Water Intake Calculator

Daily water needs depend on body weight, activity level, climate and overall health. A common starting point is 30–35 ml per kilogram of body weight. The NHS recommends 6–8 glasses (approximately 1.2 litres) of fluid per day as a minimum, though active people and those in warm climates need considerably more.

This calculator uses your weight and activity level to estimate a personalised daily target in litres. Exercise adds roughly 500–1,000 ml per hour of moderate-to-vigorous activity. Hot or humid weather, illness with fever, and pregnancy or breastfeeding all increase requirements further.

All fluids count towards your total, including tea, coffee, milk and juice, as well as water from food (fruit, vegetables and soups contribute roughly 20% of daily intake). Caffeinated drinks do count, despite common myths — their mild diuretic effect is offset by the fluid they provide.

How much water do you need daily? NHS Eatwell Guide: 6-8 glasses (1.2-1.5 L) of fluid daily, including water, lower-fat milk, sugar-free drinks, tea and coffee. EFSA reference values: 2.5 L/day men, 2.0 L/day women — including food (20-30% of intake) and all beverages. Old '8 glasses of water' rule: includes ALL fluids and food moisture, not just water. Body weight method: 30-35 ml/kg bodyweight = baseline. 70 kg person = 2.1-2.45 L total fluid.

When you need more — heat, exercise, illness. Hot weather: add 500-1,000 ml/day. Exercise: 500-750 ml/hour of moderate exercise; 1,000+ ml/hour vigorous. Fever (37.5°C+): add 200 ml/day per °C above normal. Diarrhoea/vomiting: NHS dioralyte sachets in 200 ml water — rehydration salts. Pregnancy: +300 ml/day. Breastfeeding: +700 ml/day. Diabetes: increased need from polyuria. Heart and kidney conditions: medical advice — fluid restriction may apply.

UK tap water quality — drink with confidence. UK tap water rated among world's safest. DWI (Drinking Water Inspectorate) regulates 50+ parameters. 99.97% compliance rate. Hardness varies: South-East England very hard (300+ mg/L CaCO₃); Scotland and South-West soft (under 60 mg/L). Hard water leaves limescale but is safe and possibly beneficial (calcium intake). Bottled water (NHS): unnecessary for health; environmental impact 1,000× tap. Filter jugs reduce chlorine taste; rarely improve health unless on borderline private supply.

Signs of dehydration and overhydration. Mild dehydration (1-2% body weight loss): thirsty, dark yellow urine, slight headache, fatigue. Moderate (3-4%): very thirsty, dry mouth, less urine, dizziness, irritability. Severe (5%+): heart rate up, blood pressure down, confusion — medical emergency. Urine colour chart: pale straw ideal; dark amber = drink more. Overhydration (hyponatremia): rare but possible with extreme intake (10+ L/day). Symptoms: nausea, headache, confusion, swelling. Marathon runners died from drinking too much — sodium tablets and sports drinks for endurance.

What counts as fluid — and what doesn't. Counts: water, tea, coffee (mild diuretic but net positive hydration), milk, fruit juice, sugar-free squash, soups, fruits (watermelon 92% water, cucumber 96%), vegetables. Caffeine effect on hydration: 200-300 mg caffeine (2-3 cups coffee) is mildly diuretic but net positive. Alcohol: diuretic effect — beer counts as fluid but pure alcohol causes net dehydration. Energy drinks high sugar+caffeine — limit. Best hydration drinks: plain water, herbal tea, milk (also provides electrolytes and protein post-exercise).