How to use it
- Enter your body weight in kg or lb and choose your sex (used only for the reference values).
- Add minutes of sweating exercise on a typical day, and pick a hot or very hot climate if you spend the day in it.
- Women can add pregnancy or breastfeeding, which EFSA covers with +300 ml and +700 ml respectively.
- Read the estimate in litres, millilitres, cups and fluid ounces next to the EFSA and US adequate intakes, and adjust to thirst.
Example
A 70 kg woman who runs for 45 minutes in a temperate climate: base 70 × 35 = 2,450 ml, exercise 45 ÷ 30 × 350 = 525 ml, total 2,975 ml ≈ 3.0 L, or about 12.4 cups (240 ml) and 101 US fl oz. On a hot summer day the tool adds 500 ml (3.5 L). For comparison the EFSA adequate intake for women is 2.0 L of total water a day without exercise or heat, so the extra litre is the activity and climate allowance.
How the estimate is built, and why it is only a guide
base = weight (kg) × 35 ml
exercise = minutes ÷ 30 × 350 ml
climate = 0 (temperate) · 500 ml (hot) · 1,000 ml (very hot or humid)
condition = 0 · 300 ml (pregnancy) · 700 ml (breastfeeding)
total = base + exercise + climate + condition
1 cup = 240 ml (US nutrition-label cup) · 1 US fl oz = 29.5735 mlThe 30–35 ml/kg figure is a long-standing clinical convention for healthy adults (for example in dietetic handbooks and ESPEN guidance on fluid requirements), not a measured requirement for any individual. The exercise allowance follows the common “about 12 oz per 30 minutes” guidance from fitness bodies such as the American Council on Exercise; actual sweat losses range from 0.3 to over 2 L per hour. The climate additions are flat amounts to show direction, not precise values. The EFSA and NASEM reference values are adequate intakes derived from observed intakes and urine osmolality in healthy populations, and they include water in food. People with heart failure, kidney disease or on fluid restriction must follow medical advice instead; drinking far more than needed can cause hyponatraemia, especially during endurance events. The previous version used the same 35 ml/kg and 350 ml per 30 minutes rule without climate, pregnancy or reference values.
Sources: EFSA 2010 – Scientific Opinion on Dietary Reference Values for water, NASEM 2004 – Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, ACE – How to stay hydrated during exercise.
This is a rough guide for healthy adults, not medical advice. If you have a kidney, heart or liver condition, take diuretics, or are advised to limit fluids, follow your doctor’s instructions.
Signs you are drinking the right amount
Urine that is pale straw-coloured most of the day, rarely feeling thirsty and a stable body weight before and after training (losing more than 2% during a session means you should drink more) are better indicators than any formula. Dark urine, headaches, tiredness and dizziness when standing point to too little; needing the toilet every half hour with clear urine points to more than you need.
Questions
- How much water should I drink a day?
- Most healthy adults do well on 2–3 litres of total water, roughly 1.6–2.5 litres of it from drinks. The calculator scales this with your weight (35 ml/kg) and adds allowances for exercise and heat.
- How many cups is 2.5 litres?
- About 10.4 cups of 240 ml, or 84.5 US fluid ounces. The tool shows all four units for every result.
- Do coffee, tea and food count towards water intake?
- Yes. The reference values are for total water: food typically supplies 20–30%, and caffeinated drinks hydrate almost as well as water at usual doses.
- Can I drink too much water?
- Yes. Drinking well beyond thirst – more than about a litre an hour for several hours – can dilute blood sodium (hyponatraemia). The tool warns above 5 litres a day; endurance athletes should drink to thirst or a measured sweat rate.