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Water Intake Calculator – How Much Water Should I Drink a Day?

Get a simple estimate of how much water to drink each day based on your body weight, how long you exercise and whether it’s hot. Results are shown in litres and glasses or fluid ounces and cups.

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How to use the water intake calculator

  1. Choose metric or US units.
  2. Enter your weight.
  3. Enter how many minutes of exercise you do on a typical day.
  4. Tick Hot or humid weather if it applies.

You’ll see how much to drink each day in litres and 250 ml glasses (metric) or fluid ounces and 8-ounce cups (US), with a breakdown of the base amount and the extra for exercise.

How the estimate works

Water (litres) = Weight (kg) × 0.035 + 0.35 × (exercise minutes ÷ 30) + 0.5 if hot

1 litre ≈ 33.8 US fluid ounces.

Worked examples

PersonDaily estimate
60 kg, no exerciseabout 2.1 L (71 fl oz)
75 kg, 30 minutes of exerciseabout 3.0 L (101 fl oz)
90 kg, 60 minutes of exercise in hot weatherabout 4.4 L (147 fl oz)

For the 75 kg example: 75 × 0.035 = 2.63 L, plus 0.35 L for 30 minutes of exercise = 2.98 L — about 12 glasses of 250 ml.

Where your water comes from

About 20% of the water you take in usually comes from food — fruit, vegetables, soups and yoghurt are mostly water. The rest comes from drinks, including water, tea, coffee and milk. That means you don’t need to drink the whole estimate as plain water.

Simple ways to drink more

  • Keep a bottle with you and refill it a set number of times a day.
  • Drink a glass with every meal and one when you wake up.
  • Add flavour with lemon, cucumber or mint.
  • Eat water-rich foods such as watermelon, oranges and salad.
  • Drink before, during and after exercise.

Signs of dehydration

Mild dehydration can cause thirst, dark yellow urine, a dry mouth, tiredness, headaches and poor concentration. Children and older adults are more at risk, and so is anyone who is ill with vomiting, diarrhoea or a fever. Drink small amounts often, and seek medical help for severe symptoms such as confusion, fainting or no urine for many hours.

Who needs a different amount

Pregnant and breastfeeding women usually need more fluid. Older adults may feel less thirsty and should drink regularly. People with heart failure or kidney disease may be told to limit fluids — always follow your doctor’s advice. Children’s needs depend on their age and size.

For calories and nutrition, try the calorie calculator and the macro calculator. This is general information, not medical advice — see our disclaimer.

Frequently asked questions

How much water should I drink a day?

A common rule of thumb is about 35 ml per kilogram of body weight, plus extra for exercise and hot weather. For a 75 kg adult who exercises 30 minutes a day, that is about 3 litres of fluid in total from drinks and food.

Is it really 8 glasses a day?

“Eight glasses of 8 ounces” (about 2 litres) is an easy reminder rather than a scientific rule. Needs vary with body size, activity and climate. The European Food Safety Authority suggests total water of about 2.0 litres a day for women and 2.5 litres for men, including water from food.

Do tea and coffee count?

Yes. Tea, coffee, milk and other drinks all count towards your fluid intake. Caffeine has a mild effect, but regular coffee drinkers still gain most of the water.

How much extra should I drink when exercising?

This calculator adds about 350 ml for every 30 minutes of exercise. Long or intense sessions, especially in heat, may need more, and sometimes electrolytes as well as water.

How do I know if I’m drinking enough?

Pale yellow urine and rarely feeling thirsty are good signs. Dark urine, headaches, tiredness and dizziness can mean you need more fluid.

Can you drink too much water?

Yes, though it is rare. Drinking very large amounts quickly — especially during long endurance events — can dilute the sodium in your blood. Drink to thirst and don’t force down huge amounts.

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