How Much Water Should You Drink a Day for Your Weight?
This article is general information, not medical advice. If you have a heart, kidney or liver condition, or a doctor has told you to limit fluids, follow their guidance instead.
How much water should you drink a day for your weight? The short answer most people can use: aim for roughly 30 to 35 millilitres of total water per kilogram of body weight (about half a US fluid ounce per pound), then remember that around a fifth of that usually comes from food. For a 70 kg (154 lb) adult, that works out to about 1.7 to 2.0 litres from drinks on an ordinary day. Below, we explain where that rule of thumb comes from, how it lines up with official UK, European and US guidance, and how to adjust it for exercise and hot weather, with worked examples you can follow line by line.
What the official guidelines actually say
It is worth starting with what health bodies recommend, because none of them uses body weight as the headline figure. They give daily amounts for typical adults, and weight-based rules are a way of personalising those numbers.
- NHS (UK): the Eatwell Guide advises 6 to 8 glasses of fluid a day. Water, lower-fat milk and sugar-free drinks, including tea and coffee, all count.
- EFSA (Europe): the European Food Safety Authority sets an adequate intake of 2.0 litres of total water a day for women and 2.5 litres for men, covering water from both food and drinks.
- US National Academies: the adequate intake is 2.7 litres of total water a day for women and 3.7 litres for men. Because roughly 20% typically comes from food, that is about 2.2 litres (around 9 cups) and 3.0 litres (around 13 cups) from drinks.
- CDC (US): does not set a single daily target. It encourages water as the main drink and notes that needs rise with heat and activity.
| Source | Women | Men | What it covers |
|---|---|---|---|
| NHS Eatwell Guide | 6 to 8 glasses | 6 to 8 glasses | Drinks only |
| EFSA | 2.0 L (68 US fl oz) | 2.5 L (85 US fl oz) | Food plus drinks |
| US National Academies | 2.7 L (91 US fl oz) | 3.7 L (125 US fl oz) | Food plus drinks |
| US National Academies, drinks share | about 2.2 L (74 US fl oz) | about 3.0 L (101 US fl oz) | Drinks only |
Two things jump out. First, the figures differ because each body looked at different population data, so there is no single “correct” number. Second, you must check whether a figure means total water or drinks only. Mixing them up is the most common reason people think they are falling short when they are not.
The weight-based method, step by step
The 30 to 35 ml per kg rule is a widely used rule of thumb rather than an official guideline, but it scales the averages above to your size. A heavier body has more tissue to supply and generally loses more water, so it makes sense that intake should rise with weight. Here is how to apply it:
- Find your weight in kilograms. If you know it in pounds, divide by 2.2046. If you use stones, multiply by 6.35.
- Multiply by 30 and by 35. This gives a range for total daily water in millilitres.
- Take 80% of each figure. This removes the share that typically comes from food, leaving the amount to drink.
- Add extra for exercise and heat. A practical allowance is 400 to 800 ml for each hour of moderate exercise, more if you sweat heavily.
- Convert to glasses or ounces if that helps. Divide millilitres by 250 for glasses, or by 29.57 for US fluid ounces. A UK fluid ounce is slightly smaller (28.41 ml), which is why the same bottle shows different ounce figures on each side of the Atlantic. Our guide to UK vs US gallons explains that difference in detail.
If you prefer working in pounds, the US shortcut of “half an ounce per pound” equals about 32.6 ml per kg, which sits right in the middle of the metric range.
Worked examples with real numbers
Example 1: a 70 kg (11 st, 154 lb) adult on a normal day
Total water, low end: 70 x 30 = 2,100 ml
Total water, high end: 70 x 35 = 2,450 ml
From drinks, low end: 2,100 x 0.8 = 1,680 ml
From drinks, high end: 2,450 x 0.8 = 1,960 ml
In 250 ml glasses: 1,680 / 250 = 6.7 glasses, and 1,960 / 250 = 7.8 glasses
So this person should aim for roughly 7 to 8 glasses of fluid a day, which matches the NHS advice of 6 to 8 glasses neatly. In US units, 1,680 to 1,960 ml is about 57 to 66 fluid ounces.
Example 2: a 200 lb (90.7 kg) man who runs for an hour
Weight in kg: 200 / 2.2046 = 90.7 kg
Total water: 90.7 x 30 = 2,722 ml, and 90.7 x 35 = 3,175 ml
From drinks: 2,722 x 0.8 = 2,177 ml, and 3,175 x 0.8 = 2,540 ml
Add one hour of running: 2,177 + 400 = 2,577 ml (low), and 2,540 + 800 = 3,340 ml (high)
In US fluid ounces: 2,577 / 29.57 = 87 fl oz, and 3,340 / 29.57 = 113 fl oz
That range of roughly 2.6 to 3.3 litres brackets the US National Academies drinks figure for men of about 3.0 litres, which is reassuring: the weight method and the population average agree for someone of this size and activity level.
Example 3: measuring your own sweat rate
Rules of thumb can be off by a lot for heavy sweaters, so a simple test helps. Weigh yourself before and after an hour of exercise, with minimal clothing and after using the toilet.
Weight before: 82.0 kg. Weight after: 81.2 kg. Weight lost: 0.8 kg
Water drunk during the session: 0.5 L
Sweat loss: 0.8 + 0.5 = 1.3 L in one hour
Because 1 kg of body weight lost is roughly 1 litre of fluid, this person sweats about 1.3 litres an hour (around 44 US fl oz), well above the 400 to 800 ml allowance. A common sports-nutrition guideline is to replace about 1.25 to 1.5 times the weight lost afterwards, because you keep losing some fluid as urine: 0.8 x 1.25 = 1.0 L, up to 0.8 x 1.5 = 1.2 L.
Quick reference table by body weight
The table uses the 30 to 35 ml per kg range and assumes 80% of total water comes from drinks. Figures are for a normal day with no extra exercise.
| Body weight | Total water per day | From drinks (metric) | From drinks (US fl oz) |
|---|---|---|---|
| 50 kg / 110 lb | 1.50 to 1.75 L | 1.20 to 1.40 L | 41 to 47 |
| 60 kg / 132 lb | 1.80 to 2.10 L | 1.44 to 1.68 L | 49 to 57 |
| 70 kg / 154 lb | 2.10 to 2.45 L | 1.68 to 1.96 L | 57 to 66 |
| 80 kg / 176 lb | 2.40 to 2.80 L | 1.92 to 2.24 L | 65 to 76 |
| 90 kg / 198 lb | 2.70 to 3.15 L | 2.16 to 2.52 L | 73 to 85 |
| 100 kg / 220 lb | 3.00 to 3.50 L | 2.40 to 2.80 L | 81 to 95 |
For people well above a healthy weight, the formula can overstate needs, because fat tissue holds less water than muscle. In that case, treating the official averages as a ceiling and checking your urine colour is more sensible than chasing a very high number.
When you need more, and when less
Your figure is a starting point, not a quota. You will need more on hot or humid days, at altitude, when you have a fever, vomiting or diarrhoea, and during pregnancy or breastfeeding. Physically demanding jobs and long walks count as exercise too. If you are tracking activity alongside food, our calorie burn calculator gives a sense of how hard a session was, which is a useful clue to how much you sweated.
You may need less if you eat a lot of fruit, vegetables, soups and yoghurt, which are mostly water. And some people must restrict fluids for medical reasons, such as heart failure or advanced kidney disease. More is not always better: drinking very large amounts quickly can dilute the sodium in your blood, a condition called hyponatraemia, which is a recognised risk in long endurance events.
The simplest daily check, used by the NHS, is urine colour. Pale straw yellow suggests you are drinking enough. Dark yellow or strong-smelling urine suggests you need more, while completely clear urine all day may mean you are drinking more than you need. Thirst is also a reliable signal for most healthy adults, though it becomes less sensitive with age.
Frequently asked questions
Do tea and coffee count towards my daily water intake?
Yes. Both the NHS and US guidance count tea and coffee. Caffeine has a mild diuretic effect, but at normal intakes the fluid in the cup more than makes up for it. Milk, sugar-free squash and soups count too. Sugary and alcoholic drinks also contain water but are not good choices for hydration.
Is 8 glasses a day a myth?
Not exactly, but it is a rough average rather than a rule. Eight 250 ml glasses is 2 litres, which fits a typical adult reasonably well, as Example 1 shows. A smaller, sedentary person may need less, and a large, active person in summer may need considerably more.
How much water should I drink a day in litres if I weigh 80 kg?
About 2.4 to 2.8 litres of total water, of which roughly 1.9 to 2.2 litres would come from drinks (65 to 76 US fl oz). Add 400 to 800 ml for each hour of exercise.
Work out your own number
The quickest way to get a personalised figure is our free water intake calculator, which takes your weight and activity level and gives a daily target in litres, glasses and ounces. If you also want to understand how your energy needs scale with your size, read our explainer on TDEE vs BMR. Whatever number you settle on, let thirst and urine colour be your day-to-day guide, and speak to your GP or doctor if you have a condition that affects fluid balance.
Not medical advice: these figures are general estimates for healthy adults and are not a substitute for advice from a qualified health professional.