How much water do you really need?

Few pieces of health advice are repeated as often as "drink eight glasses of water a day". It sounds precise, it is easy to remember, and it is not where the evidence comes from. The real answer is a little less tidy and a lot more relaxed.

What the official numbers say

The European Food Safety Authority sets an adequate intake of 2.0 litres of water a day for women and 2.5 litres for men, for adults living in a moderate climate with moderate activity. The important detail is what that figure includes: water from drinks of every kind, and the water in your food. Fruit, vegetables, soups, yoghurt, even bread and cooked grains all contribute, roughly a fifth to a third of the total for most people.

The Swiss Society for Nutrition does the arithmetic for everyday life. Its fact sheet on fluids puts the total at about 2 to 2.5 litres a day for healthy adults. Someone eating along the lines of the Swiss food pyramid already gets more than a litre of that from food, so the recommended amount to drink is about 1 to 2 litres a day, with water as the first choice.

Where "eight glasses" comes from

When a kidney physiologist went looking for the evidence behind eight glasses a day, his 2002 review found no scientific studies supporting it. It also noted that in healthy adults, thirst is a precise system that does its job well. That does not make eight glasses harmful. It means the number is a habit, not a rule.

Yes, coffee and tea count

The idea that coffee dehydrates you is one of the most stubborn myths in nutrition. In a trial with 50 regular coffee drinkers, four cups a day for three days hydrated them as well as the same amount of water did. Another study of 72 men compared 13 different drinks and found that coffee, tea, cola and even lager did not make people pass more urine than water. Milk was actually retained better. Moderate coffee and tea are part of your fluid, not a debt against it. Alcohol and sugary drinks are a different conversation, and not for hydration reasons.

The simplest signs

You do not need to measure anything to know roughly where you stand.

• Thirst. For most healthy adults, drinking when you are thirsty works.
• The colour of your pee. Pale straw is the goal. In one pooled analysis, drinking about a litre more a day made urine around two shades lighter on a standard colour chart, so it is a sensitive, free signal. The NHS lists dark yellow, strong-smelling pee among the signs of dehydration.

What does it matter if you are a little short? Studies show small but real effects. A meta-analysis of 33 studies found that dehydration slightly impaired attention and some other mental tasks, more clearly once people had lost more than 2 percent of their body weight in fluid. In a small study of young women, mild dehydration after exercise worsened mood and concentration and brought on headaches, even though most test scores held up. The afternoon slump is not always about water, but sometimes it is.

Why there is no single number for you

Needs genuinely differ. A study of 5,604 people in 23 countries found that daily water turnover varied with body size, activity, climate, age and more. You need more in heat, when you train, when you have a fever or are unwell, and in dry air. Some people need less, or need to be careful: if you have a heart or kidney condition, or your doctor has asked you to limit fluids, follow their advice rather than any general number, this one included.

How Kata counts it

In Kata, water is one sentence. "Two glasses of water", "a big bottle on the walk", "a coffee and a glass of water with lunch": Kata turns each into an amount and adds it to your day. Your water for the day builds up as you go, with your goal beside it, and you can always add it by hand.

Kata's starting goal is a simple rule of thumb: about 35 millilitres per kilo of body weight, plus half a litre for each training session that day, up to two. That is on the generous side of the official figures, which count the water in your food as well. So treat it as a place to start. If it feels like too much, set your own goal, and let your thirst and the colour of your pee be the better guides. If you train hard in summer, you may find it is about right.

The aim is not to hit a number perfectly. It is to notice, on the days when you forgot to drink until the evening, and to make the next day a little easier.