How Much Water You Actually Need During a Heatwave: Guidelines by Age and Weight

During a heatwave, the body loses more water through perspiration, and the risk of dehydration increases. There is, however, no universal amount that applies to everyone – fluid requirements are calculated based on weight, age, sex, humidity levels, physical exertion, diet, possible pregnancy, and health status. In practice, you start from a baseline requirement, which is then adjusted according to how much each person sweats.

How much fluid you need based on your weight

For a healthy adult, the general guideline is 30–35 ml of fluid per kilogram of body weight, daily. In concrete terms:

• 50 kg → approximately 1.5–1.75 litres per day
• 70 kg → approximately 2.1–2.45 litres per day
• 90 kg → approximately 2.7–3.15 litres per day

These figures include all fluids consumed throughout the day, not just still water, and represent the baseline requirement – not a special target for heatwave days.

For those aged 18–60, the guideline of 35 ml/kg can be used, whilst after the age of 60 the recommended amount decreases to 30 ml/kg. For people with obesity, calculating solely on the basis of total body weight may indicate an excessively high requirement, so the figures must also be adapted according to health status, any ongoing treatments, and activity level.

Water requirements by age group

• Infants aged 6–12 months: 0.8–1 litre of total water/day
• 1 year: 1.1–1.2 litres/day
• 2–3 years: approximately 1.3 litres/day
• 4–8 years: approximately 1.6 litres/day
• 9–13 years: 2.1 litres (boys), 1.9 litres (girls)
• 14–17 years: 2.5 litres (boys), 2 litres (girls)
• Adults: 2.5 litres (men), 2 litres (women)
• Pregnant women: 2.3 litres/day
• Breastfeeding women: 2.7 litres/day

These figures refer to total water obtained from both drinks and food. For infants under six months of age, the per-kilogram calculation should not be applied at home. Exclusively breastfed babies do not need additional water even during a heatwave, but may be offered the breast more frequently. Formula-fed babies may, if necessary, be given small amounts of boiled and cooled water, in accordance with their doctor's recommendations.

How much to drink during physical exertion in the heat

During physical activity in hot weather, it is advisable to drink little and often, before thirst becomes intense. For moderate exertion lasting under two hours, approximately 240 ml every 15–20 minutes is recommended – an amount that applies only during the period of exertion, not for every hour of the entire day. If sweating continues for several hours, drinks containing electrolytes may also be used.

Be mindful of excess as well: consuming a large quantity of water very rapidly can be dangerous. An intake of more than approximately 1.4 litres within a single hour should be avoided, as excess water can cause sodium levels in the blood to drop too low. The rate of hydration must be adapted to the intensity of exertion, the temperature, and how much you are sweating.

Foods and drinks that aid hydration

Water remains the primary choice during a heatwave. Milk, tea, coffee, and sugar-free drinks also contribute to daily fluid intake. Fruits, vegetables, soups, and yoghurt contain significant amounts of water. Alcohol should be avoided, and very sugary drinks consumed only in small quantities.

Signs of dehydration

Pale straw-coloured urine generally indicates good hydration. Dark-coloured urine, a dry mouth, headache, dizziness, and infrequent urination may be signs of dehydration. Confusion, fainting, and very hot skin are warning signs that require urgent medical attention. During heatwave days, the condition of children, elderly people, and those who are unwell should be monitored with extra care.

People with heart or kidney failure, liver disease, or those taking diuretics should not increase their fluid intake without medical advice. The same caution applies to those who have been advised to restrict their water consumption – for them, values calculated on the basis of weight or age may not be appropriate. A doctor's recommendation always takes precedence over general hydration guidelines.