How much water you need depends on your size, how much you move and how hot it is. This gives a rough daily target in litres, fluid ounces and glasses from your weight, your exercise time and your climate. Treat it as a guide: illness, pregnancy, medication and diet all change the real answer.
How to use it
- Enter your weight and choose kilograms or pounds.
- Enter minutes of exercise on a typical day.
- Choose your climate.
- Read the estimate.
When it helps
- Setting a daily drinking goal.
- Adjusting for hot weather or a training block.
- Building a hydration habit.
- Checking whether you're drinking roughly enough.
Good to know
- Tea, coffee and the water in food all count, often supplying a fifth or more of your fluid.
- Drink more when you're ill, pregnant, breastfeeding or sweating heavily.
- Some medical conditions require limiting fluids, so follow your doctor's advice over any calculator.
How the estimate is made
The tool starts with about 33 millilitres per kilogram of body weight, adds 350 millilitres for every 30 minutes of exercise, and adds 500 more in hot climates. A 70 kg person who exercises for 30 minutes in a mild climate comes out at about 2.7 litres.
Other guidelines give similar totals, such as about 2.7 litres for women and 3.7 for men, including water from food.
Worked example: a hot day
A 60 kg person walking for an hour in hot weather needs roughly 33 times 60, which is 1,980, plus 700 for exercise, plus 500 for heat, about 3.2 litres. Spread across the day, that's a glass every hour or so.
Sip regularly, not all at once, and include foods with high water content such as fruit, soups and vegetables.
Signs you may need more
Dark yellow urine, a dry mouth, headaches, tiredness and dizziness can all be signs of low fluid. Pale yellow urine usually means you're well hydrated. Thirst is a good guide for most healthy people, though older adults may feel it less.
For long exercise, especially in heat, consider electrolytes as well as water.
When to ask a doctor
Some conditions, such as kidney, heart and liver disease, can require fluid limits. Certain medicines change fluid needs. Pregnant and breastfeeding women usually need more. This tool can't take those into account.
If you're worried about how much you drink, or you drink very large amounts and still feel thirsty, speak to a healthcare professional.
Foods that count
About 20 percent of daily fluid comes from food. Cucumber, lettuce, watermelon and oranges are over 85 percent water, and soups, milk and yoghurt contribute too. Tea and coffee count, though very high caffeine intake can make you pass more water.
If you struggle to drink enough plain water, try flavouring it with fruit or herbal tea, and keep a bottle visible.
Questions
Is eight glasses a day right?
It's a common rule of thumb. Your own number depends on body size, activity and climate.
Can you drink too much?
In extreme cases, yes. Spread drinking through the day.