The "8 glasses a day" myth
The widely cited rule of eight glasses (roughly 2 litres) of water per day has no solid scientific foundation. It appears to trace back to a 1945 US recommendation that included all fluids from food and drink combined, not water alone. The European Food Safety Authority (EFSA) sets total daily water intake (including water from food) at 2.0 litres for women and 2.5 litres for men. The NHS advises 6 to 8 cups or glasses of fluid per day and explicitly includes tea, coffee, milk, and juice.
Caffeine and hydration: tea and coffee do count
A persistent myth holds that tea and coffee are dehydrating and should not count towards fluid intake. A 2014 meta-analysis by Ganio et al. (American Journal of Clinical Nutrition) found no meaningful difference in hydration status between coffee drinkers and water drinkers at habitual consumption levels. The mild diuretic effect of caffeine is offset by the fluid volume consumed. The NHS and the British Dietetic Association both confirm that tea and coffee contribute to daily hydration. This matters in the UK, where the average adult drinks 3 to 4 cups of tea per day.
UK tap water quality and regional differences
UK tap water meets some of the most stringent standards in the world, regulated by the Drinking Water Inspectorate (DWI) in England and Wales. The experience varies considerably by region:
- South-east England and London: very hard water (250-400 mg/L calcium carbonate). Tastes chalky and leaves limescale, but provides more dietary calcium and magnesium.
- Wales and north-west England: soft water from upland reservoirs, a lighter, cleaner taste with less calcium.
- Scotland: Highland loch water is among the purest available in Europe. Scottish Water consistently achieves near 100% compliance with all standards.
- Northern Ireland: NI Water provides generally soft water from surface sources with good compliance rates.
PFAS in UK water: an emerging concern
PFAS (per- and polyfluoroalkyl substances, sometimes called "forever chemicals") have been detected in UK water supplies near industrial sites, airports, and military bases. The Drinking Water Inspectorate updated its guidance in 2021, setting a total PFAS limit of 0.1 micrograms per litre in line with EU Directive 2020/2184. Water companies are required to monitor and treat affected sources, and must provide test results on request. The DWI publishes annual compliance reports by supplier.
Adjustments for activity and climate
- Light exercise (30-60 min), add 350-700 ml
- Intense exercise (1-2 h), add 700-1,400 ml
- Hot weather, add 500-1,000 ml above baseline
- High altitude, increase intake; altitude raises respiratory water loss
- Pregnancy, add approximately 300 ml/day
- Breastfeeding, add approximately 700 ml/day
Signs of dehydration
- Dark yellow urine (target pale straw colour)
- Infrequent urination (fewer than 4 times per day)
- Dry mouth and throat
- Headache, fatigue, difficulty concentrating
Sources: NHS, EFSA 2010 dietary reference values, Ganio et al. 2014 (AJCN), DWI annual reports, DWI PFAS guidance 2021. This tool provides general information only.