You have probably heard the rule: drink eight glasses of water a day. It is simple, easy to remember — and not quite right for everyone. A small, inactive person and a tall, active person who works outdoors do not need the same amount of water, and a single fixed target ignores body size, climate, and activity altogether. The good news is that working out your own target is straightforward. Once you know the weight-based formula, you can get a personal number in under a minute — and adjust it sensibly on hot days or workout days.
This guide explains where the eight-glasses rule came from, how to calculate your daily water intake from your body weight, how exercise and heat change the answer, and how to tell whether you are drinking enough. Use our free Water Intake Calculator if you want the number worked out for you instantly.
Where did the "8 glasses a day" rule come from?
The eight-glasses idea — eight 8-ounce (about 240 ml) glasses, or roughly 2 litres a day — has been repeated for decades, but it was never a precise scientific recommendation. It is often traced back to a 1945 recommendation that adults need about 2.5 litres of water a day, most of which was expected to come from food as well as drinks. Somewhere along the way, the "from food" part got dropped, and 2 litres of drinking water became the headline.
As a rough starting point, 2 litres is not unreasonable for many adults. But health authorities give ranges, not a single number: the U.S. National Academies suggest about 3.7 litres a day for men and 2.7 litres for women from all drinks and food, of which roughly 20% comes from food. The most useful approach is simpler: start from your body weight, then adjust for your day.
The weight-based formula for daily water intake
A widely used rule of thumb is 30 to 35 millilitres of water per kilogram of body weight per day. In imperial units, that is roughly half an ounce to two-thirds of an ounce per pound. The lower end suits smaller, less active people in cool climates; the upper end suits larger, more active people or warmer conditions.
- Metric: body weight in kg × 30 to 35 ml = daily intake in ml
- Imperial: body weight in lb × 0.5 to 0.67 fl oz = daily intake in fl oz
- Quick conversion: divide millilitres by 1,000 to get litres, or by 240 to get standard 8-ounce glasses
Take a person who weighs 70 kg (about 154 lb) and is moderately active. At the lower end: 70 × 30 ml = 2,100 ml, or 2.1 litres (about 9 glasses). At the upper end: 70 × 35 ml = 2,450 ml, or about 2.5 litres (about 10 glasses). A sensible everyday target is therefore around 2.1 to 2.5 litres from drinks. Now take a heavier person at 90 kg (about 198 lb): 90 × 30 ml = 2,700 ml, and 90 × 35 ml = 3,150 ml — roughly 2.7 to 3.2 litres. That half-litre-plus difference is exactly what the flat eight-glasses rule misses.
Daily water intake by body weight
The table below applies the 30–35 ml per kg formula. Find your starting range, then adjust it using the next section.
| Body weight | Lower end (30 ml/kg) | Upper end (35 ml/kg) | In 8-oz glasses |
|---|---|---|---|
| 50 kg (110 lb) | 1.5 litres | 1.75 litres | 6–7 glasses |
| 60 kg (132 lb) | 1.8 litres | 2.1 litres | 7–9 glasses |
| 70 kg (154 lb) | 2.1 litres | 2.45 litres | 9–10 glasses |
| 80 kg (176 lb) | 2.4 litres | 2.8 litres | 10–12 glasses |
| 90 kg (198 lb) | 2.7 litres | 3.15 litres | 11–13 glasses |
| 100 kg (220 lb) | 3.0 litres | 3.5 litres | 12–15 glasses |
These figures are for fluid from drinks. Food typically supplies about a fifth of your daily water, so fresh produce already covers part of the total.
How exercise, heat, and illness change your needs
Your baseline is only the starting point. On some days you will lose much more fluid, and those are the days to drink more — ideally before you feel very thirsty.
- Exercise: as a general guide, add about 350 to 500 ml (12 to 17 fl oz) for every 30 minutes of moderate exercise, and more for intense or long sessions. A one-hour run in warm weather can easily cost you a litre of sweat. Weighing yourself before and after a long session is a practical check: each kilogram lost is roughly a litre of fluid to replace over the following hours, along with food and electrolytes.
- Heat and humidity: hot weather increases sweat losses even without exercise. If you are outdoors or working physically, move towards the top of your weight-based range and beyond it if needed.
- Illness: fever, vomiting, or diarrhoea all increase fluid losses. Sipping water, oral rehydration drinks, or clear broths little and often is the standard general advice, and medical advice should be sought if symptoms are severe or prolonged.
- Pregnancy and breastfeeding: needs rise during pregnancy by roughly 300 ml a day, and during breastfeeding by considerably more — often around 700 ml to 1 litre extra — because breast milk itself is mostly water. These are general figures only; anyone pregnant or breastfeeding should follow their clinician's guidance.
On demanding days, drink a glass on waking, keep water within reach, and top up afterwards rather than catching up late in the evening.
Do coffee and tea count? What about food?
Yes — coffee and tea count towards your daily fluid intake. Caffeine has a mild diuretic effect, but the water in the drink more than compensates at normal intakes. Alcohol is different: it increases urine output, so alcoholic drinks should not be counted towards your target.
Food counts too: cucumber and lettuce are over 95% water, watermelon and oranges around 90%, and even cooked rice and yoghurt contribute. Fresh produce can quietly supply 500 ml or more a day.
Can you drink too much water?
Yes, though it is uncommon in healthy adults with normal kidney function. Drinking very large amounts in a short time can dilute the sodium in your blood, a condition called hyponatraemia. It is most often seen in endurance athletes who drink far more than they sweat, or in water-drinking challenges — not in someone sipping an extra glass or two through the day.
Spread your intake across the day, do not routinely exceed about 1 litre per hour, and replace electrolytes through food or a sports drink during very long, sweaty exercise. This article is general information only, not medical advice; anyone with a kidney, heart, or liver condition, or on fluid restrictions, should follow their clinician's guidance.
Signs you may need more water
Your body gives fairly clear feedback. Watch for these signs, especially in warm weather or after exercise:
- Dark yellow urine — pale straw is generally a good sign, while dark amber suggests you need more fluid
- Infrequent urination, dry mouth, or a dry, sticky feeling in the mouth
- Headache, tiredness, or difficulty concentrating that improves after drinking
- Dizziness on standing, or unusually low energy during a workout
- Dry skin and lips, or constipation that is worse than usual
None of these signs alone proves dehydration, but a cluster of them on a hot day is a strong nudge to drink. Aim for pale yellow urine; completely clear urine can mean you are overdoing it.
A simple daily plan that actually works
Knowing your number is only half the job; spreading it through the day is the other half. Drink a large glass (300–400 ml) soon after waking, a glass with each meal (roughly 750 ml across the day), and sip from a bottle kept at your desk or in your bag between meals rather than waiting until you feel parched. Around exercise, drink before you start, sip during, and top up afterwards. Taper off in the last hour or two before bed if nighttime trips to the bathroom disturb your sleep, but do not skip your evening glass entirely if you are still short of your target.
A marked reusable bottle makes this automatic: a 750 ml bottle filled three times plus a glass with a meal reaches 2.5 litres.
The bottom line
Forget a single magic number. Take your body weight, multiply by 30 to 35 ml per kilogram, and you have a personal starting range: about 1.8 to 2.1 litres for a 60 kg person, 2.4 to 2.8 litres for an 80 kg person, and 3.0 to 3.5 litres for a 100 kg person. Add more on exercise days and in heat, count tea, coffee, and water-rich foods towards the total, and let pale-yellow urine and steady energy be your everyday feedback. If you want your exact figure without the arithmetic, try the free Water Intake Calculator — enter your weight and activity level, and it will give you a daily target in seconds.
Frequently Asked Questions
Is 8 glasses of water a day a real rule?
No, it is a rough guideline, not a scientific rule. Your needs vary with body weight, activity, and climate.
How much water should I drink per day?
This guide uses a weight-based formula of roughly 30 to 35 ml per kilogram of body weight per day, adjusted upward for exercise and heat.
Do coffee and tea count toward water intake?
Yes. They contribute to daily fluid intake, and food provides roughly another 20 percent of your daily water.
Can you drink too much water?
Yes, overhydration (hyponatremia) is rare but real. Spread your intake through the day rather than drinking very large amounts at once.
Try it yourself: use our free Water Intake Calculator to get your number in seconds — no sign-up needed.