How much water should you drink a day?

Health correspondent ( 12 June 2015 )

Headaches, tiredness and constipation are just a few of the symptoms that can result from drinking too little water. Find out how much you need every day.

Take a good look at your body. Flesh, blood, bone, nails, hair and some gristly, squidgy bits that, luckily, you can’t see. All of it needs water to work well and feel good.

How much water is in our body?

‘About 72% of our weight, not including fat, is water (reducing to about 56% in older men and 47% in older women)’ says Paul McArdle, dietician and spokesperson for the British Dietetic Association (BDA). ‘That’s around 45 litres in a 70kg man, and a little less in a woman. Most of this is contained in our body cells. The rest is part of the fluid that surrounds cells and plasma.’

What does water do for the body?

Water doesn’t just sit there; it’s busy, 24 hours a day, keeping the myriad complex processes in our bodies working. The fluids carry nutrients to cells and carry waste away, help most of the chemical reactions in our bodies take place, keep our temperature regulated and help keep our eyes healthy. They also pitch in with the vital lubrication of our joints and digestive and mucous tracts.

‘The levels of our body fluids are very tightly controlled,’ explains Rachel Cooke, senior public health dietician and also a spokesperson for the BDA. ‘It only takes very small changes in our fluid balance to make us feel unwell, and to make us ill.’

Unfortunately it seems that we’re not terribly good at drinking enough, (and we’re not talking about Chablis here). ‘Fluid intake is influenced mostly by thirst,’ says Rachel Cooke. And the thirst response is very sensitive. We only need to be a few hundred mls short of our ideal water balance to feel thirsty. This may sound reasonable, but by the time we feel thirsty, we may already be slightly dehydrated – in fact it can be one of the first signs of mild dehydration.

How do our bodies lose water?

We expel fluid from our bodies every day. Around 50% goes out in our urine – along with toxic waste products. About 47% is lost through our skin, although this does depend on how hot it is and how much energy we’re expending. The more we sweat, the more water we lose – go somewhere very hot for your holidays and you could lose 10 litres a day. We lose around 2% of our water in our faeces, and a very small amount through our lungs.

An average, healthy adult can lose almost 2.5 litres of water a day. And with the water you lose electrolytes – minerals like calcium, potassium and sodium - which help maintain your body’s fluid balance. Replace the lost fluids and electrolytes and you’ll be hunky-dory. It’s when you drink less than you expel that your body starts to dry out. And that’s when you experience your body’s reaction to dehydration.

What are the signs of dehydration?

As well as feeling thirsty (although this isn’t always a reliable sign, especially in older adults and children), you may well have some of the following symptoms of dehydration:

  • Dark-coloured urine
  • Headaches
  • Constipation
  • Confusion and irritability
  • Poor concentration
  • Dry mouth
  • Tiredness
  • Dizziness

The most reliable way to check whether you’re drinking enough is to look at the colour of your urine. It should be a light straw colour. If it’s dark or strong-smelling, you’re dehydrated. (This doesn’t apply to your first wee of the morning, when you’ve been without a drink for some hours.)

Read our guide to staying hydrated

What are the causes of dehydration?

Dehydration can affect anyone, and it isn’t simply a matter of not drinking enough. Older people tend to be more at risk because they have a reduced thirst response, which means they just don’t feel as thirsty as they did when they were younger. 

Young children and people with chronic illnesses are also at increased risk. Sudden and severe diarrhoea, especially if accompanied by vomiting, a high fever and excessive sweating, can cause dehydration, as can uncontrolled or undiagnosed diabetes. Certain medicines, including some blood pressure medications, diuretics and antihistamines can also make you dehydrated.

If you are mildly dehydrated, you should be able to replenish your body’s fluid balance just by drinking more. However, if you – or anyone else – suffers more severe symptoms of dehydration, such as being very thirsty, not urinating for eight hours, having visibly dried-up skin, or being dizzy and confused, you need to get medical help immediately. 

In older people especially, confusion caused by lack of fluids can be mistaken for more serious – and possibly permanent – mental decline, so it’s important to keep an eye on how much older friends and relatives are drinking.

How much water should you drink a day?

So what should you drink, and how much should you have?

Under normal circumstances, around 1.5 litres of fluid a day (about eight glasses of liquid) should be enough to keep your liquid levels healthy. If it’s very hot, you’ve been very active and sweaty, or you’ve had diarrhoea and/or vomiting, you’ll need to drink more to keep your stocks from running low.

Plain old tap water is perfect, but you don’t have to limit yourself to this. Milk, fruit and vegetable juice, squash and fizzy drinks will all add fluid to your system. Tea and coffee do have a slight diuretic effect – the caffeine in them and in some fizzy drinks can make you produce more urine than other drinks. But in most cases it makes very little difference, so these drinks still help you stay hydrated.

Alcohol is a stronger diuretic, and drinks with a high proportion of alcohol, such as spirits, will increase your fluid loss, and add to the effect you feel the next morning, as dehydration is part of the cause of hangover symptoms. But that doesn’t mean you have to stick to mineral water. Drinks that have a low alcohol content, such as wine spritzers and many beers, won’t normally upset your fluid balance.


The opinions expressed are those of the author and are not held by Saga unless specifically stated.

The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.