Water intake calculator — how much water should you drink a day?
Estimate daily water needs from body weight, exercise time and climate, in litres, cups or fluid ounces, with the assumptions spelled out.
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149.9 lb · 10 st 9.9 lb
age 18–30: 40 ml per kg
Sex only affects the life-stage options below; the baseline is driven by body weight.
Roughly 350 ml (12 fl oz) is added for every 30 minutes of sweaty activity.
Roughly 15–24 °C. The baseline these figures are built on.
Hydration needs vary a lot between people and days. If you have heart, kidney or liver disease, or take diuretics, your fluid target should come from your doctor rather than a formula.
What that looks like
10 glasses of 250 ml. Front-load the day — most people are mildly dehydrated on waking and then try to catch up in the evening.
Where the total comes from
- Baseline 2.72 L
- Exercise 0.54 L
- Climate 0 L
The formula, with your numbers
- Baseline
- 68 kg × 40 ml/kg = 2,720 ml
- Exercise
- 45 min × 12 ml/min (≈350 ml per 30 min) = 540 ml
- Climate
- (2,720 + 540) × 1 — temperate = 3,260 ml
- From drinks
- 3,260 ml × 80% (food supplies roughly the other 20%) = 2,608 ml
- Imperial check
- 2,608 ml ÷ 29.57 ml per US fl oz = 88 fl oz
A common US rule of thumb is half your body weight in pounds as fluid ounces — for you that is about 75 fl oz, which lands close to the figure above.
What this means
Aim for roughly 2.61 litres of drinks a day — about 10 glasses — with the rest of your water coming from food. Fruit, vegetables, soup, yoghurt and even bread all contribute.
Tea and coffee count. The mild diuretic effect of caffeine at normal intakes does not outweigh the water in the cup. Alcohol is the exception: it suppresses vasopressin and genuinely increases losses.
Thirst and urine colour are better day-to-day guides than any number. Pale straw is the target; consistently dark yellow means drink more, and completely clear all day usually means you are overdoing it. If you train for more than about 90 minutes or sweat heavily, add electrolytes rather than plain water alone.
How to use the water intake calculator
- Enter your body weight — this drives the baseline, and it matters far more than any rule of thumb.
- Add your age. Kidney concentrating ability declines with age, so the per-kilogram figure tapers.
- Enter roughly how many minutes a day you spend exercising or otherwise sweating.
- Pick the climate you are actually in today, not the annual average where you live.
- Read two numbers: total water, and the smaller drinks-only figure that you have to act on.
How the estimate is built
| Component | Rule used | Source of the figure |
|---|---|---|
| Baseline, age 18–30 | 40 ml per kg | Standard clinical maintenance-fluid estimate for younger adults |
| Baseline, age 31–55 | 35 ml per kg | The most commonly cited adult figure |
| Baseline, age 56+ | 30 ml per kg | Reduced renal concentrating capacity and blunted thirst |
| Exercise | +12 ml per minute (≈350 ml per 30 min) | American College of Sports Medicine guidance on replacing sweat losses |
| Climate | ×1.00 temperate up to ×1.25 very hot or humid | Sweat losses rise steeply with heat, and further when humidity blocks evaporation |
| Pregnancy / breastfeeding | +300 ml / +700 ml | EFSA adequate-intake adjustments |
| Food contribution | 20% of total | EFSA and IOM both put food at roughly 20–30% of water intake |
Reference intakes from health authorities
The two most-cited sets of figures are total water — that is, drinks plus the water in food.
| Authority | Adult men | Adult women |
|---|---|---|
| EFSA (European Food Safety Authority) | 2.5 L per day total water | 2.0 L per day total water |
| US National Academies (IOM) | 3.7 L per day total water (about 13 cups from drinks) | 2.7 L per day total water (about 9 cups from drinks) |
They differ because they were derived from different population survey data, not because one is wrong. Both are population averages for sedentary adults in temperate climates — which is exactly why a body-weight-based estimate with adjustments is more useful for an individual.
Where "eight glasses a day" came from
The slogan appears to trace back to a 1945 US Food and Nutrition Board recommendation of about 2.5 litres a day — which included the sentence, usually omitted afterwards, that "most of this quantity is contained in prepared foods". The number survived; the caveat did not. There is no trial evidence that eight glasses specifically is the right target for anyone.
Does coffee count?
Yes. Caffeine has a mild diuretic effect at doses above about 250–300 mg in people who are not habitual consumers, but the water in the drink comfortably outweighs the extra urine output. Controlled studies comparing coffee with water at equal volumes find no meaningful difference in hydration status. Tea, milk, juice and soft drinks all count too.
Alcohol is the genuine exception. It suppresses vasopressin, so you lose more fluid than you take in — roughly 10 ml of extra urine per gram of alcohol. Alternating drinks with water is not folklore.
How to tell whether you are actually hydrated
- Urine colour. Pale straw is the target. Dark yellow or amber means drink more. Completely colourless all day usually means you are drinking more than you need. B vitamins turn urine bright yellow and will fool you.
- Frequency. Four to seven trips a day is typical for a well-hydrated adult.
- Thirst. For healthy adults under normal conditions, drinking to thirst is a reliable strategy. It becomes less reliable in older adults, in extreme heat, and during prolonged exercise.
- Weight around exercise. Weigh yourself before and after a long session. Each kilogram lost is about a litre of sweat; replace 1.25 to 1.5 litres per kilogram over the following hours.
When plain water is not enough
Sweat contains sodium — roughly 0.5 to 1.5 g per litre, and more in unacclimatised people. For efforts over about 90 minutes, in heat, or for anyone who finishes sessions with salt crusted on their skin, replacing water without sodium dilutes blood sodium and can cause hyponatraemia. Symptoms are headache, nausea, confusion and swelling, and in endurance events it is more dangerous than mild dehydration. Use an electrolyte drink, or add salt to food after long sessions.
Practical habits that work better than a target
- Drink 400–500 ml on waking; you are mildly dehydrated after a night's sleep.
- Keep a filled bottle where you work — visibility beats intention.
- Attach drinking to existing habits: every meal, every coffee, every time you stand up.
- Front-load the day so you are not catching up at 10 pm and then sleeping badly.
- Eat water: cucumber, watermelon, tomatoes, soup and yoghurt are 80 to 95 percent water.
Questions
Is eight glasses a day right?
It is a convenient slogan with no strong evidence behind it. Needs scale with body size, sweat rate and climate, which is why this calculator starts from 30–40 ml per kg of body weight, depending on age, and then adds for exercise and heat.
Does tea, coffee or food count?
Yes. Roughly 20 to 30 percent of total water intake usually comes from food, and caffeinated drinks in normal amounts are still net hydrating. The figure shown here is total water; the drink-only target subtracts the food share.
How do I tell if I am drinking enough?
Pale straw-coloured urine and no strong thirst are the practical signs. Dark yellow urine, headaches or dizziness after exercise point the other way. Weighing yourself before and after a long session shows exactly how much you lost.
Can you drink too much water?
Yes, though it is rare. Drinking litres per hour well beyond sweat losses can dilute blood sodium (hyponatraemia), which is genuinely dangerous. Endurance athletes should drink to thirst and include electrolytes on long efforts.