About the Water Intake Calculator
Water Intake Calculator suggests how much fluid to aim for each day. Enter your body weight, the minutes you exercise, the weather where you live and whether you are pregnant or breastfeeding. The tool starts from a base of 30 to 35 millilitres per kilogram, adds allowances for exercise, warm weather and life stage, and shows a range in litres or fluid ounces with a good middle target. It also works out about how many glasses or bottles of your chosen size that is, and suggests evenly spread times through the day. The figures are general guidance for healthy adults and not medical advice, and all drinks count, not only plain water. Nothing you enter is stored or sent anywhere.
Updated September 2026
How to use the Water Intake Calculator
- 1Enter your weight and choose kilograms or pounds.
- 2Enter the minutes of exercise you do on a normal day.
- 3Choose the weather where you live and, if it applies, pregnancy or breastfeeding.
- 4Enter the size of your usual glass or bottle in millilitres.
- 5Read your daily range, the glasses per day and the suggested times, and switch between litres and fluid ounces if you like.
Why weight is the starting point
Water makes up more than half of the adult body, and larger bodies hold and lose more of it, so fluid needs grow with weight. The old advice of eight glasses a day is easy to remember but ignores this, and it treats a small person and a large athlete the same. Using millilitres per kilogram gives a sensible range that scales with you.
The tool shows a range instead of a single number because healthy people differ in what they need. Use the low end when you are sitting in a cool room and the high end when you are active, ill or in the heat.
What adds to your need
- Exercise: sweat loss, roughly 12 ml per minute of active time in this tool.
- Weather: 250 ml extra when warm and 500 ml when hot or humid.
- Pregnancy: about 300 ml extra a day.
- Breastfeeding: about 700 ml extra a day.
- Illness, high altitude and dry indoor air can also raise needs, but are not modelled.
Turning litres into habits
A figure in litres is hard to picture, so the tool divides it by your glass or bottle size and adds evenly spaced times between 08:00 and 21:00. Use them as a guide: set a phone reminder, keep a bottle at your desk and drink a glass with each meal. Front-load the day if you do not want to wake at night.
You do not need to force every drop. Thirst is a good guide for healthy adults, and the water in food, tea and coffee counts. If your urine is pale yellow through the day, you are probably doing well.
Safety and next steps
These numbers are general and not medical advice. Some medicines, kidney, heart and liver conditions and some hormone disorders change how much fluid is safe, so follow your doctor's advice if you have one.
For related health sums, BMI Calculator checks weight against height, Calorie Calculator estimates daily energy needs, Macro Calculator splits calories into carbs, protein and fat, Unit Converter switches between litres and fluid ounces and Sleep Calculator helps plan rest.
Features
- Range based on 30 to 35 ml per kg of body weight
- Extra for exercise, warm or hot weather, pregnancy and breastfeeding
- Litres or US fluid ounces
- Number of glasses or bottles of your size
- Times spread through the day
- Kilograms or pounds
- Clear safety notes
- Runs in your browser
Frequently asked questions
How much water should I drink a day?
A common rule of thumb is 30 to 35 millilitres per kilogram of body weight for a healthy adult. For 70 kg that is 2.1 to 2.45 litres, before extra for exercise or heat. Needs vary from person to person, so treat it as a starting point.
Does coffee, tea and food count?
Yes. Tea, coffee, milk, juice and other drinks all add to your fluids, and water-rich foods such as fruit, vegetables and soup add more. This tool's figure is for drinks, so food comes on top. Very sugary drinks are not ideal for health, so plain water should make up most of it.
How much extra should I drink when I exercise?
The tool adds about 12 millilitres for each minute of exercise, roughly 350 millilitres for half an hour of sweaty activity. Long or intense sessions, and hot conditions, need more, and endurance athletes may need to replace salts too.
Does hot weather change how much I need?
Yes. You lose more water as sweat in warm and humid conditions. The tool adds 250 millilitres for warm weather and 500 millilitres for hot or humid weather, which is a modest allowance. If you are outdoors all day in extreme heat, you may need much more.
How much extra do pregnancy and breastfeeding need?
Health bodies suggest roughly 300 millilitres extra during pregnancy and about 700 millilitres extra while breastfeeding. The tool adds those amounts. Ask your midwife or doctor for advice that fits you.
Can you drink too much water?
Yes, although it is rare. Drinking very large amounts in a short time can lower the salt level in the blood, a condition called hyponatremia. Spread drinks through the day, drink to thirst, and follow your doctor if you have kidney, heart or liver disease or a fluid limit.
How can I tell if I am drinking enough?
Pale yellow urine, regular toilet trips and not feeling thirsty most of the day are good signs. Dark urine, headaches, dry mouth and dizziness suggest you need more fluids. Illness with vomiting or diarrhoea raises needs a lot.