"Drink 8 glasses a day" gets repeated everywhere — but is it actually what the NHS recommends? And does your tea, coffee or fizzy water count? Here's the straight answer for UK adults.
The NHS recommendation
The NHS Eatwell Guide recommends adults drink 6 to 8 cups or glasses of fluid a day — that's roughly 1.5 to 2 litres.
How to work out your personal target
The 2-litre rule is a starting point, but it doesn't account for body size or activity. A simple formula many UK dietitians use:
Bodyweight in kg × 33ml
So a 70kg adult would aim for 70 × 33 = 2,310ml (~2.3 litres). A 90kg adult would aim for ~3 litres.
Add roughly 500ml extra per hour of exercise, or more in hot weather.
Signs you're not drinking enough
The NHS lists these as common signs of dehydration in adults:
- Feeling thirsty (by the time you feel thirsty, you're already mildly dehydrated)
- Dark yellow, strong-smelling urine
- Peeing less often than usual (fewer than 4 times a day)
- Feeling dizzy or lightheaded
- Feeling tired
- A dry mouth, lips and eyes
The simplest test: glance at the toilet bowl. Pale straw colour = well hydrated. Dark amber = drink more.
Can you drink too much water?
Yes, though it's rare. Drinking massive amounts of water in a short time can dilute the sodium in your blood — a condition called hyponatraemia. It's been documented mainly in marathon runners and military recruits drinking 4+ litres in a few hours.
For a normal day, you'd struggle to drink "too much" — your kidneys can clear up to a litre an hour. If you ever feel headachy or nauseous after a lot of water, ease off and add a pinch of salt to your next meal.
Does tea and coffee count?
Yes. The NHS, British Nutrition Foundation and the European Food Safety Authority all agree: tea and coffee in normal amounts (up to 4 cups a day) contribute to hydration, despite caffeine's mild diuretic effect. The water content far outweighs the diuretic effect.
What doesn't count well:
- Alcohol — it's a net dehydrator
- Sugary fizzy drinks — they contribute fluid but bring 100+ calories per can
- Energy drinks — same problem
Special situations
Exercise
Add 500ml–1L per hour of moderate exercise. For longer sessions or hot weather, an electrolyte tablet helps replace the salt lost in sweat.
Hot UK summers
The NHS advises drinking more during heatwaves — aim for 2.5–3 litres if temperatures are above 25°C, especially for over-65s and children.
Pregnancy and breastfeeding
NHS guidance is roughly an extra 300ml/day during pregnancy and an extra 700ml/day while breastfeeding.
Older adults
Thirst sensation reduces with age. Over-65s should drink to a schedule (e.g. a glass with each meal, plus one mid-morning and mid-afternoon) rather than waiting to feel thirsty.
Tricks to drink more
- Keep a 750ml bottle on your desk and aim to refill it twice
- Drink a glass first thing in the morning before tea/coffee
- Have a glass with every meal and snack
- If plain water is boring, add a slice of lemon, cucumber, or use sugar-free squash
- Set a phone reminder if you forget
The bottom line
The NHS says 1.5–2 litres a day for most adults, with personalised needs around 33ml per kg of body weight. Tea and coffee count. Watch your urine colour — it's the most reliable indicator. Use the calculator below to get your personal target.
Sources: NHS Eatwell Guide, NHS Live Well, British Nutrition Foundation, European Food Safety Authority. This article is general information only and does not replace medical advice.