand: Exercise and Fluid Replacement (PubMed) — Fluid replacement needs during exercise
- Valtin H — "Drink at least eight glasses of water a day." Really? Am J Physiol Regul Integr Comp Physiol (2002) — Finds no scientific basis for the "8 x 8" rule
The thresholds and formulas used by this calculator follow the primary sources above. This tool provides general information, not personalised medical advice.