How much water should you drink a day?

How much water should I drink a day?

The NHS suggests 6 to 8 cups or glasses of fluid a day, a figure that comes from the government's Eatwell Guide. It calls this "just a guide" and says you may need more if you're pregnant or breastfeeding, in a hot environment, physically active for long periods, or ill or recovering from illness.

In litres, the British Dietetic Association (BDA) puts the amount you need from drinks at about 1.6 litres a day for women and 2 litres for men. With a medium glass holding about 200 ml, by the BDA's measure, eight glasses is 1.6 litres.

Those figures come from the European Food Safety Authority (EFSA), which set adequate intakes for total water, meaning drinks plus the water in food. Food supplies about 20 to 30% of it, from things like soup, stews, fruit and vegetables, according to the BDA.

EFSA's figures assume a moderate climate and moderate activity. They aren't scaled to body weight: EFSA sets one figure for adult women and one for adult men, from age 14. Rules of thumb you may see online, such as a set number of millilitres per kilo, aren't part of UK or EFSA guidance.

Do tea and coffee count towards your water intake?

Yes. The NHS says water, lower-fat milk and lower-sugar drinks, including tea and coffee, all count. The BDA explains that caffeine has a small dehydrating effect, but the water in a cup of tea or coffee more than makes up for it. Keep caffeine moderate: the NHS limit in pregnancy is 200 mg a day.

A few exceptions are worth knowing:

• If you're already dehydrated, the NHS advises avoiding caffeinated drinks and alcohol until you've recovered.
• In a heatwave, the NHS advises drinking extra fluids but avoiding alcohol, caffeine and hot drinks.
• The NHS lists drinking too much alcohol as a cause of dehydration, so don't count it towards your intake.
• Fruit juice and smoothies count but are high in sugar. The NHS limit is one small glass (150 ml) a day, with a meal.

How do you know if you're drinking enough?

Look at your pee. The NHS says most people should drink enough that their pee is a clear pale yellow. Dark yellow, strong-smelling pee is one of the signs of dehydration it lists, along with thirst, a headache, feeling dizzy or light-headed, tiredness, peeing less often and a dry mouth. The BDA notes that dark yellow pee occasionally has other causes, so speak to your GP if it doesn't change when you drink more.

Thirst is a useful signal but a slow one: the BDA notes you stop feeling thirsty before your body has fully rehydrated.

How much more should you drink in hot weather or when you exercise?

More, by an amount that depends on how much you sweat. EFSA notes that in extreme heat combined with hard exercise, daily water losses can reach about 8 litres, and that lost salts need replacing too.

For exercise, the American College of Sports Medicine's fluid replacement position stand suggests weighing yourself before and after a session to estimate your sweat rate, and aiming to avoid losing more than 2% of your body weight in fluid during exercise. For a 70 kg person that's 1.4 kg. Because sweat rates vary so much between people, it recommends a plan that fits you rather than a fixed amount.

In long endurance events, drinking too much is a real risk. A 2017 review of exercise-associated hyponatraemia recommends drinking according to thirst, and says fixed drinking targets aren't appropriate (Hew-Butler and colleagues). Our guide to electrolytes and hydration covers when salt matters as well as water.

Can you drink too much water?

Yes, though it's rare. EFSA notes that healthy kidneys can clear at most about 0.7 to 1 litre of water an hour. Drink much faster than that and the sodium in your blood becomes diluted, which in severe cases causes swelling of the brain. In sport, a 2017 review found drinking beyond thirst to be the most common cause (Hew-Butler and colleagues). At the 2003 London Marathon, 14 runners were treated at St Thomas' Hospital for low blood sodium, 11 of them confused, and some seem to have become confused only after they'd finished (Goudie and colleagues). Confusion after a long event is a medical emergency.

How much water do older adults need?

The same as other adults, according to EFSA, but it's harder to get right. EFSA notes that ageing kidneys are less able to concentrate urine, so older people need more water for each calorie they eat. The NHS lists older adults among those at higher risk of dehydration.

The BDA's advice for older adults is to drink regularly rather than wait for thirst, because you may not notice thirst the way you used to, and because the usual signs (pee colour, thirst and a dry mouth) don't reliably show dehydration in older people. It suggests at least 1,600 ml a day for women and 2,000 ml for men, a full glass with medicines, and a drink with and between meals. If you worry that drinking more will cause bladder problems, it suggests talking to a healthcare professional, because drinking less won't stop them.

When should you drink less, or more, than usual?

For heart failure, NICE guidance says people shouldn't be routinely told to restrict fluids, but some will be, for example if their blood sodium is low. People on kidney dialysis are often advised to limit how much they drink, because extra fluid builds up between sessions: North Bristol NHS Trust lists rapid weight gain, swollen ankles, breathlessness and higher blood pressure as signs of overload. The opposite applies after kidney stones: the NHS advises adults trying to prevent them to drink 2.5 to 3 litres of water a day.

How much water should I drink on creatine?

There's no official water target for people taking creatine, so the general NHS advice applies. Studies reviewed in the International Society of Sports Nutrition's creatine position stand found no sign that creatine causes dehydration or muscle cramps. Water still matters for mixing, because creatine dissolves poorly: our creatine calculator shows how much water dissolves your dose, and our guide to how to take creatine covers the rest.

What's the best drink to prevent muscle cramps?

No drink has good evidence behind it. The NHS lists not drinking enough among possible causes of leg cramps, alongside ageing, straining muscles during exercise, pregnancy and some medicines, and says the reason for some cramps is unknown.

For cramps during exercise, the case for dehydration or lost salt is weak. A 2009 review found that the evidence for those explanations came mainly from anecdotes, case series totalling 18 cases and one small study, while four prospective cohort studies didn't support them (Schwellnus). A 2022 review concluded that cramps come from several factors at once and that individual prevention plans are likely to work better than general advice such as drinking more (Miller and colleagues).

Two other studies are worth knowing about:

• In a small lab study, a small drink of pickle juice (about 75 ml) shortened electrically induced cramps by about 49 seconds compared with water, too fast to be explained by replacing fluid or salt. The authors suspect a reflex triggered in the mouth and throat (Miller and colleagues, 2010).
• A 2020 Cochrane review found magnesium is unlikely to prevent cramps in older adults in any meaningful way, and found no trials in exercise cramps (Garrison and colleagues).

The NHS suggests regular calf stretching, which may reduce cramps but may not stop them completely. See a GP if cramps disturb your sleep, last longer than 10 minutes, or come with numbness or swelling.

Where Natura fits

For most people on most days, tap water, tea and coffee do the job. Daily Electrolytes, in the Natura cabinet (coming soon) on our drinks shelf, gives 1,000 mg of sodium, 250 mg of potassium, 100 mg of magnesium and 50 mg of calcium per sachet, with no sugar, and is meant for long, sweaty sessions or heat rather than everyday drinking. Magnesium contributes to electrolyte balance.