How to use
- Enter your body weight and the minutes you exercise on a typical day (leave 0 if none).
- Switch weather to hot or humid when it applies.
- Pick a cup size to see your daily target in fl oz or liters, cups and bottles.
This calculator provides general information only and is not medical advice.
How it is calculated
| Part | Rule |
|---|---|
| Base | 30–35 ml per kg of body weight (a common clinical nutrition rule of thumb — about 0.5 fl oz per lb) |
| Exercise | About 350 ml (≈ 12 fl oz) for every 30 minutes |
| Hot or humid weather | +500 ml (≈ 17 fl oz), an allowance this calculator uses |
The exercise and heat allowances are rules of thumb, not official standards. Sweat losses vary widely between people and with intensity, temperature and humidity, so the most accurate check is to weigh yourself before and after exercise. The American College of Sports Medicine (2007) suggests drinking about 1.5 L for every kilogram lost (roughly 23 fl oz per pound).
For comparison, official daily reference values for total water — including water from food — are:
| Organization | Women | Men |
|---|---|---|
| US National Academies (2004) | 2.7 L (91 fl oz) | 3.7 L (125 fl oz) |
| European Food Safety Authority (2010) | 2.0 L | 2.5 L |
The National Academies estimate that about 20% of total water comes from food, which works out to roughly 9 cups of beverages a day for women and 13 for men.
Examples
- 150 lb, 30 minutes of exercise, mild weather: about 81–92 fl oz (2.4–2.7 L), or 10–11.5 cups of 8 fl oz.
- 180 lb, 1 hour of exercise on a hot day: about 123–137 fl oz (3.6–4.1 L).
- 70 kg, no exercise: 2,100–2,450 ml.
Things to keep in mind
- If you have heart failure, kidney disease or liver cirrhosis, or take diuretics, you may be told to limit fluids. Follow your doctor’s instructions instead of this estimate.
- Drinking a very large amount in a short time can dilute the sodium in your blood (hyponatremia). Spread your intake across the day.
- The NHS suggests drinking enough that your urine is a pale straw color; darker urine means you should drink more.
- Coffee, tea, milk and the water in food all count toward your daily fluid.
FAQ
Do I really need eight glasses a day?
The “8 × 8” rule has little scientific backing (Valtin, 2002). Needs vary with body size, activity and climate, so use the estimate above alongside thirst and urine color.
Does coffee count?
Yes. Caffeinated drinks still contribute to hydration. Sugary drinks add calories, which you can track with the calorie calculator.
How much should I drink when running?
It depends on duration and intensity. Plan sessions with the running pace calculator, and weigh yourself before and after long runs to see how much to replace.