How to estimate your water needs
- Choose kilograms or pounds, and your sex for the reference intakes.
- Enter your weight and how many minutes you exercise on a typical day.
- Choose hot if you exercise or work in the heat.
- Read your daily total, how much of it to drink, and how it compares with the official reference intakes.
How the estimate works
total water = 30 mL × weight (kg) + exercise allowance
- 30 mL per kg is the weight-based guide in the Nordic nutrition recommendations, one of the national approaches EFSA reviewed; others range from 25 to 35 mL per kg.
- Exercise: 0.5 litres per hour, the bottom of the 0.5 to 4.0 litres an hour that adults sweat during exercise, according to the National Athletic Trainers' Association.
- In heat: about 0.71 litres per hour of activity, from NIOSH's advice to drink a cup (8 US fl oz) every 15 to 20 minutes when working in the heat.
- To drink: 80% of the total, because about a fifth of the water we take in comes from food.
Worked example
A woman weighing 60 kg who exercises for 30 minutes in mild weather:
- 30 mL × 60 = 1,800 mL, plus 0.5 L × 0.5 hours = 250 mL for exercise.
- Total water: 2,050 mL, about 2.1 litres a day from food and drinks.
- To drink: 80% of that is about 1.6 litres, or 7 glasses of 250 mL.
Official reference intakes
| Adults | EFSA (Europe) | US National Academies |
|---|---|---|
| Women | 2.0 L a day | 2.7 L a day |
| Men | 2.5 L a day | 3.7 L a day |
Both are total water from food and drinks, based on what healthy people actually take in, and both apply to a mild climate with moderate activity. EFSA adds about 300 mL a day in pregnancy and about 700 mL when breastfeeding. The National Academies note that people who are very active in hot weather often need 6 litres or more.
Needs vary
No single number fits everyone. Body size, food, weather and activity all change how much water you lose, and sweat rates differ a lot from person to person. To learn yours, weigh yourself before and after an hour of exercise: each kilogram lost, plus whatever you drank, is about a litre of sweat. For most healthy people, drinking when thirsty and with meals is enough.
Not medical advice
This is a general estimate, not medical advice. If you have heart, kidney or liver disease, are pregnant or breastfeeding, or a doctor has asked you to limit fluids, follow their guidance instead.
Sources
Each link was last checked on .
- Scientific Opinion on Dietary Reference Values for water, European Food Safety Authority (2010): adequate total water of 2.0 L for women and 2.5 L for men, about 20% of it from food, and the national 25–35 mL per kg guidance it reviews.
- Report sets dietary intake levels for water, salt, and potassium, US National Academies (Institute of Medicine) (2004): adequate total water of 2.7 L for women and 3.7 L for men, thirst as a guide, and higher needs in heat.
- Position Statement: Fluid Replacement for the Physically Active, National Athletic Trainers' Association, Journal of Athletic Training (2017): adult sweat rates of 0.5 to 4.0 L an hour during exercise, and weighing to find yours.
- Heat Stress: Hydration (publication 2017-126), US National Institute for Occupational Safety and Health (CDC) (2017): 1 cup every 15–20 minutes when working in heat, and no more than 1.4 L an hour.
Common uses
- Starting a new exercise routine. See how much extra water an hour of exercise adds to your day, and weigh yourself before and after a session to learn your own sweat rate.
- Working or training in the heat. Switch to hot weather to see the larger allowance for activity in the heat, and keep to NIOSH's limit of about 1.4 litres an hour.
- Setting a daily water bottle goal. Turn the drinks estimate into a number of glasses or bottle refills to spread through the day.