How much water should I drink per day in the UK?

The European Food Safety Authority (EFSA) recommends 2.0 litres per day for women and 2.5 litres per day for men as a population average. However, a more accurate method is 30–35ml per kg of body weight, adjusted upward for physical activity and hot weather. These figures cover total fluid intake, including water from food and other drinks.

Does the '8 glasses a day' rule have any scientific basis?

Not really. It traces back to a 1945 US Food and Nutrition Board recommendation that suggested 2.5 litres of water daily — but the same document noted that most of this would come from food. That second part was ignored, and '8 glasses' became the folk version of the advice. There is no peer-reviewed study that has ever validated 8 glasses as a universal human requirement.

Does coffee count towards my daily water intake?

Yes. The claim that caffeine dehydrates you is overstated for moderate consumption. Research shows that the mild diuretic effect of caffeine in regular drinkers is more than offset by the fluid in the drink itself. Tea and coffee both count towards your daily total. Very high caffeine intake (multiple energy drinks, for example) is a different matter.

How do I know if I'm drinking enough water?

Urine colour is the most practical check. Pale straw yellow means you're well hydrated. Dark yellow or amber means you need to drink more. Clear urine can occasionally indicate over-hydration. Thirst is also a reasonable signal in healthy adults — it's not as delayed as some people claim, though it can be blunted in older adults.

Should I drink more water when exercising?

Yes. For moderate exercise lasting under an hour, an additional 500–750ml is a reasonable guideline. For intense or prolonged exercise, especially in warm conditions, you may need significantly more. The weight-based calculation (30–35ml per kg) is a resting baseline — use our Water Intake Calculator to get an activity-adjusted figure.

Health

How Much Water Should You Actually Drink Per Day? (It's Not 8 Glasses)

Someone out there is diligently counting their eighth glass of water at 9pm, despite weighing 58kg, sitting at a desk all day, and living in Scotland. The number is wrong for them. Here's what the science actually says.

7 April 2026

By Krishna Chaitanya, Software Engineer

Picture someone filling their eighth glass of water at 9:47pm. They weigh 58kg. They sit at a desk in Edinburgh. They have not exercised today. It is 12°C outside. They are slightly annoyed at themselves for falling behind on their water count and are now trying to get two glasses down before bed, which will mean at least one trip to the bathroom at 2am.

The number they are targeting was never calibrated to them. It was not calibrated to anyone in particular. It is a round number that escaped from a sentence no one finished reading, then got repeated in wellness articles, on branded water bottles, and in GP waiting rooms until questioning it started to feel almost rebellious.

It should not feel rebellious. It should feel obvious.

Where the "8 Glasses" Rule Actually Came From

In 1945, the US Food and Nutrition Board recommended that adults consume about 2.5 litres of water per day. The same document, in the very next sentence, clarified that "most of this quantity is contained in prepared foods." That second sentence disappeared. What remained was the number.

The result was a recommendation stripped of context and turned into a universal prescription, one that ignores body weight, activity level, climate, diet, and whether you are 28 or 78.

Under-hydration is a real problem. Mild dehydration impairs concentration, increases fatigue, and in older adults it raises the risk of urinary tract infections and falls. But the fix is not a fixed glass count applied to everyone regardless of their circumstances.

Over-hydration is less discussed but also real. The kidneys can process roughly 0.8 to 1 litre of water per hour. Drinking significantly more than that, particularly all at once, can dilute sodium levels in the blood: a condition called hyponatraemia. In rare cases it is dangerous. Obsessively hitting arbitrary targets is not a neutral habit.

Clear glass of water — hydration doesn't need to be complicated

What the Science Actually Says

The most authoritative source for European adults is the European Food Safety Authority. In their 2010 dietary reference values (EFSA Journal, 8(3)), EFSA set adequate intake levels at 2.0 litres per day for women and 2.5 litres per day for men, covering total fluid intake from all sources, including food. These are population averages, not targets for every individual.

The more useful number for individuals is weight-based: 30 to 35ml per kilogram of body weight per day, at rest, in a temperate climate. A larger body has more cells requiring fluid, a larger surface area losing fluid through the skin, and higher metabolic activity generating waste that needs excreting.

That 30 to 35ml/kg range is where I think most hydration advice should start. It is the baseline used in clinical nutrition and hospital fluid management. It scales with the person rather than pretending everyone is the same.

Two adjustments matter beyond the baseline. Exercise increases fluid loss through sweat; even a 45-minute brisk walk can add 500ml or more to your daily requirement depending on conditions. And temperature matters even at rest: in summer, or if you work in a warm environment, the baseline should shift upward.

One more thing worth saying clearly: coffee and tea count. The claim that caffeine dehydrates you has been exaggerated into a myth. Moderate caffeine intake, up to around 400mg per day (roughly four standard cups of coffee), does not produce a net dehydrating effect in regular consumers. The fluid in the drink more than compensates for any mild diuretic action. This has been established in multiple peer-reviewed studies, and EFSA's 2011 caffeine opinion is consistent with it.

How the Water Intake Calculator Solves This

Rather than applying a fixed number to everyone, the Water Intake Calculator takes your weight, activity level, and climate into account and gives you a personalised daily target in both litres and glasses.

It takes about 20 seconds to use. Enter your weight (metric or imperial), select your typical activity level, and indicate your climate. The calculator applies the 30-35ml/kg formula with appropriate adjustments and shows you a specific daily target. Not a generic one.

The eight-glasses rule cannot give you a figure calibrated to your body and your life. A weight-based, activity-adjusted calculator can.

A Worked Example

Take a 75kg person who works an office job but goes for a run three or four times a week. They live in the UK, temperate climate, not particularly hot most of the year.

At rest, the baseline is: 75kg x 32.5ml = 2,437ml per day, using the midpoint of the 30-35ml range. That is approximately 2.4 litres, or just over 8 standard 250ml glasses.

Eight glasses, but personalised to this person rather than derived from a stripped-out 1945 recommendation. The number happens to look similar. For someone who weighs 60kg or 95kg, the result would diverge significantly.

Now add activity. On days this person runs for 45 minutes, they might add 500-750ml to their target. On a rest day, the baseline applies. The total shifts depending on the day.

Water Intake Calculator showing result for a 75kg moderately active person

The calculator handles this automatically when you select your activity level. If you want to understand the activity-level classifications in more depth, the TDEE Calculator uses the same activity multipliers and can help you work out which category fits your lifestyle.

What to Do With the Result

Once you have a daily target, do not start counting glasses. That is the behaviour the eight-glasses myth created, and it is not a particularly useful way to manage hydration.

A more practical approach:

Check urine colour as your real-time guide. Pale straw yellow is where you want to be. Dark yellow means drink more. Consistently clear can mean you are drinking more than you need, though this is rarely harmful.
Spread intake across the day. The kidneys can only process about 0.8 to 1 litre per hour. Drinking a litre of water in one go because you forgot earlier does not achieve the same thing as consistent intake. It mostly just produces a bathroom trip.
Drink ahead of exercise, not just during it. If you know you are going for a run in two hours, start hydrating before you leave.

Your daily target is a planning tool, not a score. The goal is consistent hydration, not hitting a number at 11:59pm.

Three Common Mistakes

Drinking all your water at once is probably the most common one. The kidneys have a processing limit. Large amounts of water drunk quickly are excreted rapidly rather than absorbed gradually. If you have a habit of drinking a litre of water when you first notice you are thirsty, you are not catching up. You are mostly just emptying your bladder an hour later.

Ignoring food water content is the second. Food contributes roughly 20% of daily fluid intake for most people. Fruit, vegetables, soups, and even bread contain water. Salads are mostly water. This is part of why the EFSA figures refer to total water from all sources, not just what you drink. If your diet runs to whole foods, your drinking target is slightly lower than if you eat mostly dry, processed food.

The third: applying the same target year-round. Your baseline in December in Glasgow is not the same as in July. Temperature, humidity, and air conditioning all affect sweat rate and fluid loss. A weight-based starting point is useful, but it should shift slightly with the seasons. The calculator accounts for climate, which is one reason it is worth using rather than memorising a single number.

For informational purposes only. Consult a healthcare professional for personalised advice.

Hydration does not need to be complicated. You have a weight. You have an activity level. There is a formula that puts those together and gives you a number that is actually relevant to you. Use the Water Intake Calculator and stop counting eight glasses that were never meant for you.