You’ve heard it your whole life: drink eight glasses of water a day. It’s simple, easy to remember, and — according to actual science — not really based on much.
The “8×8 rule” (eight 8-ounce glasses, totalling about 2 litres) became popular decades ago, but researchers have struggled to find solid evidence behind it. Your actual water needs depend on your body size, how active you are, where you live, and what you eat. Two people with completely different lives can have very different hydration needs — and both can be perfectly healthy.
This post explains how to figure out the right amount for you specifically.
Why Your Body Needs Water
Before getting into numbers, it’s worth understanding what water actually does in your body — because it’s a lot more than quenching thirst.
Water makes up around 60% of your body weight. It’s involved in almost every biological function: regulating your body temperature through sweat, transporting nutrients through your bloodstream, flushing waste products through your kidneys, lubricating your joints, and keeping your cells functioning properly.
When you don’t drink enough, even mild dehydration — as little as 1-2% of body weight — can cause noticeable effects: reduced concentration, headaches, fatigue, and a drop in physical performance. By the time you feel thirsty, you’re already slightly dehydrated.
What the Research Actually Says
Different health organisations give slightly different recommendations, but they all point to the same general range:
| Organisation | Men | Women |
|---|---|---|
| National Academies of Medicine (US) | 3.7 litres (about 15.5 cups) total fluids/day | 2.7 litres (about 11.5 cups) total fluids/day |
| European Food Safety Authority | 2.5 litres/day | 2.0 litres/day |
| NHS (UK) | 6-8 glasses/day (roughly 1.5-2 litres) | 6-8 glasses/day |
One important detail: these numbers include all fluid intake — not just plain water. About 20% of your daily water comes from food, particularly fruits and vegetables. Tea, coffee, milk, and other beverages all contribute too.
So if you’re eating a diet with plenty of fruits and vegetables and drinking other beverages through the day, your plain water target is lower than the headline numbers suggest.
How to Calculate Your Personal Water Intake
The most commonly used formula for estimating individual water needs is based on body weight:
Drink 35 ml of water per kilogram of body weight per day.
Or in imperial: roughly 0.5 oz per pound of body weight.
Example:
- Person weighing 70 kg → 70 × 35 = 2,450 ml (2.45 litres) per day
- Person weighing 90 kg → 90 × 35 = 3,150 ml (3.15 litres) per day
This is your baseline — the amount for a moderately active person in a temperate climate. From here, you adjust upward based on your individual factors.
Want a quick, accurate result? Use our Water Intake Calculator — enter your weight, activity level, and climate, and it gives you a personalised daily target in seconds.
5 Factors That Change How Much You Need
1. Activity Level
Every hour of moderate exercise causes you to lose an extra 500–1,000 ml of water through sweat, depending on intensity and conditions. If you work out regularly, add at least 500 ml to your daily intake on exercise days — more if you sweat heavily or train in heat.
A practical approach: weigh yourself before and after exercise. Every 1 kg of weight lost during a session equals roughly 1 litre of fluid that needs replacing.
2. Climate and Weather
Heat dramatically increases your water needs. People living in hot climates or working outdoors in summer may need 1–2 extra litres per day compared to someone in a cool, temperate environment. High altitude also increases fluid loss through faster breathing.
3. Body Size
Larger bodies contain more water and require more daily intake to maintain balance. This is why the body-weight formula is more useful than flat recommendations — a 50 kg person and a 100 kg person have genuinely different needs.
4. Diet
A diet rich in water-heavy foods — cucumbers, lettuce, watermelon, soups, fruits — contributes meaningfully to your daily intake. If you eat plenty of these, you can drink slightly less water and still stay hydrated. A diet heavy in salty, processed foods has the opposite effect.
5. Health and Medical Conditions
Illness that causes fever, vomiting, or diarrhoea sharply increases fluid loss and requires more water to compensate. Pregnant women need about 300 ml more per day than usual. Breastfeeding mothers need roughly 700 ml more. Certain kidney conditions, on the other hand, may require limiting fluid intake — always follow medical advice if this applies to you.
Signs You’re Not Drinking Enough
Your body gives fairly clear signals when water intake is low:
Urine colour is one of the most reliable indicators. Pale yellow means you’re well-hydrated. Dark yellow or amber means drink more. Clear is fine but suggests you may be slightly overdoing it.
Frequency also matters. If you’re going more than 4 hours between bathroom trips during the day, you’re probably not drinking enough.
Other signs of mild dehydration: persistent headaches, afternoon energy crashes, dry mouth, difficulty concentrating, feeling hungry when you’ve eaten recently (thirst is often mistaken for hunger).
The “8 Glasses” Rule — Is It Useless?
Not entirely. For a moderately active person of average size in a temperate climate who eats a typical diet, eight 8-ounce glasses (about 2 litres) of water lands in a reasonable range. It’s a decent default if you have no better information.
The problem is that it gets applied uniformly — to a 50 kg woman sitting at a desk in Edinburgh and a 90 kg man doing physical work in Dubai — as if they have the same needs. They don’t, by a significant margin.
Use it as a rough starting point if helpful, but the body-weight formula gives you something closer to your actual personal target.
Practical Tips to Hit Your Daily Target
Start the day with water. A glass of water first thing in the morning, before coffee or food, is an easy habit to build and starts your intake early before the day gets busy.
Drink before meals. Having a glass of water 15–20 minutes before eating helps with both hydration and portion control — the stomach has limited space, and water occupies some of it.
Carry a measured bottle. Knowing that your bottle holds 750 ml or 1 litre makes it easy to track how many you’ve had. Most people find that seeing the container makes them drink more consistently.
Eat water-rich foods. Cucumber, watermelon, strawberries, oranges, celery, and soups all contribute to your daily water intake without requiring you to drink anything extra.
Don’t wait for thirst. Thirst is a late signal — by the time you notice it, you’re already mildly dehydrated. Drink at regular intervals through the day rather than in response to thirst alone.
A Quick Reference by Body Weight
| Body Weight | Estimated Daily Water Intake |
|---|---|
| 50 kg (110 lbs) | ~1.75 litres (7 cups) |
| 60 kg (132 lbs) | ~2.1 litres (9 cups) |
| 70 kg (154 lbs) | ~2.45 litres (10 cups) |
| 80 kg (176 lbs) | ~2.8 litres (12 cups) |
| 90 kg (198 lbs) | ~3.15 litres (13 cups) |
| 100 kg (220 lbs) | ~3.5 litres (15 cups) |
Add 500 ml for every hour of moderate exercise. Add 500–1,000 ml if you live in a hot climate.
Bottom Line
There is no single right answer for everyone — but there is a right answer for you, and it’s closer to 35 ml per kilogram of body weight than it is to the generic 8-glasses rule.
The basics: start with your body weight formula, adjust for your activity level and climate, watch your urine colour as feedback, and build a few simple habits that keep your intake consistent through the day.
Use our Water Intake Calculator to get your personalised daily target instantly. And if you’re tracking your overall health alongside hydration, our BMI Calculator and Calorie Calculator give you a fuller picture of where you stand.