No formula can calculate your water needs exactly. A practical method: take the EFSA figure for total water (2.0 L for women, 2.5 L for men), subtract about 20% for water from food, add a margin for heat, exercise or illness — and treat the result as an estimate you check in practice.
Step 1: the starting point
EFSA considers about 2.0 L of total water adequate for adult women and about 2.5 L for adult men in moderate conditions. US values (NASEM) are higher — about 2.7 L and 3.7 L. Use a figure close to the European one as your reference and watch how you feel.
Step 2: subtract water from food
Around 20% of daily water usually comes from food. From 2.0 L that leaves about 1.6 L, and from 2.5 L about 2.0 L to drink. If you eat plenty of soup, fruit and vegetables that share is larger; with a diet low in them it is smaller.
Step 3: adapt to your day
- Heat and humidity: drink more, especially if you sweat a lot.
- Exercise: weigh yourself before and after — each kilogram lost is roughly a litre to replace (see How Much Water Should You Drink During Exercise?).
- Pregnancy and breastfeeding: EFSA suggests roughly 0.3 L extra in pregnancy and 0.7 L extra while breastfeeding.
- Illness with fever, vomiting or diarrhoea: losses rise — contact a doctor if in doubt.
Examples (illustrative, not recommendations)
- A woman with an office job in a moderate climate: 2.0 L of total water, minus about 20% from food — roughly 1.6 L from drinks.
- A man with an hour of training on a hot day: 2.5 L of total water, about 2.0 L from drinks on a day without exertion, plus extra fluid depending on sweat losses (check by weighing).
- A pregnant woman: about 2.0 L plus about 0.3 L according to EFSA, so around 2.3 L of total water — agree your target with your doctor.
Weight-based formulas — their limits
In healthcare a rule of around 30 mL per kilogram of body weight per day is sometimes used, but it is a simplification. It ignores body composition, climate, diet, activity and illness, and can give unrealistic results for people who are very light or have a high body weight. For older adults the ESPEN guideline points to at least about 1.6 L (women) and 2.0 L (men) of fluids a day. None of these formulas is a universal medical standard.
When you need individual advice
Discuss your fluid intake with a doctor if you have heart failure, kidney or liver disease, a sodium disorder, take diuretics, are pregnant with complications, or are planning changes for a child. In these situations both too little and too much fluid can be dangerous.
How to check your target in practice
Set a target, watch your thirst, energy and urine colour for a week, then adjust. In the WODA app you set your own daily goal and log the water you drink, which makes this kind of observation easier. Also read How Much Water Should You Drink Per Day? and Signs of Dehydration.
Frequently asked questions
Is there a calculator that gives the exact amount?
No. Calculators give an estimate based on simplifications. Treat the result as a starting point and adapt it to your day — and, with medical conditions, to your doctor’s advice.
Does the requirement depend on body weight?
To some extent, but it also depends on activity, climate, diet and health. Weight-based formulas alone are only an approximation.
Do I need much more in hot weather?
Usually yes, because you lose more fluid in sweat. How much more depends on temperature, humidity and activity.
Sources
- EFSA — Scientific Opinion on Dietary Reference Values for water (EFSA Journal 2010;8(3):1459)
- National Academies — Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005)
- Volkert D. et al. — ESPEN guideline on clinical nutrition and hydration in geriatrics (Clin Nutr 2019;38(1):10–47)
- Sawka M.N. et al. — ACSM Position Stand: Exercise and Fluid Replacement (Med Sci Sports Exerc 2007;39(2):377–390)
This article is for information only and does not replace advice from a doctor or dietitian. If you are unsure or have worrying symptoms, contact a doctor.