Water Intake Calculator
The "eight glasses a day" rule has no real evidence behind it and takes no account of your size. This calculator scales the figure to your body weight instead.
Formula
Daily water ≈ weight (kg) × 0.033 litres
- Add roughly 0.5 to 1 litre per hour of exercise
- Food contributes perhaps 20% of total intake
70 kg: 70 × 0.033 = 2.3 litres a day before exercise
How Much You Need
A widely used starting point is 33 ml of fluid per kilogram of body weight per day:
Daily intake (litres) = weight (kg) × 0.033
A 70 kg adult comes out at about 2.3 litres, or roughly ten 250 ml glasses.
When You Need More
- Exercise: add roughly 500–750 ml per hour of activity, more in heat.
- Hot weather: sweat losses rise sharply, and thirst lags behind need.
- Illness: fever, vomiting and diarrhoea all increase requirements considerably.
- Pregnancy and breastfeeding both raise daily needs.
Food and Other Drinks Count
Around 20–30% of most people’s fluid intake comes from food, particularly fruit and vegetables. Tea and coffee count too: the diuretic effect of moderate caffeine is far smaller than the volume of fluid consumed with it.
The simplest practical check is urine colour. Pale straw suggests you are well hydrated; dark yellow suggests you should drink more. Very large intakes are not harmless, so use thirst and colour rather than forcing a target.
Quick Reference
| Body weight | Daily water | Roughly |
|---|---|---|
| 50 kg | 1.7 L | 7 glasses |
| 60 kg | 2.0 L | 8 glasses |
| 70 kg | 2.3 L | 9 glasses |
| 80 kg | 2.6 L | 11 glasses |
| 90 kg | 3.0 L | 12 glasses |
| 100 kg | 3.3 L | 13 glasses |
Frequently Asked Questions
- How much water should I drink a day?
- Roughly 33 ml per kilogram of body weight, so about 2.3 litres for a 70 kg adult, plus extra for exercise, heat, illness or pregnancy.
- Does tea or coffee count?
- Yes. The mild diuretic effect of moderate caffeine does not offset the fluid volume, so both contribute to daily intake.
- Is eight glasses a day correct?
- It is a rule of thumb with no strong evidence base and no adjustment for body size. Scaling to weight is a better starting point.
- Can I drink too much water?
- Yes. Drinking far beyond need can dilute blood sodium, which is dangerous. This is rare, but it is a reason to treat any target as a guide rather than a quota.