The Complete Guide to Daily Water Intake
By Arshi Khan, Founder of Just4Marketing · Last reviewed October 2026
'Drink 8 glasses a day' is a rough rule of thumb that ignores body size and activity level entirely. This guide explains the weight-based formula this calculator uses, how exercise changes the target, and the factors that affect how much water you actually need.
How much water should I drink a day?
A good starting point is about 33 ml of water for every kilogram you weigh (roughly half an ounce per pound), plus 350 ml for light exercise, 700 ml for moderate exercise, or 1,050 ml for intense exercise. Without exercise, that works out to:
| Body weight | Liters per day | Fluid ounces | Cups (8 oz) |
|---|---|---|---|
| 50 kg (110 lb) | 1.6 L | 56 oz | 7 |
| 60 kg (132 lb) | 2.0 L | 67 oz | 8.4 |
| 70 kg (154 lb) | 2.3 L | 78 oz | 9.8 |
| 80 kg (176 lb) | 2.6 L | 89 oz | 11.2 |
| 90 kg (198 lb) | 3.0 L | 100 oz | 12.6 |
| 100 kg (220 lb) | 3.3 L | 112 oz | 13.9 |
A 70 kg person who exercises moderately would aim for about 3.0 liters (102 oz). Hot weather, illness, pregnancy, and breastfeeding all raise your needs, so use the calculator above for your own target.
What determines daily water intake?
This calculator estimates a daily water target using a weight-based formula: roughly 33 ml of water per kilogram of body weight, plus an additional amount depending on how much you exercise. A 70 kg (154 lb) person with no exercise would land around 2.3 liters (about 78 fl oz) per day before any activity adjustment.
Weight-based formulas are more useful than the generic "8 glasses a day" rule because water needs scale with body size — a 100 kg person and a 55 kg person don't need the same amount of fluid to stay properly hydrated.
How exercise changes your target
| Exercise level | Description | Extra water |
|---|---|---|
| None | No structured exercise | +0 ml |
| Light | Under 30 minutes/day | +350 ml |
| Moderate | 30–60 minutes/day | +700 ml |
| Intense | 60+ minutes/day | +1,050 ml |
Factors that affect your real needs
Climate and altitude increase needs
Hot, humid weather and higher altitudes both increase fluid loss through sweat and respiration, pushing real needs above the baseline estimate.
Food contributes to hydration too
Fruits, vegetables, soups, and other water-rich foods can supply a meaningful portion of daily fluid needs, which this calculator doesn't subtract out.
Certain health conditions change the target
Kidney disease, heart failure, and some medications can require either restricted or increased fluid intake — always follow a doctor's specific guidance over a general formula.
Thirst is still a useful signal
For most healthy adults, thirst combined with pale-yellow urine color remains a reliable everyday indicator alongside a calculated target.
Frequently asked questions
How much water should I drink a day?
Roughly 33 ml per kilogram of body weight, plus 350–1,050 ml depending on how much you exercise. For a 70 kg (154 lb) adult with no exercise, that's about 2.3 liters (78 fl oz) a day.
Where does the 33 ml per kg guideline come from?
It's a commonly cited general hydration guideline — roughly 30–35 ml of water per kilogram of body weight per day — used by dietitians as a simple starting point before adjusting for climate, activity, and individual factors.
Does this include water from food and other drinks?
No, this calculator estimates total fluid intake as if it came entirely from water. In practice, food (especially fruits and vegetables) and other beverages contribute a meaningful share of daily fluid intake, so your direct water needs may be somewhat lower.
Why does exercise add so much extra water?
Sweat loss during exercise can range from roughly 0.5 to over 2 liters per hour depending on intensity, heat, and individual sweat rate. The calculator's exercise add-ons (350–1,050 ml) are a general estimate — heavy sweaters in hot conditions may need more.
Can you drink too much water?
Yes — drinking far more water than your body can process can dilute sodium levels and cause a rare but serious condition called hyponatremia, most often seen in endurance athletes. Following a reasonable, body-weight-based target avoids this risk for the vast majority of people.