Daily water intake is the total amount of water your body needs each day — from drinks and food combined — to replace the fluid you lose through sweat, breathing and urine.
“Drink eight glasses a day” is the advice everyone has heard — but your real water needs depend on your body weight, how active you are, and how hot it is where you live. Enter those three things below and this calculator will estimate how much water you should drink each day, in litres, millilitres and glasses.
Roughly 20% of daily water normally comes from food, so your drinking target is a little below the total shown. If you are pregnant, breastfeeding, ill, or have kidney or heart conditions, follow your doctor’s advice instead.
How the Water Intake Calculator Works
The baseline of this calculator is the widely used rule of about 33 ml of water per kilogram of body weight per day — so a 70 kg adult starts at roughly 2.3 litres. That figure lines up well with official references: the European Food Safety Authority suggests total water intakes of about 2.0 litres a day for women and 2.5 litres for men from all food and drink, and the US National Academies put the figures slightly higher at 2.7 and 3.7 litres. On top of the baseline, the calculator adds water for the two things that reliably increase loss: exercise (about 350 ml for every 30 minutes, replacing sweat) and heat (an extra 300–600 ml on hot or hot-and-humid days, when you sweat even at rest).
Daily water targets by body weight (baseline, before adding extra for exercise and hot weather):
50 kg — about 1.7 litres a day
60 kg — about 2.0 litres a day
70 kg — about 2.3 litres a day
80 kg — about 2.6 litres a day
90 kg — about 3.0 litres a day
Remember that “total water” is not the same as “glasses of plain water”. Around a fifth of your daily fluid typically comes from food — fruit, vegetables, rice, yoghurt and even bread all contain water — and tea, coffee and milk count towards the rest. Despite the old myth, normal coffee and tea consumption hydrates you overall; caffeine’s mild diuretic effect does not cancel out the fluid in the cup.
How to Use Your Result
- Spread your intake across the day — a glass on waking, one with each meal, and small sips in between — rather than drinking a litre at once.
- Use urine colour as a live check: pale straw is ideal; dark yellow means drink more; completely clear all day may mean you can ease off slightly.
- Drink extra before, during and after exercise: roughly 500 ml in the two hours before, sips every 15–20 minutes during, and enough afterwards to replace the weight you lost.
- Increase your intake when you are ill with fever, vomiting or diarrhoea, and in very hot weather — those are the situations where dehydration actually becomes dangerous.
Signs of Dehydration to Watch For
Mild dehydration creeps up quietly: thirst, a dry mouth, headaches, tiredness, difficulty concentrating and urine that is darker than usual. Because thirst lags behind actual need — and older adults feel it less strongly — waiting until you are thirsty is not always enough on hot days or during exercise. Severe dehydration is a medical emergency: dizziness, confusion, a racing heart, very low urine output or fainting all need prompt medical care. At the other extreme, drinking far beyond your needs in a short period can dilute blood sodium (hyponatraemia), which is why this calculator’s target is a guide for the whole day, not a challenge to finish as fast as possible.
Frequently Asked Questions
Is the “8 glasses a day” rule correct?
It is a rough average that happens to suit a mid-sized adult in a mild climate. Your personal number may be well below or above it — which is exactly why this calculator adjusts for weight, activity and climate.
Does coffee count towards my water intake?
Yes. Tea, coffee, milk and juice are mostly water and count towards your daily total. Very strong caffeine in large amounts can slightly increase urine output, but not enough to make the drink a net loss.
Can I drink too much water?
Yes, though it is uncommon. Drinking several litres in a short time can overwhelm the kidneys and dilute blood sodium. Sip steadily through the day and let thirst plus the urine-colour check guide you.
How does water fit with weight management?
Replacing sugary drinks with water removes empty calories, and drinking a glass before meals modestly reduces how much people eat. Combine your hydration target with your calorie target from the TDEE & Calorie Calculator and your baseline from the BMI Calculator.