Calculate your daily intake.
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STEP 01
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Your personalized hydration result will appear here.
How much water per day — the science.
The 8×8 rule — and why it's outdated
The popular "8 glasses per day" rule has no specific scientific origin. A 2002 review in the American Journal of Physiology found no evidence supporting this one-size-fits-all guideline. Individual needs vary by up to 300% based on body size, activity, and environment.
What the science actually says
The Institute of Medicine recommends an adequate intake (AI) of 3.7 liters per day for men and 2.7 liters per day for women — but this includes water from all food and beverages. Plain water accounts for about 20% of intake from food alone, so the drinking target is roughly 3.0L for men and 2.2L for women as a baseline.
The formula we use
Our calculator uses a weight-based formula adjusted for gender: Weight (kg) × 33ml for men, Weight (kg) × 31ml for women. This is then multiplied by activity and climate factors, with additional exercise bonuses (+500ml per 30 min). This approach is endorsed by multiple sports medicine organizations.
Daily water intake by body weight
| Weight | Men (L/day) | Women (L/day) | Cups (avg) |
|---|---|---|---|
| 50 kg (110 lbs) | 1.6L | 1.5L | ~6–7 |
| 60 kg (132 lbs) | 2.0L | 1.9L | ~8 |
| 70 kg (154 lbs) | 2.3L | 2.2L | ~9 |
| 80 kg (176 lbs) | 2.6L | 2.5L | ~10–11 |
| 90 kg (198 lbs) | 3.0L | 2.8L | ~12 |
| 100 kg (220 lbs) | 3.3L | 3.1L | ~13 |
* Baseline figures for moderate activity in temperate climate. Add 10–35% for heat, and 500ml per 30 min of exercise.
FAQ — daily intake.
The American College of Sports Medicine (ACSM) provides the following general guidance for exercise hydration: drink approximately 400–600ml (14–22 oz) around 2 hours before exercise to begin well-hydrated; consume approximately 150–350ml (5–12 oz) every 15–20 minutes during exercise to match sweat losses; and after exercise, drink approximately 1.5 times the fluid lost (a practical estimate is around 750ml per pound of body weight lost) to support recovery. Our calculator adds approximately 500ml per 30 minutes of daily exercise to your baseline estimate, based on this general guidance. These are population-level estimates; actual sweat rates vary considerably between individuals.
For most exercise sessions lasting under 60 minutes at moderate intensity, plain water is generally sufficient for healthy adults. For longer or more intense sessions, particularly in hot conditions, sports drinks containing electrolytes (especially sodium and potassium) may help replace what is lost in sweat and may help prevent hyponatremia — a condition that can occur when large amounts of plain water are consumed during prolonged exercise. The threshold is not absolute; individual sweat rate, heat conditions, and intensity all influence when electrolytes become useful. If you exercise for more than 60–90 minutes, especially in heat, discussing electrolyte strategy with a sports dietitian or healthcare provider may be beneficial.
For endurance events such as marathons, general sports medicine guidance suggests aiming for approximately 400–800ml of fluid per hour, adjusted to your sweat rate and conditions. Drinking to thirst — rather than following a rigid schedule — is increasingly recommended by sports medicine experts, including ACSM, to reduce overhydration risk. For runs longer than 90 minutes, including sodium (via sports drinks or electrolyte supplements) is widely recommended to prevent dilutional hyponatremia. It is advisable to practise your hydration strategy during training rather than trying something new on race day. Individual needs vary considerably — consulting a sports dietitian for personalised advice is worthwhile for competitive events.
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