Dehydration happens when your body loses more fluid than it takes in. Mild dehydration is common and easy to fix, especially on hot days, after exercise, or when you've been unwell. The trick is noticing it early.
At a glance: drink more, or get help?
Early signs Drink more
- Thirst
- Dark yellow or strong-smelling urine
- Peeing less often
- Dry mouth, lips or eyes
- Headache
- Tiredness, trouble concentrating
- Dizzy or light-headed
Most get better quickly once you drink more.
Urgent signs Get help now
- Confused, disoriented or unusually drowsy
- Fast heartbeat or fast breathing
- No urine all day
- Fainting, or can't stay standing
- Can't keep fluids down
- In babies: sunken soft spot, no tears, few wet nappies
Get medical help straight away. In Australia, call 000.
Early signs of dehydration
- Feeling thirsty. Thirst is a useful signal, but by the time you notice it you're usually already a little behind.
- Dark yellow, strong-smelling urine. Pale straw-coloured urine is the goal.
- Peeing less often than usual, or only small amounts.
- Dry mouth, lips or eyes.
- Headache.
- Tiredness and trouble concentrating.
- Feeling dizzy or light-headed, especially when standing up.
Most of these get better quickly once you drink more.
Signs that need urgent medical help
Get medical help straight away, or call emergency services (000 in Australia), if someone:
- is confused, disoriented or unusually drowsy;
- has a fast heartbeat or fast breathing;
- hasn't passed urine all day;
- faints or can't stay standing;
- can't keep fluids down because of vomiting or diarrhoea;
- is a baby or young child with a sunken soft spot on the head, no tears when crying, or few wet nappies.
Who is most at risk?
- Babies and young children, who lose fluid quickly when unwell.
- Older adults, whose sense of thirst weakens with age.
- People with vomiting, diarrhoea or fever.
- People exercising hard or working outdoors, especially in heat.
- People with conditions such as diabetes, or on medicines that make you pass more urine.
Test yourself: early, urgent or risk factor?
Knowing the difference is the useful part. For each one, guess whether it's an early sign, a sign that needs urgent help, or something that puts you more at risk.
Early sign, urgent, or risk factor?
Dark yellow, strong-smelling urine
Early signPale straw-coloured urine is the goal. Most early signs get better quickly once you drink more.
Being confused or unusually drowsy
UrgentGet medical help straight away, or call 000 in Australia.
Being an older adult
Risk factorOlder adults are more at risk because the sense of thirst weakens with age.
Feeling dizzy when you stand up
Early signLike most early signs, it usually gets better quickly once you drink more.
Not passing urine all day
UrgentThis one needs medical help straight away.
Working outdoors on a hot day
Risk factorExercising hard or working outdoors, especially in heat, puts you at higher risk.
A baby with no tears when crying
UrgentIn babies and young children, watch for no tears, a sunken soft spot and few wet nappies.
Taking medicines that make you pass more urine
Risk factorSome medicines, and conditions such as diabetes, raise your risk.
How to rehydrate
- Mild dehydration: drink water steadily, in sips rather than all at once.
- After vomiting or diarrhoea: an oral rehydration solution from a pharmacy replaces salts as well as water.
- After heavy sweating: water plus food is usually enough. For long, sweaty sessions a sports drink can help. See how much to drink when you exercise or it's hot, and our hot-weather dehydration checklist.
- Skip alcohol while you rehydrate. It works against you. Here's how different drinks compare.
How to prevent it
The simplest prevention is drinking steadily through the day rather than catching up in the evening, and drinking more when it's hot or you're active. Our guide on how to drink more water has practical ways to make that automatic.
That's also the idea behind Hydration Hero. It sets a personal daily goal, reminds you at times you choose, and weights drinks honestly, so a coffee or a beer isn't counted as if it were water. Its optional weather feature adds water on hot or humid days, and Health Power Up raises your goal on active days. You can try the App Clip on iPhone without installing anything.
Frequently asked questions
What are the first signs of dehydration?
Common early signs are thirst, dark yellow or strong-smelling urine, peeing less often, a dry mouth, headache, tiredness and feeling dizzy or light-headed. They usually improve quickly once you drink more.
When is dehydration an emergency?
Seek urgent medical help if someone is confused or very drowsy, has a fast heartbeat or breathing, hasn't urinated all day, faints, or can't keep fluids down. In babies, watch for a sunken soft spot, no tears and few wet nappies. In Australia, call 000 in an emergency.
What is the fastest way to rehydrate?
For mild dehydration, sip water steadily. After vomiting or diarrhoea, an oral rehydration solution replaces salts as well as fluid. Avoid alcohol while rehydrating.
Can a water tracker help prevent dehydration?
It can help by setting a daily goal and reminding you to drink before you fall behind (see how to set up a drink water reminder app). Hydration Hero also adds water to your goal on hot or humid days and active days, and weights drinks like coffee and alcohol for their real hydration effect.
Read next
- Dehydration in hot weatherA checklist for heatwave days
- How to drink more water12 habits that prevent it
- Water for exercise and heatHow much extra you need when you sweat
- Does coffee count as water?Which drinks help and which work against you
Sources
- Dehydration: signs, symptoms and treatment, healthdirect
- Dehydration, NHS
- Hot weather risks and staying cool, healthdirect
Last updated 10 November 2026. This is general information, not medical advice. If you have a heart, kidney or liver condition, or have been told to limit fluids, follow your doctor's advice on how much to drink.