Daily Water Intake Calculator

Roughly 60% of your body is water, and you lose around two to three litres of it every day through breathing, sweating and urine. Replacing that consistently — rather than drinking nothing all morning and then catching up at lunch — keeps your blood volume, joint lubrication and brain function steady.

How we calculate your target

The base figure is about 35ml of water per kilogram of body weight. Then we add an activity allowance of around 350ml for moderate exercise (a few sessions a week) and 700ml for high-activity days, to replace what you lose through sweat. The result is converted into litres, fluid ounces and the equivalent number of 250ml glasses.

A worked example

A 70kg adult who exercises a few times a week: 70 × 35ml = 2,450ml baseline, plus 350ml activity bonus, gives around 2.8 litres a day, or about 11 standard glasses. On a hot summer afternoon or a long run, expect to need noticeably more.

Does it have to be plain water?

No. The NHS counts tea, coffee, milk, sugar-free drinks and water from food in your daily fluid total. Fruit and vegetables contribute around 20% of most people's intake — a cucumber is 96% water, an apple about 85%. Alcohol is the only one that works against you on balance, because it suppresses the hormone that helps you retain fluid.

Why this matters

Mild dehydration — just 1–2% of body weight lost in fluid — measurably reduces concentration, reaction time and mood, and is a common hidden cause of afternoon headaches and tiredness. Knowing roughly how much you need turns "I should drink more" into a target you can actually meet: a glass with each meal, a 500ml bottle at your desk, and a top-up around exercise will get most people there without thinking about it.

When to drink more

• Hot weather or heated indoor environments — add at least 500ml.
• Pregnancy and breastfeeding — the NHS suggests an extra 300ml and 700ml a day respectively.
• Fever, vomiting or diarrhoea — rehydration is more important than the exact volume.
• High-altitude travel or long flights — cabin air is very dry; aim for an extra glass per hour airborne.

Common mistakes

• Treating the target as a hard rule. Thirst and urine colour are better real-time guides than a daily total.
• Slamming a litre in one go. You absorb fluid faster when you sip steadily.
• Counting only water and ignoring tea, coffee and food.
• Forgetting to scale up for exercise or hot weather.

Related tools: Calorie Needs Calculator · BMI Calculator · Ideal Weight Calculator