Water Intake Calculator
Work out how much water you should drink a day based on your body weight, age, activity level, climate and life stage — then get a personalised hourly drinking schedule you can actually follow.
- Instant result
- Hourly schedule
- Litres, oz, cups & bottles
- No sign-up, no data stored
About 8 glasses of 250 ml
How your target is built
Your hourly drinking schedule
Why this water intake calculator is different
Most hydration calculators multiply your weight by one number and stop there. Real fluid needs move with your age, your training, the heat you sit in and what your body is doing that week.
A real drinking schedule
You get times, not just a number. The tool spreads your target between your wake-up and bedtime in even slots, capped so you never front-load a litre at once.
Food is counted properly
Roughly a fifth of your daily fluid comes from food. We separate total fluid from the water you actually need to drink, so your glass target is not inflated.
Every adjustment is visible
A line-by-line breakdown shows exactly how much each factor added: baseline, exercise, heat, altitude, pregnancy, illness. No black box.
Bottles, glasses and cups
Targets in litres, millilitres, US fluid ounces and cups, plus how many 500 ml, 1 L, 16.9 oz and 32 oz bottles that works out to.
Safety limits built in
If your inputs push the target unusually high, the tool warns you instead of quietly telling you to drink five litres. Over-drinking has real risks.
Fast and private
Everything runs in your browser. Nothing is uploaded, nothing is stored, there is no sign-up, and the page loads without heavy scripts or trackers.
How to use the water intake calculator
- 1
Pick your units
Switch between kilograms and millilitres or pounds and fluid ounces. Every result on the page converts instantly.
- 2
Enter weight, age and exercise
Use your average day, not your hardest one. Exercise means minutes where you are genuinely sweating.
- 3
Set climate and life stage
Choose the temperature you actually spend the day in, then tick pregnancy, breastfeeding, altitude or illness if they apply.
- 4
Follow the schedule
Read your plain-water target, then use the hourly slots as reminders. Most people find spacing it out far easier than chasing a total.
How much water should you drink a day?
The old advice was eight glasses a day for everyone. It is easy to remember and wrong for most people, because a 55 kg office worker in a cool climate and a 95 kg roofer in July do not have the same needs.
A more useful starting point is body weight. Healthy adults need roughly 30–35 ml of total fluid per kilogram of body weight per day, which is close to the 0.5–0.6 US fl oz per pound rule you see quoted in imperial units. That number then moves with age, because kidney concentrating ability and thirst sensitivity both decline over time, and it moves with heat and exercise, because sweat losses are the single biggest variable in the equation.
National bodies publish population averages rather than personal numbers. The US National Academies set an adequate total water intake of about 3.7 litres a day for men and 2.7 litres for women, and the European Food Safety Authority uses 2.5 litres and 2.0 litres. Both figures include water from food. This calculator starts from the same evidence base but scales it to your body instead of handing you an average.
Water from food counts
Around 20% of daily fluid intake comes from food in a typical mixed diet. Cucumber, watermelon, tomatoes, soup, yoghurt and fruit are mostly water. That is why the calculator shows a total fluid target and a smaller plain-water target — the second one is the number you actually pour into a glass.
Benefits of hitting your daily water target
- Sharper thinking. Losing as little as 1–2% of body mass in fluid is enough to measurably reduce concentration, short-term memory and mood in controlled studies.
- Better exercise performance. Dehydration raises heart rate and perceived effort at the same workload, so training feels harder for no extra benefit.
- Fewer headaches. Dehydration is a well documented headache trigger, and rehydration relieves it for a meaningful share of people who get them.
- Lower kidney stone risk. Higher fluid intake dilutes urine and is one of the few interventions consistently shown to reduce stone recurrence.
- More regular digestion. Adequate fluid works with dietary fibre; without it, extra fibre can make constipation worse rather than better.
- Steadier appetite signals. Thirst is often misread as hunger, particularly in the afternoon, and drinking to a schedule removes the guesswork.
Signs you are behind
Dark yellow urine, infrequent urination, dry mouth, fatigue in the mid-afternoon, light-headedness on standing and a dull headache are the common early flags. Pale straw-coloured urine several times a day is the simplest daily check there is.
What changes your daily water needs
These are the adjustments the calculator applies on top of your weight-based baseline.
| Factor | Typical adjustment | Why it happens |
|---|---|---|
| Exercise | +400 ml (13.5 oz) per 30 minutes | Sweat and respiratory losses rise sharply with sustained effort. |
| Hot or humid weather | +8% to +28% | Humidity slows sweat evaporation, so you sweat more for the same cooling. |
| High altitude | +500 ml (17 oz) | Dry air, faster breathing and altitude diuresis all increase losses. |
| Pregnancy | +300 ml (10 oz) | Higher blood volume and amniotic fluid production. |
| Breastfeeding | +700 ml (24 oz) | Milk is roughly 87% water, and output is around 750–800 ml a day. |
| Alcohol | +150 ml (5 oz) per drink | Alcohol suppresses vasopressin, so you lose more fluid than you took in. |
| Fever, vomiting or diarrhoea | +500 ml (17 oz) or more | Losses climb fast; severe cases need oral rehydration salts, not plain water. |
| Age 65 and over | Lower ml per kg, higher priority | Thirst signalling weakens with age, so scheduled drinking matters more. |
Coffee and tea are not subtracted. The old belief that caffeine dehydrates you does not hold up at normal intakes — moderate coffee consumption contributes to daily fluid balance much like water does. Sugary drinks technically hydrate too, but they bring calories you probably do not want, so the calculator assumes plain water.
Water intake calculator FAQs
How much water should I drink a day for my weight?
A practical baseline for healthy adults is 30–35 ml of total fluid per kilogram of body weight, or about 0.5–0.6 US fluid ounces per pound. For a 70 kg (154 lb) adult that is roughly 2.2–2.5 litres of total fluid, of which about 1.8–2.0 litres is water you drink and the rest comes from food. Add roughly 400 ml for every 30 minutes of exercise and more again in hot weather.
Is 8 glasses of water a day actually enough?
Eight 250 ml glasses comes to 2 litres, which is a reasonable plain-water target for an average-sized, lightly active adult in a temperate climate. It is too little for larger people, anyone training hard, anyone working in heat, and pregnant or breastfeeding women. It can be more than needed for a small, sedentary person in a cool climate. The rule survives because it is memorable, not because it is personalised.
Do coffee, tea and other drinks count towards my water intake?
Yes. Water in coffee, tea, milk, juice and soft drinks all contributes to your fluid balance. Caffeine has a mild diuretic effect, but at normal intakes it does not outweigh the fluid in the drink itself. Alcohol is the exception: it suppresses the hormone that helps you retain water, so this calculator adds about 150 ml of extra water for each alcoholic drink.
How much water should I drink to lose weight?
There is no separate weight-loss target — your normal calculated intake is the right one. Water helps indirectly: drinking around 500 ml before a meal has been shown in trials to modestly reduce how much people eat, replacing sugary drinks with water removes calories outright, and staying hydrated stops thirst being misread as hunger. It does not burn fat on its own.
If you are managing weight, pair this with our BMI calculator and macro calculator for a fuller picture.
Can you drink too much water?
Yes, and it is genuinely dangerous. Drinking far more than your kidneys can excrete — they clear roughly 0.8 to 1.0 litres per hour at most — dilutes blood sodium and can cause hyponatraemia, with symptoms ranging from nausea and confusion to seizures. Cases usually involve endurance events or drinking several litres in a short window. Spread intake through the day, do not force large volumes quickly, and stop if your urine is completely clear for hours.
How much water should a pregnant or breastfeeding woman drink?
Pregnancy typically adds about 300 ml a day to your usual requirement to support increased blood volume and amniotic fluid. Breastfeeding adds substantially more, around 700 ml a day, because breast milk is roughly 87% water and daily output is close to 750–800 ml. Tick the relevant box in the calculator and it will apply these adjustments and reschedule your day accordingly.
How do I know if I am drinking enough?
Urine colour is the easiest daily check. Pale straw is the target; dark yellow or amber means you are behind, and completely colourless urine for hours can mean you are overdoing it. Frequency matters too — most well hydrated adults urinate around every three to four hours during the day. Persistent thirst, a dry mouth and afternoon fatigue are late signals rather than early ones.
Does the calculator work for children and teenagers?
It accepts ages from 14 upward and applies a slightly higher millilitres-per-kilogram figure for teenagers, who have a higher metabolic rate relative to body size. For children under 14 the weight-based formulas used in paediatrics differ enough that you should follow guidance from a paediatrician or your national health service rather than an adult calculator.
This tool provides general information and is not medical advice. If you have kidney disease, heart failure, liver disease, or take diuretics or lithium, your fluid intake may need to be limited — follow your doctor’s instructions rather than a calculator.
Stop guessing how much water you need
Enter your weight, activity and climate once, get a target that fits your body, and follow a schedule that spreads it across your actual waking hours.