Early signs
Mild dehydration often shows up before you feel very thirsty:
- Dark yellow urine — the simplest, most reliable check (aim for pale straw).
- Thirst and a dry mouth or lips
- Headache
- Tiredness and low energy
- Trouble concentrating or feeling foggy
- Less frequent urination
More serious signs
Significant dehydration needs prompt attention:
- Dizziness or light-headedness
- Rapid heartbeat
- Very dark urine or barely urinating
- Sunken eyes, dry skin that's slow to bounce back
- Confusion (seek medical help)
What causes it
Simply not drinking enough is the most common cause, but heat, exercise and sweating, illness (vomiting, diarrhoea, fever), alcohol, and high altitude all increase fluid loss. Older adults and children are more vulnerable.
How to rehydrate
For mild dehydration, sip water steadily — don't gulp huge amounts at once. After heavy sweating or illness, you also lose electrolytes (sodium, potassium), so an oral rehydration drink or a salty snack with your water helps your body hold onto the fluid. Foods like fruit and soup count too.
How much you need
A practical target is around 35 ml per kg of body weight plus extra for exercise and heat — for many adults that's 2–3 litres. Our water intake calculator gives your personal number, and the easiest daily check is simply keeping your urine pale.
When to get help
Seek medical care for severe symptoms — confusion, fainting, a racing heart, or no urination — or for dehydration in a baby, young child, or older adult that doesn't improve with fluids.