How much water should you drink?
Your fluid needs depend on how much you sweat, the weather, what you eat and your health. Water is a useful default drink, but tea, coffee, milk and other non-alcoholic drinks also contribute. You do not have to meet your entire fluid intake through plain water.
The NHS uses six to eight cups or glasses a day as a general guide, with more needed in some situations. Glass sizes differ, which is why this calculator lets you choose the size of the glass or bottle you actually use.
How we estimate your daily fluids
The European Food Safety Authority gives adult total-water references of 2.5 litres for men and 2.0 litres for women in moderate conditions. Those figures include water in food as well as drinks.
To make a practical drinks estimate, this tool assumes around 20% comes from food, leaving starting drinks amounts of 2.0 litres and 1.6 litres respectively. That food contribution is an assumption, not a measurement of your diet. Eating more water-rich food can change the amount you need to drink.
What exercise changes
For each hour of exercise you enter, the tool adds an illustrative range of 0.4–0.8 litres to your daily drinks estimate. It is based on a commonly used exercise fluid range and is not a personalised sweat-replacement prescription. For example, a male reference with one hour of exercise gives 2.4–2.8 litres from drinks over the day.
Sweat losses differ substantially. Heat, intensity, clothing and individual physiology all matter. If you select hot conditions, the result reminds you that needs may increase; it does not invent a precise weather adjustment. Long endurance sessions need a more individual fluid and electrolyte plan.
Use thirst and context
Spread drinks across the day and take account of thirst and urine colour. Pale yellow urine is a useful rough guide for many people, though supplements and medicines can change its colour. Avoid forcing large volumes of water, particularly during exercise.
If you have a prescribed fluid limit, kidney or heart disease, or need a specific hydration plan, follow your clinician’s advice. This calculator is for adults and is not designed for pregnancy, breastfeeding or illness.
Questions about this calculator
Is the result only plain water?
No. It is an estimate of fluid from drinks, including water, tea, coffee, milk and other non-alcoholic drinks. Water in food is additional.
Why does this use sex rather than body weight?
It starts with EFSA adult total-water reference values, then makes a transparent assumption about the share coming from drinks. It does not pretend that body weight alone measures your fluid needs.
Does hot weather change the number?
Heat and sweat losses can increase needs. The tool flags that context but does not add a precise allowance that it cannot individualise.
Can I use this if my doctor has limited my fluids?
No. Follow your prescribed fluid limit and ask your clinician for advice rather than using a general calculator.