How much water should you drink in hot weather? A practical guide

Start by distinguishing total water from drinking water

Reference figures need to specify what they count. In its opinion on nutritional reference values, Anses, the French food safety agency, describes around 2.5 litres of total water a day, including about 1.5 litres from drinks. This population reference therefore includes the water in food.

The reference values from EFSA, the European Food Safety Authority, also concern total water intake: water from both beverages and food. Its adequate intakes are 2.0 litres a day for adult women and 2.5 litres for adult men, in moderate conditions of physical activity and temperature.

This distinction matters. EFSA's figures are not simply the amount of water to drink from a glass or bottle. Fruit, vegetables, soups, yoghurt, prepared dishes and other beverages all contribute. Check what a quoted figure includes: total water is not an extra volume to add to your bottle. EFSA's values describe moderate conditions; intense heat and substantial physical activity require intake to be adjusted to losses.

Why heat increases water needs

In hot weather, the body works to keep its internal temperature stable. One of its main mechanisms is sweating: sweat evaporates from the skin and dissipates heat. That regulation comes at a cost: water and mineral salts are lost.

A large study published in Science in 2022 by Yamada and colleagues examined water turnover in 5,604 people, aged from eight days to 96 years, across several countries and very different ways of life.

Water turnover varied with age, body size, body composition, physical activity, pregnancy, air temperature, humidity, altitude and socioeconomic circumstances. Temperature plays a real part, but cannot predict water needs on its own.

One finding is particularly useful. In a subgroup measured in two seasons, turnover was about 3.7 litres a day in summer at 29°C, compared with 3.0 litres in spring at 18°C. Turnover includes water from drinks and food, as well as water produced by metabolism. These figures describe a measured seasonal difference in that subgroup; they do not mean everyone should drink 3.7 litres as soon as the temperature reaches 29°C.

Can drinking-water needs be calculated from temperature?

Temperature tells you something about your surroundings, but cannot calculate the contents of your water bottle. Spending a day somewhere cool and carrying loads outdoors produce different losses, even if the thermometer reads 30°C in both cases.

French hot-weather recommendations give a general reference of 1.5 to 2 litres of water a day, taken regularly. This is neither a ceiling for every activity nor an amount to impose on someone with a medically prescribed fluid restriction. Activity, sweating and advice specific to your circumstances remain decisive.

Instead of automatically adding a fixed amount for every five-degree rise, plan the day: keep water within reach, drink regularly, reduce exertion during the hottest hours and take breaks somewhere cool. The table below brings these reference points together by situation.

If you have heart, kidney or liver failure, take diuretics or have a prescribed fluid restriction, ask your clinician how much to drink during hot weather. Infants and pregnant or breastfeeding people also have specific needs; general adult reference figures are not their hydration plan.

In intense heat, drink before you feel thirsty

French hot-weather advice emphasises a simple habit: drink water regularly without waiting to feel thirsty. This matters particularly in hot weather or during physical activity, when water losses increase.

It is especially important for older adults. Thirst can become less reliable with age, so dehydration may develop without a clear feeling of thirst. For an older or dependent person, offer drinks regularly, even if they do not ask. Keeping a glass or bottle within reach makes this easier. Individual medical instructions take priority.

Infants, young children, pregnant people, those with chronic conditions and people working or exercising outdoors also require greater attention to hydration.

Physical work in heat: a specific reference point

Activity makes a substantial difference. Walking slowly in shade at 28°C is a different situation from running, gardening, carrying loads or working on a construction site at 32°C.

For workers doing moderate activity in heat for less than two hours, NIOSH recommends one 8-US-fluid-ounce cup, about 237 ml, every 15 to 20 minutes. That amounts to approximately 0.7 to 0.95 litres an hour in this specific occupational setting. For exercise, the amount needs to reflect duration, intensity and losses; one hourly rate does not suit every activity.

NIOSH also says fluid intake should generally not exceed six cups an hour, or approximately 1.4 litres an hour. This upper limit is not a target. Drinking too much water quickly can become dangerous, particularly when sodium replacement is insufficient.

For prolonged activity with heavy sweating, water alone may not be enough. When sweating continues for several hours, NIOSH's recommendations include drinks containing balanced electrolytes to replace mineral salts.

Drinking too much can also be dangerous

“More is better” is the wrong approach to water. Excessive intake can cause hyponatraemia, an excessive dilution of sodium in the blood. This can occur when fluid intake exceeds losses, particularly during prolonged exertion. Some illnesses and medicines also affect the balance of water and sodium. Hydration should replace losses, rather than greatly exceed them.

The practical approach is to drink regularly, in small amounts throughout the day, while taking heat, sweating and activity into account, rather than drinking a very large amount at once.

Useful signs: urine, thirst, tiredness and context

For a healthy adult, several simple signs can help guide adjustments.

Pale-yellow urine is generally consistent with adequate hydration. Strong thirst, dry lips, reduced urine output, unusual tiredness, headache, dizziness or weakness can indicate developing dehydration. The French health insurance service, Ameli, lists thirst, dry lips, unusual fatigue and weakness among the early signs in hot weather.

Consider urine colour alongside other symptoms and what you have drunk: colour alone cannot diagnose overhydration. Avoid forcing down large volumes quickly to obtain completely clear urine. Confusion, severe faintness or loss of consciousness during intense heat requires emergency help. In France, call 15 or 112; elsewhere, use the local emergency number.

What should you drink in hot weather?

Water remains the usual choice: tap, filtered or mineral water, depending on preference and local quality. For variety, try homemade infused water, unsweetened cold herbal tea, sparkling water or cold broth. Water-rich fruit and vegetables also contribute through meals.

Hot weather is a good occasion for cool, lightly sweetened or unsweetened drinks without alcohol. To vary the menu beyond sugary soft drinks, browse our refreshing mocktail recipes or non-alcoholic aperitif drinks.

Health authorities advise avoiding alcohol in intense heat. Ameli explains that alcohol reduces the body's ability to cope with heat and promotes dehydration; NIOSH also recommends avoiding alcohol and drinks very high in caffeine or sugar during heat exposure.

Be mindful of very sugary or highly caffeinated drinks. They may feel refreshing immediately, but are not always helpful for a regular hydration routine, particularly when they replace water entirely.

A practical table to keep in mind

Scroll the table to read all columns.