How Much Water to Drink a Day Calculator — UK Water Intake Tool

The NHS recommends drinking 6–8 glasses of fluid per day — approximately 1.5–2 litres for adults. Water, lower-fat milk, and sugar-free drinks all count. Tea and coffee count too, but should not be your only source. Hot weather and exercise increase your needs.

A common formula is 35ml per kilogram of body weight. Example: 70kg × 35ml = 2,450ml (2.45 litres) per day. Add 500–750ml extra for every hour of moderate exercise. Our calculator applies this formula automatically based on your weight and activity level.

Early signs include dark yellow urine, thirst, dry mouth, headache, tiredness, and reduced concentration. Severe dehydration causes dizziness, rapid heartbeat, and confusion. The simplest indicator is urine colour — pale straw yellow means well hydrated, dark amber means drink more.

Yes — despite the mild diuretic effect, coffee and tea do contribute to overall fluid intake. However, they should not replace plain water. The NHS includes tea and coffee in its recommended 6–8 glasses per day, but also advises limiting caffeine to 400mg per day (about 4 cups of coffee).

The "8 glasses a day" rule is a popular guideline but not scientifically precise. The NHS recommends 6–8 cups or glasses of fluid per day (about 1.5–2 litres). Your actual needs depend on body weight, activity level, and the weather. Heavier individuals and those who exercise regularly need more than 8 glasses.

During moderate exercise lasting under an hour, drink 500ml before, 150–200ml every 20 minutes during, and 500ml after. For intense sessions over an hour, increase to 500–750ml extra per hour of exercise. Sports drinks containing electrolytes are beneficial for sessions lasting over 90 minutes.

Yes — drinking excessive water in a short period can cause hyponatraemia (low blood sodium), a serious and potentially dangerous condition. This is rare in healthy adults following normal guidelines. Stick to the amounts calculated based on your body weight and activity level, and drink to thirst.

The NHS recommends drinking 6–8 cups or glasses of fluid every day — approximately 1.5–2 litres for adults. Water, lower-fat milk, and sugar-free drinks all count, as do tea and coffee. The NHS advises limiting sugary drinks due to their impact on dental health and weight. Our NHS water intake calculator uses these guidelines to give you a personalised daily target.

Yes — fruits and vegetables contain significant water and contribute to your overall daily hydration. Cucumber, celery, and lettuce are over 90% water; oranges and strawberries are around 85–87% water. While they add to your fluid balance, most health professionals recommend not relying on fruits and vegetables alone — plain water should still be your primary source to stay hydrated. People who eat a diet rich in fruits and vegetables may naturally need slightly less from drinks alone.

Yes — various health conditions significantly affect how much water you need. Kidney stones benefit from high intake (often 2.5–3 litres daily to flush the kidneys). Heart failure and kidney disease may require restricting fluids — always follow medical advice. Fever, vomiting, or diarrhoea causes the body to lose water rapidly, sharply increasing your recommended water intake. Pregnancy and breastfeeding also raise daily requirements. If you have any health conditions affecting fluid balance, consult your GP rather than relying solely on this calculator.

This NHS water intake calculator is based on official NHS hydration guidance — 6–8 glasses (1.5–2 litres) of fluid per day for most adults — combined with the 35ml-per-kg body weight formula used by UK dietitians, plus extra for exercise. Enter your weight and activity level above to get a personalised daily target in litres and glasses, instantly and free.