Water Intake Calculator
Find how much water you should drink each day based on your weight and activity.
The water intake calculator estimates how much fluid you need each day. A common guideline is 35 ml per kilogram of body weight, plus extra for exercise and hot weather. Remember that tea, milk, soups and juicy fruit also contribute to your fluid intake.
How to use the calculator
- Enter your weight.
- Add the minutes you spend exercising in a day.
- Choose whether you live in a hot climate.
- The result shows your target in litres, millilitres and 250 ml glasses.
How it is calculated
Exercise add-on = 12 ml × minutes · Hot climate adds about 15%
Target = (base + add-on) × climate factor
Worked example
70 kg, 30 minutes of exercise, normal climate
Base = 2,450 ml and exercise = 360 ml
Daily target ≈ 2.8 litres, about 11 glasses of 250 ml.
Who should use this calculator
- Anyone exercising in Indian summer heat, where sweat losses are much higher than in temperate climates.
- People who drink very little during the day, feel tired by the afternoon and want a simple target to work towards.
- Older adults, whose sense of thirst becomes less reliable with age.
- Anyone recovering from a fever, vomiting or diarrhoea, where fluid needs are temporarily much higher.
Common mistakes to avoid
- Forcing eight glasses regardless of size, activity or climate. A 50 kg office worker and a 95 kg person training outdoors have very different needs. The 30–35 ml per kilogram guideline used here is a better starting point.
- Drinking large volumes in a short time. Drinking several litres within an hour or two can dilute blood sodium, which is dangerous. Spread intake across the day.
- Forgetting that food counts. Tea, coffee, milk, soup and fruit all contribute. Only about 70–80% of your fluid typically comes from drinks.
- Over-drinking during long endurance events. Replacing every gram of sweat loss with plain water without electrolytes is a recognised cause of exercise-related hyponatraemia.
- Ignoring illness. Fever, diarrhoea and vomiting raise requirements sharply, and in those cases an oral rehydration solution is usually better than plain water.
How much is too much
A 70 kg person exercising for thirty minutes in a normal climate lands at roughly 2.8 litres a day: about 2.45 litres for body size plus 360 ml for the workout. That is a target, not a rule to hit exactly. Thirst, urine colour — pale straw is the goal — and how you feel are the practical guides.
The danger sign is not drinking too little but drinking too much, too fast. Kidneys can typically process around 800 ml to a litre of fluid an hour. Beyond that, sodium in the blood becomes diluted, which can cause headache, nausea, confusion and, in extreme cases, seizures. If you are sweating heavily for over an hour, choose a drink that replaces some electrolytes rather than plain water alone.
Hydration terms explained
- Hyponatraemia
- Abnormally low sodium in the blood, most often caused by drinking large volumes of plain water too quickly.
- Electrolytes
- Sodium, potassium and other minerals lost in sweat. They need replacing during long, heavy sweating sessions.
- Climate factor
- An adjustment for hot or humid conditions, where sweat losses are greater even at the same activity level.
- Fluid deficit
- The gap between what you have lost and what you have replaced. A 2% loss of body weight from sweat can already reduce performance.
- Urine colour check
- A practical hydration gauge: pale straw suggests good hydration, dark yellow suggests drinking more.
Simple signs you are running short
Thirst is a late signal, especially in older adults. Earlier signs include dark yellow urine, a dry mouth, headaches in the afternoon, difficulty concentrating and a sudden drop in energy during a workout. Constipation and recurring kidney stones are longer-term consequences worth discussing with a doctor. On the other side, drinking steadily past your thirst until you feel bloated or light-headed means you have gone too far the other way — spread the same volume across the day instead of finishing it in one sitting.
Frequently asked questions
Do I really need 8 glasses of water a day?
The "8 glasses" rule is a rough average, not a scientific requirement. Needs vary with body size, activity, weather and diet. Using 30–35 ml per kg, as this calculator does, is a more personalised guideline.
Can drinking too much water be harmful?
Yes, though it is rare. Drinking several litres in a short time can dilute blood sodium (hyponatraemia). Spread your intake through the day and drink to thirst during normal activities.
Does coffee or tea count as water?
Yes, most of it counts. The mild diuretic effect of caffeine does not offset the fluid you take in, so tea, coffee, milk and soups all add to your daily total.