“How much water should I drink?” has no single answer, however many posters say otherwise. A smaller person in a cool office needs less than a larger person who spends the day on their feet in the heat. This calculator gives you a personal starting point. Enter your weight, how active you are and the kind of weather you live in, and it estimates how much fluid you may need in a day.
Enter your weight in kilograms or pounds.
Enter how much exercise you do on a typical day, in minutes.
Choose your climate: cool or temperate, or hot and humid.
Read the result in litres, fluid ounces and cups.
The calculator uses a simple rule of thumb, not a medical measurement:
Base amount = body weight in kg × 33 ml (or, in imperial units, about half an ounce per pound of body weight)
Exercise = add about 350 ml for every 30 minutes of activity
Hot or humid weather = add roughly 10% to the total
A quiet day. Someone who weighs 60 kg, doesn’t exercise and lives in a temperate climate.
60 × 33 = 1,980 ml
Result: about 2.0 litres, or roughly 8 cups
An active day. Someone who weighs 70 kg and exercises for 45 minutes.
Base: 70 × 33 = 2,310 ml
Exercise: 350 × 1.5 = 525 ml
Result: about 2.8 litres, or roughly 12 cups
A hot day. The same 70 kg person, with no exercise, in hot weather.
Base: 2,310 ml
Add 10%: 2,310 × 1.10 = 2,541 ml
Result: about 2.5 litres
In imperial units. Someone who weighs 160 lb.
160 × 0.5 = 80 fl oz
Result: about 80 ounces, or roughly 2.4 litres
No. Your total includes:
Water and other drinks, such as milk, juice, herbal tea and sparkling water
Tea and coffee, in moderate amounts. They count towards your fluid intake despite their caffeine
Food. Fruit, vegetables, soup, yoghurt and many other foods contain a lot of water, and they typically supply around a fifth of what you take in each day
Most health authorities give general figures for total fluid intake from all of these sources combined. A commonly quoted guide for healthy adults is around 2.7 litres a day for women and 3.7 litres for men, including the water in food. Your own number can sit above or below that depending on your size, activity and surroundings.
Your result is a starting point, and several things can push it up:
Exercise and sweating. The longer and harder you work out, the more you lose.
Heat, humidity and altitude. Hot and dry conditions both raise how much you lose through sweat and breathing.
Illness. Fever, vomiting and diarrhoea all drain fluid quickly.
Pregnancy and breastfeeding. Needs go up during both.
A high-salt or high-protein diet, or a lot of caffeine or alcohol.
Some medicines, especially water tablets (diuretics).
Thirst. It’s a perfectly good signal for most healthy people
Dark yellow urine. Aim for pale yellow
A dry mouth or lips
A headache, tiredness or trouble concentrating
Passing urine less often than usual
The urine colour check is the easiest one to use. Pale straw colour usually means you’re doing well. Note that some vitamins and foods can change urine colour, so use it as a guide, not as proof.
Yes, though it’s uncommon. Drinking very large amounts in a short time, especially during long endurance events, can dilute the sodium in your blood, a condition called hyponatraemia. It can cause nausea, headaches and confusion, and in severe cases it’s dangerous. Sip steadily through the day instead of forcing down big amounts at once, and drink to thirst during long exercise.
Keep a bottle where you can see it, at your desk or in your bag
Have a glass with each meal and when you wake up
Add a slice of lemon, cucumber or mint if plain water bores you
Eat more water-rich foods such as cucumber, oranges, melon and soup
Take a few sips whenever you take a break
Drink a little more before, during and after exercise
Treating the number as a target to force. It’s an estimate. If you’re not thirsty and your urine is pale, you don’t need to keep drinking.
Waiting until you’re very thirsty during exercise or in the heat. Start drinking earlier and keep sipping.
Counting only plain water. Other drinks and food count too.
Relying on “eight glasses a day”. It’s an easy rule to remember, not a scientific requirement, and your own needs may be higher or lower.
Ignoring salt when you sweat a lot. For long or intense exercise in the heat, you lose electrolytes as well as water.
How much water should I drink a day?
For many healthy adults, somewhere between about 2 and 3.5 litres of total fluid from all drinks and food is typical. The calculator narrows that down using your weight, activity and climate.
Does coffee or tea count?
Yes. In moderate amounts they contribute to your fluid intake. Too much caffeine can cause other problems, such as poor sleep, so don’t rely on them alone.
Is my result a medical recommendation?
No. It’s a rough estimate for healthy adults. If you have a medical condition, speak to your doctor.
Do I need more water to lose weight?
Drinking enough can help you tell thirst from hunger, and a glass before a meal may help some people eat a little less. But water isn’t a weight-loss treatment on its own.
Should I drink more if I’m pregnant or breastfeeding?
Needs are generally higher during both. Your midwife or doctor can tell you what’s right for you.
Are there people who should be careful about drinking more?
Yes. People with kidney, heart or liver conditions, and some people taking particular medicines, may have been given fluid limits. If that applies to you, follow your doctor’s advice and not the calculator.
Can I use this for children?
No. Children’s needs differ by age and size. A paediatrician or a school nurse can advise.
This calculator provides estimates for general information only and is not medical advice. If you have a medical condition, are pregnant or breastfeeding, take medication, or have been told to limit your fluids, please speak to a doctor first. Seek medical help for signs of severe dehydration, such as confusion, a very fast heartbeat, or being unable to keep fluids down.