Signs of dehydration, and what to do about them
This article is general information, not medical advice. It is not a substitute for a doctor, dietitian or other qualified professional who knows your circumstances. If you have a medical condition, are pregnant, or are changing your diet or exercise significantly, talk to one first.
Dehydration happens when you lose more fluid than you take in. Mild dehydration is common, easy to fix and easy to miss; severe dehydration is a medical emergency. Knowing the signs matters more than hitting a daily number, though the water intake tracker helps if you tend to forget to drink.
What dehydration is
Water makes up more than half of an adult's body and is lost constantly through breathing, sweat, urine and stools. Normally thirst and the kidneys keep the balance: when you are short of water, you feel thirsty and the kidneys concentrate urine to save fluid. Dehydration occurs when losses outrun intake, for example through vomiting and diarrhoea, heavy sweating in heat or exercise, fever, drinking too much alcohol, or simply not drinking enough. It also involves the loss of salts such as sodium and potassium, which is why rehydration drinks contain them.
Signs in adults
Early signs, listed by the NHS among others, include:
- feeling thirsty;
- dark yellow, strong-smelling urine;
- peeing less often than usual, for example fewer than four times a day;
- feeling dizzy or light-headed, especially on standing;
- feeling tired, with a headache;
- a dry mouth, lips and eyes.
Urine colour is a handy everyday check: pale straw-yellow suggests you are well hydrated, while dark yellow or amber suggests you need more fluid. Some vitamins and foods change urine colour, so it is a guide rather than a test.
Signs in babies and children
Babies and young children become dehydrated faster than adults because they are small and lose fluid quickly when unwell. Signs include a sunken soft spot (fontanelle) on a baby's head, few or no wet nappies over several hours, few or no tears when crying, a dry mouth, sunken eyes, unusual drowsiness or irritability, and cold, blotchy hands and feet. A baby or child with vomiting or diarrhoea who shows these signs needs medical advice promptly. The baby tracker can log feeds and nappies if you are keeping an eye on an unwell baby.
Who is most at risk
Besides babies and young children, older adults are at higher risk because the sense of thirst weakens with age and some medicines increase fluid loss. People with diabetes, kidney disease or conditions affecting swallowing, people who are ill with vomiting, diarrhoea or fever, and anyone exercising hard or working in heat also lose fluid faster than usual. Alcohol increases urine output, which is part of why a hangover feels the way it does. How much water you should drink a day — what the evidence says, and where 8 glasses came from covers how needs change with weight, exercise and climate.
Treating it and when to get help
Mild dehydration is treated by drinking more fluids, little and often if you feel sick. Water is fine for most people; oral rehydration solutions from a pharmacy replace salts as well and are recommended after vomiting or diarrhoea, especially for children and older adults. Avoid sugary fizzy drinks for children with diarrhoea. Get urgent medical help if someone has signs of severe dehydration: confusion or disorientation, not peeing all day, a fast heartbeat, fainting, fits, or a baby or child who is very drowsy or floppy. Drinking far more than you need can also be harmful, diluting the body's sodium, so there is no benefit in forcing large amounts. For an everyday habit, the water tracker suggests a goal from your weight, exercise and climate; the calorie calculator is useful alongside it if you are adjusting your diet.
Sources and further reading
The claims in this guide rest on these references, which were checked when the guide was last updated. Spotted an error? The contact page says how to report it.