How much water should you drink a day?
An adult needs about 35 ml of water to drink a day for every kilogram of body weight: 2.1 L at 60 kg, 2.5 L at 70 kg, 2.8 L at 80 kg.
Add water for exercise (around 0.5 L per hour), 0.3 L during pregnancy and 0.7 L while breastfeeding. The result is close to the EFSA values for adults.
How is water intake calculated?
The calculator multiplies your weight by 35 ml, the practical rule dietitians use for adults (30–35 ml per kilogram), then adds 0.3 L during pregnancy and 0.7 L while breastfeeding (EFSA) and about 0.5 L for each hour of moderate exercise.
The 0.5 L per hour is an average estimate: according to the ACSM, sweat rates range from about 0.3 to 2.4 L an hour, depending on effort, heat and the person. The result is a guide only.
Formula
- Water to drink = weight (kg) × 35 ml + 0.3 L pregnancy or 0.7 L breastfeeding + 0.5 L × hours of exercise
- Total water (including food) ≈ water to drink ÷ 0.8
Worked example
A 70 kg person who exercises 45 minutes a day: 70 × 35 ml = 2.45 L, plus 0.5 × 0.75 L for exercise, so about 2.8 L of water to drink, around 11 glasses of 250 ml. With the water from food, the total is about 3.5 L.
How much water to drink a day, by weight?
| Weight | Water to drink per day | 250 ml glasses |
|---|---|---|
| 50 kg | 1.75 L | 7 |
| 60 kg | 2.1 L | 8 |
| 70 kg | 2.45 L | 10 |
| 80 kg | 2.8 L | 11 |
| 90 kg | 3.15 L | 13 |
| 100 kg | 3.5 L | 14 |
Why is the result only a guide?
- The EFSA values are for moderate temperatures and moderate activity; in hot weather your needs rise.
- Sweat rates vary a lot from person to person; weighing yourself before and after training is the best measure (1 kg lost ≈ 1 L).
- It does not apply to people with heart or kidney disease, where a doctor may limit fluids.
- A practical sign: pale urine usually means you are drinking enough.
When should you talk to a specialist?
Ask your doctor if you have heart failure or kidney disease, take diuretics, or have been told to limit fluids: then the amount is set individually.
See a doctor too if you are very thirsty and urinate often for no reason, as these can be signs of diabetes.
Working with a nutritionist? See how they follow you in Rod